erichkeane updated this revision to Diff 419750.
erichkeane added a comment.

Do a rebase, only conflict was with ReleaseNotes.rst.


CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D119544/new/

https://reviews.llvm.org/D119544

Files:
  clang/docs/ReleaseNotes.rst
  clang/include/clang/Sema/Sema.h
  clang/include/clang/Sema/Template.h
  clang/lib/Sema/SemaConcept.cpp
  clang/lib/Sema/SemaTemplate.cpp
  clang/lib/Sema/SemaTemplateDeduction.cpp
  clang/lib/Sema/SemaTemplateInstantiate.cpp
  clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
  clang/lib/Sema/TreeTransform.h
  clang/test/CXX/temp/temp.constr/temp.constr.constr/non-function-templates.cpp
  clang/test/SemaTemplate/deferred-concept-inst.cpp
  clang/test/SemaTemplate/instantiate-requires-clause.cpp
  clang/test/SemaTemplate/trailing-return-short-circuit.cpp

Index: clang/test/SemaTemplate/trailing-return-short-circuit.cpp
===================================================================
--- /dev/null
+++ clang/test/SemaTemplate/trailing-return-short-circuit.cpp
@@ -0,0 +1,61 @@
+// RUN: %clang_cc1 -std=c++20 -verify %s
+
+template <class T>
+requires(sizeof(T) > 2) || T::value // #FOO_REQ
+    void Foo(T){};                  // #FOO
+
+template <class T>
+    void TrailingReturn(T)     // #TRAILING
+    requires(sizeof(T) > 2) || // #TRAILING_REQ
+    T::value{};                // #TRAILING_REQ_VAL
+template <bool B>
+struct HasValue {
+  static constexpr bool value = B;
+};
+static_assert(sizeof(HasValue<true>) <= 2);
+
+template <bool B>
+struct HasValueLarge {
+  static constexpr bool value = B;
+  int I;
+};
+static_assert(sizeof(HasValueLarge<true>) > 2);
+
+void usage() {
+  // Passes the 1st check, short-circuit so the 2nd ::value is not evaluated.
+  Foo(1.0);
+  TrailingReturn(1.0);
+
+  // Fails the 1st check, but has a ::value, so the check happens correctly.
+  Foo(HasValue<true>{});
+  TrailingReturn(HasValue<true>{});
+
+  // Passes the 1st check, but would have passed the 2nd one.
+  Foo(HasValueLarge<true>{});
+  TrailingReturn(HasValueLarge<true>{});
+
+  // Fails the 1st check, fails 2nd because there is no ::value.
+  Foo(true);
+  // expected-error@-1{{no matching function for call to 'Foo'}}
+  // expected-note@#FOO{{candidate template ignored: constraints not satisfied [with T = bool]}}
+  // expected-note@#FOO_REQ{{because 'sizeof(_Bool) > 2' (1 > 2) evaluated to false}}
+  // expected-note@#FOO_REQ{{because substituted constraint expression is ill-formed: type 'bool' cannot be used prior to '::' because it has no members}}
+
+  TrailingReturn(true);
+  // expected-error@-1{{no matching function for call to 'TrailingReturn'}}
+  // expected-note@#TRAILING{{candidate template ignored: constraints not satisfied [with T = bool]}}
+  // expected-note@#TRAILING_REQ{{because 'sizeof(_Bool) > 2' (1 > 2) evaluated to false}}
+  // expected-note@#TRAILING_REQ_VAL{{because substituted constraint expression is ill-formed: type 'bool' cannot be used prior to '::' because it has no members}}
+
+  // Fails the 1st check, fails 2nd because ::value is false.
+  Foo(HasValue<false>{});
+  // expected-error@-1 {{no matching function for call to 'Foo'}}
+  // expected-note@#FOO{{candidate template ignored: constraints not satisfied [with T = HasValue<false>]}}
+  // expected-note@#FOO_REQ{{because 'sizeof(HasValue<false>) > 2' (1 > 2) evaluated to false}}
+  // expected-note@#FOO_REQ{{and 'HasValue<false>::value' evaluated to false}}
+  TrailingReturn(HasValue<false>{});
+  // expected-error@-1 {{no matching function for call to 'TrailingReturn'}}
+  // expected-note@#TRAILING{{candidate template ignored: constraints not satisfied [with T = HasValue<false>]}}
+  // expected-note@#TRAILING_REQ{{because 'sizeof(HasValue<false>) > 2' (1 > 2) evaluated to false}}
+  // expected-note@#TRAILING_REQ_VAL{{and 'HasValue<false>::value' evaluated to false}}
+}
Index: clang/test/SemaTemplate/instantiate-requires-clause.cpp
===================================================================
--- clang/test/SemaTemplate/instantiate-requires-clause.cpp
+++ clang/test/SemaTemplate/instantiate-requires-clause.cpp
@@ -40,6 +40,18 @@
 
 static_assert(S<void>::f(1));
 
+// Similar to the 'S' test, but tries to use 'U' in the requires clause.
+template <typename T2>
+struct S1 {
+  // expected-note@+3 {{candidate template ignored: constraints not satisfied [with U = int]}}
+  // expected-note@+2 {{because substituted constraint expression is ill-formed: type 'int' cannot be used prior to '::' because it has no members}}
+  template <typename U>
+  static constexpr auto f(U const index) requires(U::foo) { return true; }
+};
+
+// expected-error@+1 {{no matching function for call to 'f'}}
+static_assert(S1<void>::f(1));
+
 constexpr auto value = 0;
 
 template<typename T>
Index: clang/test/SemaTemplate/deferred-concept-inst.cpp
===================================================================
--- /dev/null
+++ clang/test/SemaTemplate/deferred-concept-inst.cpp
@@ -0,0 +1,23 @@
+// RUN: %clang_cc1 -std=c++2a -x c++ %s -verify -Wno-unused-value
+// expected-no-diagnostics
+
+namespace GithubBug44178 {
+template <typename D>
+struct CRTP {
+  void call_foo() requires
+      requires(D &v) { v.foo(); }
+  {
+    static_cast<D *>(this)->foo();
+  }
+};
+
+struct Test : public CRTP<Test> {
+  void foo() {}
+};
+
+int main() {
+  Test t;
+  t.call_foo();
+  return 0;
+}
+} // namespace GithubBug44178
Index: clang/test/CXX/temp/temp.constr/temp.constr.constr/non-function-templates.cpp
===================================================================
--- clang/test/CXX/temp/temp.constr/temp.constr.constr/non-function-templates.cpp
+++ clang/test/CXX/temp/temp.constr/temp.constr.constr/non-function-templates.cpp
@@ -90,3 +90,24 @@
 
 static_assert(C<int>{}); // expected-note{{while checking constraint satisfaction for template 'C<int>' required here}}
 static_assert(D<int>{}); // expected-note{{while checking constraint satisfaction for template 'D<int>' required here}}
+
+// Test the delayed instantiation, the 'foo' implementation shouldn't cause the
+// constraint failure(or crash!) until the use to create 'y'.
+namespace DelayedInst {
+template <unsigned I>
+struct AAA {
+  template <typename T>
+  requires(sizeof(T) == I) // expected-note {{because 'sizeof(int) == 5U' (4 == 5) evaluated to false}}
+      struct B {
+    static constexpr int a = 0;
+  };
+
+  static constexpr auto foo() {
+    return B<int>::a; // expected-error{{constraints not satisfied for class template 'B' [with T = int]}}
+  }
+};
+
+constexpr auto x = AAA<4>::foo();
+constexpr auto y = AAA<5>::foo(); // expected-note {{in instantiation of member function 'DelayedInst::AAA<5>::foo' requested here}}
+
+} // namespace DelayedInst
Index: clang/lib/Sema/TreeTransform.h
===================================================================
--- clang/lib/Sema/TreeTransform.h
+++ clang/lib/Sema/TreeTransform.h
@@ -12974,11 +12974,8 @@
   }
 
   // Transform the trailing requires clause
-  ExprResult NewTrailingRequiresClause;
-  if (Expr *TRC = E->getCallOperator()->getTrailingRequiresClause())
-    // FIXME: Concepts: Substitution into requires clause should only happen
-    //                  when checking satisfaction.
-    NewTrailingRequiresClause = getDerived().TransformExpr(TRC);
+  ExprResult NewTrailingRequiresClause =
+      E->getCallOperator()->getTrailingRequiresClause();
 
   // Create the local class that will describe the lambda.
 
Index: clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
===================================================================
--- clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
+++ clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
@@ -2037,19 +2037,7 @@
       return nullptr;
   }
 
-  // FIXME: Concepts: Do not substitute into constraint expressions
   Expr *TrailingRequiresClause = D->getTrailingRequiresClause();
-  if (TrailingRequiresClause) {
-    EnterExpressionEvaluationContext ConstantEvaluated(
-        SemaRef, Sema::ExpressionEvaluationContext::Unevaluated);
-    ExprResult SubstRC = SemaRef.SubstExpr(TrailingRequiresClause,
-                                           TemplateArgs);
-    if (SubstRC.isInvalid())
-      return nullptr;
-    TrailingRequiresClause = SubstRC.get();
-    if (!SemaRef.CheckConstraintExpression(TrailingRequiresClause))
-      return nullptr;
-  }
 
   // If we're instantiating a local function declaration, put the result
   // in the enclosing namespace; otherwise we need to find the instantiated
@@ -2394,22 +2382,7 @@
       return nullptr;
   }
 
-  // FIXME: Concepts: Do not substitute into constraint expressions
   Expr *TrailingRequiresClause = D->getTrailingRequiresClause();
-  if (TrailingRequiresClause) {
-    EnterExpressionEvaluationContext ConstantEvaluated(
-        SemaRef, Sema::ExpressionEvaluationContext::Unevaluated);
-    auto *ThisContext = dyn_cast_or_null<CXXRecordDecl>(Owner);
-    Sema::CXXThisScopeRAII ThisScope(SemaRef, ThisContext,
-                                     D->getMethodQualifiers(), ThisContext);
-    ExprResult SubstRC = SemaRef.SubstExpr(TrailingRequiresClause,
-                                           TemplateArgs);
-    if (SubstRC.isInvalid())
-      return nullptr;
-    TrailingRequiresClause = SubstRC.get();
-    if (!SemaRef.CheckConstraintExpression(TrailingRequiresClause))
-      return nullptr;
-  }
 
   DeclContext *DC = Owner;
   if (isFriend) {
@@ -2741,9 +2714,6 @@
       // Invented template parameter type constraints will be instantiated with
       // the corresponding auto-typed parameter as it might reference other
       // parameters.
-
-      // TODO: Concepts: do not instantiate the constraint (delayed constraint
-      // substitution)
       if (SemaRef.SubstTypeConstraint(Inst, TC, TemplateArgs))
         return nullptr;
     }
@@ -3986,18 +3956,7 @@
   if (Invalid)
     return nullptr;
 
-  // FIXME: Concepts: Substitution into requires clause should only happen when
-  // checking satisfaction.
-  Expr *InstRequiresClause = nullptr;
-  if (Expr *E = L->getRequiresClause()) {
-    EnterExpressionEvaluationContext ConstantEvaluated(
-        SemaRef, Sema::ExpressionEvaluationContext::Unevaluated);
-    ExprResult Res = SemaRef.SubstExpr(E, TemplateArgs);
-    if (Res.isInvalid() || !Res.isUsable()) {
-      return nullptr;
-    }
-    InstRequiresClause = Res.get();
-  }
+  Expr *InstRequiresClause = L->getRequiresClause();
 
   TemplateParameterList *InstL
     = TemplateParameterList::Create(SemaRef.Context, L->getTemplateLoc(),
Index: clang/lib/Sema/SemaTemplateInstantiate.cpp
===================================================================
--- clang/lib/Sema/SemaTemplateInstantiate.cpp
+++ clang/lib/Sema/SemaTemplateInstantiate.cpp
@@ -57,7 +57,8 @@
 /// friend function template specializations.
 MultiLevelTemplateArgumentList Sema::getTemplateInstantiationArgs(
     const NamedDecl *D, const TemplateArgumentList *Innermost,
-    bool RelativeToPrimary, const FunctionDecl *Pattern) {
+    bool RelativeToPrimary, const FunctionDecl *Pattern, bool LookBeyondLambda,
+    bool IncludeContainingStructArgs) {
   // Accumulate the set of template argument lists in this structure.
   MultiLevelTemplateArgumentList Result;
 
@@ -153,11 +154,13 @@
           break;
 
         // If this function is a generic lambda specialization, we are done.
-        if (isGenericLambdaCallOperatorOrStaticInvokerSpecialization(Function))
+        if (!LookBeyondLambda &&
+            isGenericLambdaCallOperatorOrStaticInvokerSpecialization(Function))
           break;
 
       } else if (Function->getDescribedFunctionTemplate()) {
-        assert(Result.getNumSubstitutedLevels() == 0 &&
+        assert((IncludeContainingStructArgs ||
+                Result.getNumSubstitutedLevels() == 0) &&
                "Outer template not instantiated?");
       }
 
@@ -174,10 +177,18 @@
       }
     } else if (const auto *Rec = dyn_cast<CXXRecordDecl>(Ctx)) {
       if (ClassTemplateDecl *ClassTemplate = Rec->getDescribedClassTemplate()) {
-        assert(Result.getNumSubstitutedLevels() == 0 &&
+        assert((IncludeContainingStructArgs ||
+                Result.getNumSubstitutedLevels() == 0) &&
                "Outer template not instantiated?");
         if (ClassTemplate->isMemberSpecialization())
           break;
+        if (IncludeContainingStructArgs) {
+          QualType RecordType = Context.getTypeDeclType(Rec);
+          QualType Injected = cast<InjectedClassNameType>(RecordType)
+                                  ->getInjectedSpecializationType();
+          const auto *InjectedType = cast<TemplateSpecializationType>(Injected);
+          Result.addOuterTemplateArguments(InjectedType->template_arguments());
+        }
       }
     }
 
@@ -2304,6 +2315,17 @@
     const MultiLevelTemplateArgumentList &TemplateArgs) {
   const ASTTemplateArgumentListInfo *TemplArgInfo =
       TC->getTemplateArgsAsWritten();
+
+  // If we're not checking a constraint, we shouldn't be instantiating the type
+  // constraint, so we should just create a copy of the previous one.
+  if (!IsEvaluatingAConstraint()) {
+    Inst->setTypeConstraint(TC->getNestedNameSpecifierLoc(),
+                            TC->getConceptNameInfo(), TC->getNamedConcept(),
+                            TC->getNamedConcept(), TemplArgInfo,
+                            TC->getImmediatelyDeclaredConstraint());
+    return false;
+  }
+
   TemplateArgumentListInfo InstArgs;
 
   if (TemplArgInfo) {
@@ -3488,6 +3510,14 @@
   return Instantiator.TransformExpr(E);
 }
 
+ExprResult
+Sema::SubstConstraintExpr(Expr *E,
+                          const MultiLevelTemplateArgumentList &TemplateArgs) {
+
+  ConstraintEvalRAII EvalRAII(*this);
+  return SubstExpr(E, TemplateArgs);
+}
+
 ExprResult Sema::SubstInitializer(Expr *Init,
                           const MultiLevelTemplateArgumentList &TemplateArgs,
                           bool CXXDirectInit) {
Index: clang/lib/Sema/SemaTemplateDeduction.cpp
===================================================================
--- clang/lib/Sema/SemaTemplateDeduction.cpp
+++ clang/lib/Sema/SemaTemplateDeduction.cpp
@@ -2791,8 +2791,10 @@
                                 TemplateDeductionInfo& Info) {
   llvm::SmallVector<const Expr *, 3> AssociatedConstraints;
   Template->getAssociatedConstraints(AssociatedConstraints);
-  if (S.CheckConstraintSatisfaction(Template, AssociatedConstraints,
-                                    DeducedArgs, Info.getLocation(),
+  MultiLevelTemplateArgumentList MLTAL;
+  MLTAL.addOuterTemplateArguments(DeducedArgs);
+  if (S.CheckConstraintSatisfaction(Template, AssociatedConstraints, MLTAL,
+                                    Info.getLocation(),
                                     Info.AssociatedConstraintsSatisfaction) ||
       !Info.AssociatedConstraintsSatisfaction.IsSatisfied) {
     Info.reset(TemplateArgumentList::CreateCopy(S.Context, DeducedArgs));
@@ -4572,8 +4574,11 @@
   if (S.CheckTemplateArgumentList(Concept, SourceLocation(), TemplateArgs,
                                   /*PartialTemplateArgs=*/false, Converted))
     return Sema::DAR_FailedAlreadyDiagnosed;
+
+  MultiLevelTemplateArgumentList MLTAL;
+  MLTAL.addOuterTemplateArguments(Converted);
   if (S.CheckConstraintSatisfaction(Concept, {Concept->getConstraintExpr()},
-                                    Converted, TypeLoc.getLocalSourceRange(),
+                                    MLTAL, TypeLoc.getLocalSourceRange(),
                                     Satisfaction))
     return Sema::DAR_FailedAlreadyDiagnosed;
   if (!Satisfaction.IsSatisfied) {
Index: clang/lib/Sema/SemaTemplate.cpp
===================================================================
--- clang/lib/Sema/SemaTemplate.cpp
+++ clang/lib/Sema/SemaTemplate.cpp
@@ -4698,9 +4698,11 @@
   bool AreArgsDependent =
       TemplateSpecializationType::anyDependentTemplateArguments(*TemplateArgs,
                                                                 Converted);
+  MultiLevelTemplateArgumentList MLTAL;
+  MLTAL.addOuterTemplateArguments(Converted);
   if (!AreArgsDependent &&
       CheckConstraintSatisfaction(
-          NamedConcept, {NamedConcept->getConstraintExpr()}, Converted,
+          NamedConcept, {NamedConcept->getConstraintExpr()}, MLTAL,
           SourceRange(SS.isSet() ? SS.getBeginLoc() : ConceptNameInfo.getLoc(),
                       TemplateArgs->getRAngleLoc()),
           Satisfaction))
@@ -5917,13 +5919,20 @@
   if (UpdateArgsWithConversions)
     TemplateArgs = std::move(NewArgs);
 
-  if (!PartialTemplateArgs &&
-      EnsureTemplateArgumentListConstraints(
-        Template, Converted, SourceRange(TemplateLoc,
-                                         TemplateArgs.getRAngleLoc()))) {
-    if (ConstraintsNotSatisfied)
-      *ConstraintsNotSatisfied = true;
-    return true;
+  if (!PartialTemplateArgs) {
+    TemplateArgumentList StackTemplateArgs(TemplateArgumentList::OnStack,
+                                           Converted);
+    MultiLevelTemplateArgumentList MLTAL = getTemplateInstantiationArgs(
+        Template, &StackTemplateArgs, /*RelativeToPrimary*/ true,
+        /*Pattern*/ nullptr,
+        /*LookBeyondLambda*/ true, /*IncludeContainingStruct*/ true);
+    if (EnsureTemplateArgumentListConstraints(
+            Template, MLTAL,
+            SourceRange(TemplateLoc, TemplateArgs.getRAngleLoc()))) {
+      if (ConstraintsNotSatisfied)
+        *ConstraintsNotSatisfied = true;
+      return true;
+    }
   }
 
   return false;
@@ -7453,9 +7462,28 @@
       //   are not considered.
       if (ParamsAC.empty())
         return false;
+
       Template->getAssociatedConstraints(TemplateAC);
+
+      // Attn Reviewers: This works and fixes the constraint comparison issues,
+      // but I don't have a good idea why this is, nor if it is the 'right'
+      // thing.  I THINK it is pulling off the 'template template' level of the
+      // constraint, but I have no idea if that is the correct thing to do.
+      SmallVector<const Expr *, 3> ConvertedParamsAC;
+      TemplateArgumentList Empty(TemplateArgumentList::OnStack, {});
+      MultiLevelTemplateArgumentList MLTAL{Empty};
+      for (const auto *E : ParamsAC) {
+        // Both SubstExpr and SubstConstraintExpr work here, but I'm not sure
+        // which is correct. I THINK SubstConstraintExpr should be right, but I
+        // honestly have no idea.
+        ExprResult Res = SubstConstraintExpr(const_cast<Expr *>(E), MLTAL);
+        if (Res.isInvalid())
+          return true;
+        ConvertedParamsAC.push_back(Res.get());
+      }
+
       bool IsParamAtLeastAsConstrained;
-      if (IsAtLeastAsConstrained(Param, ParamsAC, Template, TemplateAC,
+      if (IsAtLeastAsConstrained(Param, ConvertedParamsAC, Template, TemplateAC,
                                  IsParamAtLeastAsConstrained))
         return true;
       if (!IsParamAtLeastAsConstrained) {
Index: clang/lib/Sema/SemaConcept.cpp
===================================================================
--- clang/lib/Sema/SemaConcept.cpp
+++ clang/lib/Sema/SemaConcept.cpp
@@ -30,6 +30,7 @@
 
 namespace {
 class LogicalBinOp {
+  SourceLocation Loc;
   OverloadedOperatorKind Op = OO_None;
   const Expr *LHS = nullptr;
   const Expr *RHS = nullptr;
@@ -40,12 +41,14 @@
       Op = BinaryOperator::getOverloadedOperator(BO->getOpcode());
       LHS = BO->getLHS();
       RHS = BO->getRHS();
+      Loc = BO->getExprLoc();
     } else if (auto *OO = dyn_cast<CXXOperatorCallExpr>(E)) {
       // If OO is not || or && it might not have exactly 2 arguments.
       if (OO->getNumArgs() == 2) {
         Op = OO->getOperator();
         LHS = OO->getArg(0);
         RHS = OO->getArg(1);
+        Loc = OO->getOperatorLoc();
       }
     }
   }
@@ -56,6 +59,22 @@
 
   const Expr *getLHS() const { return LHS; }
   const Expr *getRHS() const { return RHS; }
+
+  ExprResult recreateBinOp(Sema &SemaRef, ExprResult LHS) {
+    return recreateBinOp(SemaRef, LHS, const_cast<Expr *>(getRHS()));
+  }
+
+  ExprResult recreateBinOp(Sema &SemaRef, ExprResult LHS, ExprResult RHS) {
+    assert((isAnd() || isOr()) && "Not the right kind of op?");
+    assert((!LHS.isInvalid() && !RHS.isInvalid()) && "not good expressions?");
+    assert(LHS.isUsable() && RHS.isUsable() && "Side not usable?");
+    // We should just be able to 'normalize' these to the builtin Binary
+    // Operator, since that is how they are evaluated in constriant checks.
+    return BinaryOperator::Create(SemaRef.Context, LHS.get(), RHS.get(),
+                                  BinaryOperator::getOverloadedOpcode(Op),
+                                  SemaRef.Context.BoolTy, VK_PRValue,
+                                  OK_Ordinary, Loc, FPOptionsOverride{});
+  }
 };
 }
 
@@ -122,16 +141,18 @@
 }
 
 template <typename AtomicEvaluator>
-static bool
+static ExprResult
 calculateConstraintSatisfaction(Sema &S, const Expr *ConstraintExpr,
                                 ConstraintSatisfaction &Satisfaction,
                                 AtomicEvaluator &&Evaluator) {
   ConstraintExpr = ConstraintExpr->IgnoreParenImpCasts();
 
   if (LogicalBinOp BO = ConstraintExpr) {
-    if (calculateConstraintSatisfaction(S, BO.getLHS(), Satisfaction,
-                                        Evaluator))
-      return true;
+    ExprResult LHSRes = calculateConstraintSatisfaction(
+        S, BO.getLHS(), Satisfaction, Evaluator);
+
+    if (LHSRes.isInvalid())
+      return ExprError();
 
     bool IsLHSSatisfied = Satisfaction.IsSatisfied;
 
@@ -142,7 +163,7 @@
       //    is checked. If that is satisfied, the disjunction is satisfied.
       //    Otherwise, the disjunction is satisfied if and only if the second
       //    operand is satisfied.
-      return false;
+      return LHSRes.isUsable() ? BO.recreateBinOp(S, LHSRes) : ExprEmpty();
 
     if (BO.isAnd() && !IsLHSSatisfied)
       // [temp.constr.op] p2
@@ -151,12 +172,21 @@
       //    is checked. If that is not satisfied, the conjunction is not
       //    satisfied. Otherwise, the conjunction is satisfied if and only if
       //    the second operand is satisfied.
-      return false;
+      return LHSRes.isUsable() ? BO.recreateBinOp(S, LHSRes) : ExprEmpty();
 
-    return calculateConstraintSatisfaction(
+    ExprResult RHSRes = calculateConstraintSatisfaction(
         S, BO.getRHS(), Satisfaction, std::forward<AtomicEvaluator>(Evaluator));
+    if (RHSRes.isInvalid())
+      return ExprError();
+
+    if (!LHSRes.isUsable() || !RHSRes.isUsable())
+      return ExprEmpty();
+    return BO.recreateBinOp(S, LHSRes, RHSRes);
   } else if (auto *C = dyn_cast<ExprWithCleanups>(ConstraintExpr)) {
-    return calculateConstraintSatisfaction(S, C->getSubExpr(), Satisfaction,
+    // These aren't evaluated, so we don't care about cleanups, so we can just
+    // evaluate these as if the cleanups didn't exist.
+    return calculateConstraintSatisfaction(
+        S, C->getSubExpr(), Satisfaction,
         std::forward<AtomicEvaluator>(Evaluator));
   }
 
@@ -164,11 +194,11 @@
   ExprResult SubstitutedAtomicExpr = Evaluator(ConstraintExpr);
 
   if (SubstitutedAtomicExpr.isInvalid())
-    return true;
+    return ExprError();
 
   if (!SubstitutedAtomicExpr.isUsable())
     // Evaluator has decided satisfaction without yielding an expression.
-    return false;
+    return ExprEmpty();
 
   EnterExpressionEvaluationContext ConstantEvaluated(
       S, Sema::ExpressionEvaluationContext::ConstantEvaluated);
@@ -185,7 +215,7 @@
         << SubstitutedAtomicExpr.get()->getSourceRange();
     for (const PartialDiagnosticAt &PDiag : EvaluationDiags)
       S.Diag(PDiag.first, PDiag.second);
-    return true;
+    return ExprError();
   }
 
   assert(EvalResult.Val.isInt() &&
@@ -195,13 +225,13 @@
     Satisfaction.Details.emplace_back(ConstraintExpr,
                                       SubstitutedAtomicExpr.get());
 
-  return false;
+  return SubstitutedAtomicExpr;
 }
 
-static bool calculateConstraintSatisfaction(
-    Sema &S, const NamedDecl *Template, ArrayRef<TemplateArgument> TemplateArgs,
-    SourceLocation TemplateNameLoc, MultiLevelTemplateArgumentList &MLTAL,
-    const Expr *ConstraintExpr, ConstraintSatisfaction &Satisfaction) {
+static ExprResult calculateConstraintSatisfaction(
+    Sema &S, const NamedDecl *Template, SourceLocation TemplateNameLoc,
+    const MultiLevelTemplateArgumentList &MLTAL, const Expr *ConstraintExpr,
+    ConstraintSatisfaction &Satisfaction) {
   return calculateConstraintSatisfaction(
       S, ConstraintExpr, Satisfaction, [&](const Expr *AtomicExpr) {
         EnterExpressionEvaluationContext ConstantEvaluated(
@@ -219,8 +249,8 @@
             return ExprError();
           // We do not want error diagnostics escaping here.
           Sema::SFINAETrap Trap(S);
-          SubstitutedExpression = S.SubstExpr(const_cast<Expr *>(AtomicExpr),
-                                              MLTAL);
+          SubstitutedExpression =
+              S.SubstConstraintExpr(const_cast<Expr *>(AtomicExpr), MLTAL);
           // Substitution might have stripped off a contextual conversion to
           // bool if this is the operand of an '&&' or '||'. For example, we
           // might lose an lvalue-to-rvalue conversion here. If so, put it back
@@ -268,56 +298,70 @@
       });
 }
 
-static bool CheckConstraintSatisfaction(Sema &S, const NamedDecl *Template,
-                                        ArrayRef<const Expr *> ConstraintExprs,
-                                        ArrayRef<TemplateArgument> TemplateArgs,
-                                        SourceRange TemplateIDRange,
-                                        ConstraintSatisfaction &Satisfaction) {
+static bool CheckConstraintSatisfaction(
+    Sema &S, const NamedDecl *Template, ArrayRef<const Expr *> ConstraintExprs,
+    llvm::SmallVectorImpl<Expr *> &Converted,
+    const MultiLevelTemplateArgumentList &TemplateArgsList,
+    SourceRange TemplateIDRange, ConstraintSatisfaction &Satisfaction) {
   if (ConstraintExprs.empty()) {
     Satisfaction.IsSatisfied = true;
     return false;
   }
 
-  for (auto& Arg : TemplateArgs)
-    if (Arg.isInstantiationDependent()) {
-      // No need to check satisfaction for dependent constraint expressions.
-      Satisfaction.IsSatisfied = true;
-      return false;
-    }
+  if (TemplateArgsList.isAnyArgInstantiationDependent()) {
+    //  No need to check satisfaction for dependent constraint expressions.
+    Satisfaction.IsSatisfied = true;
+    return false;
+  }
+
+  ArrayRef<TemplateArgument> TemplateArgs =
+      TemplateArgsList.getNumSubstitutedLevels() > 0
+          ? TemplateArgsList.getOutermost()
+          : ArrayRef<TemplateArgument>{};
 
-  Sema::InstantiatingTemplate Inst(S, TemplateIDRange.getBegin(),
+  Sema::InstantiatingTemplate Inst(
+      S, TemplateIDRange.getBegin(),
       Sema::InstantiatingTemplate::ConstraintsCheck{},
       const_cast<NamedDecl *>(Template), TemplateArgs, TemplateIDRange);
   if (Inst.isInvalid())
     return true;
 
-  MultiLevelTemplateArgumentList MLTAL;
-  MLTAL.addOuterTemplateArguments(TemplateArgs);
-
   for (const Expr *ConstraintExpr : ConstraintExprs) {
-    if (calculateConstraintSatisfaction(S, Template, TemplateArgs,
-                                        TemplateIDRange.getBegin(), MLTAL,
-                                        ConstraintExpr, Satisfaction))
+    ExprResult Res = calculateConstraintSatisfaction(
+        S, Template, TemplateIDRange.getBegin(), TemplateArgsList,
+        ConstraintExpr, Satisfaction);
+    if (Res.isInvalid())
       return true;
-    if (!Satisfaction.IsSatisfied)
-      // [temp.constr.op] p2
+
+    Converted.push_back(Res.get());
+    if (!Satisfaction.IsSatisfied) {
+      // Backfill the 'converted' list with nulls so we can keep the Converted
+      Converted.append(ConstraintExprs.size() - Converted.size(), nullptr);
+      // and unconverted lists in sync. [temp.constr.op] p2
       //   [...] To determine if a conjunction is satisfied, the satisfaction
       //   of the first operand is checked. If that is not satisfied, the
       //   conjunction is not satisfied. [...]
       return false;
+    }
   }
   return false;
 }
 
 bool Sema::CheckConstraintSatisfaction(
     const NamedDecl *Template, ArrayRef<const Expr *> ConstraintExprs,
-    ArrayRef<TemplateArgument> TemplateArgs, SourceRange TemplateIDRange,
-    ConstraintSatisfaction &OutSatisfaction) {
+    llvm::SmallVectorImpl<Expr *> &ConvertedConstraints,
+    const MultiLevelTemplateArgumentList &TemplateArgsList,
+    SourceRange TemplateIDRange, ConstraintSatisfaction &OutSatisfaction) {
   if (ConstraintExprs.empty()) {
     OutSatisfaction.IsSatisfied = true;
     return false;
   }
 
+  ArrayRef<TemplateArgument> TemplateArgs =
+      TemplateArgsList.getNumSubstitutedLevels() > 0
+          ? TemplateArgsList.getOutermost()
+          : ArrayRef<TemplateArgument>{};
+
   llvm::FoldingSetNodeID ID;
   void *InsertPos;
   ConstraintSatisfaction *Satisfaction = nullptr;
@@ -334,8 +378,8 @@
     Satisfaction = &OutSatisfaction;
   }
   if (::CheckConstraintSatisfaction(*this, Template, ConstraintExprs,
-                                    TemplateArgs, TemplateIDRange,
-                                    *Satisfaction)) {
+                                    ConvertedConstraints, TemplateArgsList,
+                                    TemplateIDRange, *Satisfaction)) {
     if (ShouldCache)
       delete Satisfaction;
     return true;
@@ -353,20 +397,83 @@
 bool Sema::CheckConstraintSatisfaction(const Expr *ConstraintExpr,
                                        ConstraintSatisfaction &Satisfaction) {
   return calculateConstraintSatisfaction(
-      *this, ConstraintExpr, Satisfaction,
-      [](const Expr *AtomicExpr) -> ExprResult {
-        return ExprResult(const_cast<Expr *>(AtomicExpr));
-      });
+             *this, ConstraintExpr, Satisfaction,
+             [](const Expr *AtomicExpr) -> ExprResult {
+               return ExprResult(const_cast<Expr *>(AtomicExpr));
+             })
+      .isInvalid();
+}
+
+// This function collects all of the template arguments for the purposes of
+// constraint-instantiation and checking.
+llvm::Optional<MultiLevelTemplateArgumentList>
+Sema::SetupConstraintCheckingTemplateArgumentsAndScope(
+    FunctionDecl *FD, llvm::Optional<ArrayRef<TemplateArgument>> TemplateArgs,
+    LocalInstantiationScope &Scope) {
+  MultiLevelTemplateArgumentList MLTAL;
+
+  // Collect the list of template arguments relative to the 'primary' template.
+  // We need the entire list, since the constraint is completely uninstantiated
+  // at this point.
+  MLTAL = getTemplateInstantiationArgs(FD, nullptr, /*RelativeToPrimary*/ true,
+                                       /*Pattern*/ nullptr,
+                                       /*LookBeyondLambda*/ true);
+
+  if (FD->isTemplateInstantiation() && FD->getPrimaryTemplate()) {
+    FunctionTemplateDecl *PrimaryTemplate = FD->getPrimaryTemplate();
+    InstantiatingTemplate Inst(
+        *this, FD->getPointOfInstantiation(),
+        InstantiatingTemplate::ConstraintsCheck{}, PrimaryTemplate,
+        TemplateArgs ? *TemplateArgs : ArrayRef<TemplateArgument>{},
+        SourceRange());
+    if (Inst.isInvalid())
+      return {};
+
+    // addInstantiatedParametersToScope creates a map of 'uninstantiated' to
+    // 'instantiated' parameters and adds it to the context. For the case where
+    // this function is a template being instantiated NOW, we also need to add
+    // the list of current template arguments to the list so that they also can
+    // be picked out of the map.
+    if (TemplateArgs) {
+      MultiLevelTemplateArgumentList JustTemplArgs(
+          *FD->getTemplateSpecializationArgs());
+      if (addInstantiatedParametersToScope(
+              FD, PrimaryTemplate->getTemplatedDecl(), Scope, JustTemplArgs))
+        return {};
+    }
+
+    // If this is a member function, make sure we get the parameters that
+    // reference the original primary template.
+    if (const auto *FromMemTempl =
+            PrimaryTemplate->getInstantiatedFromMemberTemplate()) {
+      if (addInstantiatedParametersToScope(FD, FromMemTempl->getTemplatedDecl(),
+                                           Scope, MLTAL))
+        return {};
+    }
+  }
+
+  return MLTAL;
 }
 
 bool Sema::CheckFunctionConstraints(const FunctionDecl *FD,
                                     ConstraintSatisfaction &Satisfaction,
                                     SourceLocation UsageLoc) {
-  const Expr *RC = FD->getTrailingRequiresClause();
-  if (RC->isInstantiationDependent()) {
+  // Don't check constraints if the function is dependent. Also don't check if
+  // this is a function template specialization, as the call to
+  // CheckinstantiatedFunctionTemplateConstraints after this will check it
+  // better.
+  if (FD->isDependentContext() ||
+      FD->getTemplatedKind() ==
+          FunctionDecl::TK_FunctionTemplateSpecialization) {
     Satisfaction.IsSatisfied = true;
     return false;
   }
+
+  LocalInstantiationScope Scope(*this);
+  llvm::Optional<MultiLevelTemplateArgumentList> MLTAL =
+      SetupConstraintCheckingTemplateArgumentsAndScope(
+          const_cast<FunctionDecl *>(FD), {}, Scope);
+
   Qualifiers ThisQuals;
   CXXRecordDecl *Record = nullptr;
   if (auto *Method = dyn_cast<CXXMethodDecl>(FD)) {
@@ -377,14 +484,27 @@
   // We substitute with empty arguments in order to rebuild the atomic
   // constraint in a constant-evaluated context.
   // FIXME: Should this be a dedicated TreeTransform?
-  return CheckConstraintSatisfaction(
-      FD, {RC}, /*TemplateArgs=*/{},
-      SourceRange(UsageLoc.isValid() ? UsageLoc : FD->getLocation()),
-      Satisfaction);
+  const Expr *RC = FD->getTrailingRequiresClause();
+  llvm::SmallVector<Expr *, 1> Converted;
+
+  if (CheckConstraintSatisfaction(
+          FD, {RC}, Converted, *MLTAL,
+          SourceRange(UsageLoc.isValid() ? UsageLoc : FD->getLocation()),
+          Satisfaction))
+    return true;
+
+  // Attn Reviewers: we need to do this for the function constraints for
+  // comparison of constraints to work, but do we also need to do it for
+  // CheckInstantiatedFunctionConstraints?  That one is more difficult, but we
+  // seem to always just pick up the constraints from the primary template.
+  assert(Converted.size() <= 1 && "Got more expressions converted?");
+  if (!Converted.empty() && Converted[0] != nullptr)
+    const_cast<FunctionDecl *>(FD)->setTrailingRequiresClause(Converted[0]);
+  return false;
 }
 
 bool Sema::EnsureTemplateArgumentListConstraints(
-    TemplateDecl *TD, ArrayRef<TemplateArgument> TemplateArgs,
+    TemplateDecl *TD, MultiLevelTemplateArgumentList TemplateArgs,
     SourceRange TemplateIDRange) {
   ConstraintSatisfaction Satisfaction;
   llvm::SmallVector<const Expr *, 3> AssociatedConstraints;
@@ -397,7 +517,8 @@
     SmallString<128> TemplateArgString;
     TemplateArgString = " ";
     TemplateArgString += getTemplateArgumentBindingsText(
-        TD->getTemplateParameters(), TemplateArgs.data(), TemplateArgs.size());
+        TD->getTemplateParameters(), TemplateArgs.getInnermost().data(),
+        TemplateArgs.getInnermost().size());
 
     Diag(TemplateIDRange.getBegin(),
          diag::err_template_arg_list_constraints_not_satisfied)
@@ -429,21 +550,13 @@
   Sema::ContextRAII savedContext(*this, Decl);
   LocalInstantiationScope Scope(*this);
 
-  // If this is not an explicit specialization - we need to get the instantiated
-  // version of the template arguments and add them to scope for the
-  // substitution.
-  if (Decl->isTemplateInstantiation()) {
-    InstantiatingTemplate Inst(*this, Decl->getPointOfInstantiation(),
-        InstantiatingTemplate::ConstraintsCheck{}, Decl->getPrimaryTemplate(),
-        TemplateArgs, SourceRange());
-    if (Inst.isInvalid())
-      return true;
-    MultiLevelTemplateArgumentList MLTAL(
-        *Decl->getTemplateSpecializationArgs());
-    if (addInstantiatedParametersToScope(
-            Decl, Decl->getPrimaryTemplate()->getTemplatedDecl(), Scope, MLTAL))
-      return true;
-  }
+  Optional<MultiLevelTemplateArgumentList> MLTAL =
+      SetupConstraintCheckingTemplateArgumentsAndScope(Decl, TemplateArgs,
+                                                       Scope);
+
+  if (!MLTAL)
+    return true;
+
   Qualifiers ThisQuals;
   CXXRecordDecl *Record = nullptr;
   if (auto *Method = dyn_cast<CXXMethodDecl>(Decl)) {
@@ -451,7 +564,8 @@
     Record = Method->getParent();
   }
   CXXThisScopeRAII ThisScope(*this, Record, ThisQuals, Record != nullptr);
-  return CheckConstraintSatisfaction(Template, TemplateAC, TemplateArgs,
+  llvm::SmallVector<Expr *, 1> Converted;
+  return CheckConstraintSatisfaction(Template, TemplateAC, Converted, *MLTAL,
                                      PointOfInstantiation, Satisfaction);
 }
 
Index: clang/include/clang/Sema/Template.h
===================================================================
--- clang/include/clang/Sema/Template.h
+++ clang/include/clang/Sema/Template.h
@@ -158,6 +158,14 @@
       return !(*this)(Depth, Index).isNull();
     }
 
+    bool isAnyArgInstantiationDependent() const {
+      for (ArgList List : TemplateArgumentLists)
+        for (const TemplateArgument &TA : List)
+          if (TA.isInstantiationDependent())
+            return true;
+      return false;
+    }
+
     /// Clear out a specific template argument.
     void setArgument(unsigned Depth, unsigned Index,
                      TemplateArgument Arg) {
@@ -197,6 +205,9 @@
     const ArgList &getInnermost() const {
       return TemplateArgumentLists.front();
     }
+
+    /// Retrieve the outermost template argument list.
+    const ArgList &getOutermost() const { return TemplateArgumentLists.back(); }
   };
 
   /// The context in which partial ordering of function templates occurs.
Index: clang/include/clang/Sema/Sema.h
===================================================================
--- clang/include/clang/Sema/Sema.h
+++ clang/include/clang/Sema/Sema.h
@@ -6966,7 +6966,24 @@
       LocalInstantiationScope &Scope,
       const MultiLevelTemplateArgumentList &TemplateArgs);
 
+  llvm::Optional<MultiLevelTemplateArgumentList>
+  SetupConstraintCheckingTemplateArgumentsAndScope(
+      FunctionDecl *FD, llvm::Optional<ArrayRef<TemplateArgument>> TemplateArgs,
+      LocalInstantiationScope &Scope);
+
+  // Keep track of whether we are evaluating a constraint.
+  unsigned ConstraintEvaluationDepth = 0;
+
+  class ConstraintEvalRAII {
+    Sema &S;
+
+  public:
+    ConstraintEvalRAII(Sema &S) : S(S) { ++S.ConstraintEvaluationDepth; }
+    ~ConstraintEvalRAII() { --S.ConstraintEvaluationDepth; }
+  };
+
 public:
+  bool IsEvaluatingAConstraint() { return ConstraintEvaluationDepth > 0; }
   const NormalizedConstraint *
   getNormalizedAssociatedConstraints(
       NamedDecl *ConstrainedDecl, ArrayRef<const Expr *> AssociatedConstraints);
@@ -6996,8 +7013,10 @@
   /// check (either a concept or a constrained entity).
   /// \param ConstraintExprs a list of constraint expressions, treated as if
   /// they were 'AND'ed together.
-  /// \param TemplateArgs the list of template arguments to substitute into the
-  /// constraint expression.
+  /// \param TemplateArgList the multi-level list of template arguments to
+  /// substitute into the constraint expression. This should be relative to the
+  /// top-level (hence multi-level), since we need to instantiate fully at the
+  /// time of checking.
   /// \param TemplateIDRange The source range of the template id that
   /// caused the constraints check.
   /// \param Satisfaction if true is returned, will contain details of the
@@ -7007,7 +7026,40 @@
   /// false otherwise.
   bool CheckConstraintSatisfaction(
       const NamedDecl *Template, ArrayRef<const Expr *> ConstraintExprs,
-      ArrayRef<TemplateArgument> TemplateArgs,
+      const MultiLevelTemplateArgumentList &TemplateArgList,
+      SourceRange TemplateIDRange, ConstraintSatisfaction &Satisfaction) {
+    llvm::SmallVector<Expr *, 4> Converted;
+    return CheckConstraintSatisfaction(Template, ConstraintExprs, Converted,
+                                       TemplateArgList, TemplateIDRange,
+                                       Satisfaction);
+  }
+
+  /// \brief Check whether the given list of constraint expressions are
+  /// satisfied (as if in a 'conjunction') given template arguments.
+  /// Additionally, takes an empty list of Expressions which is populated with
+  /// the instantiated versions of the ConstraintExprs.
+  /// \param Template the template-like entity that triggered the constraints
+  /// check (either a concept or a constrained entity).
+  /// \param ConstraintExprs a list of constraint expressions, treated as if
+  /// they were 'AND'ed together.
+  /// \param ConvertedConstraints a out parameter that will get populated with
+  /// the instantiated version of the ConstraintExprs if we successfully checked
+  /// satisfaction.
+  /// \param TemplateArgList the multi-level list of template arguments to
+  /// substitute into the constraint expression. This should be relative to the
+  /// top-level (hence multi-level), since we need to instantiate fully at the
+  /// time of checking.
+  /// \param TemplateIDRange The source range of the template id that
+  /// caused the constraints check.
+  /// \param Satisfaction if true is returned, will contain details of the
+  /// satisfaction, with enough information to diagnose an unsatisfied
+  /// expression.
+  /// \returns true if an error occurred and satisfaction could not be checked,
+  /// false otherwise.
+  bool CheckConstraintSatisfaction(
+      const NamedDecl *Template, ArrayRef<const Expr *> ConstraintExprs,
+      llvm::SmallVectorImpl<Expr *> &ConvertedConstraints,
+      const MultiLevelTemplateArgumentList &TemplateArgList,
       SourceRange TemplateIDRange, ConstraintSatisfaction &Satisfaction);
 
   /// \brief Check whether the given non-dependent constraint expression is
@@ -7043,9 +7095,9 @@
   ///
   /// \returns true if the constrains are not satisfied or could not be checked
   /// for satisfaction, false if the constraints are satisfied.
-  bool EnsureTemplateArgumentListConstraints(TemplateDecl *Template,
-                                       ArrayRef<TemplateArgument> TemplateArgs,
-                                             SourceRange TemplateIDRange);
+  bool EnsureTemplateArgumentListConstraints(
+      TemplateDecl *Template, MultiLevelTemplateArgumentList TemplateArgs,
+      SourceRange TemplateIDRange);
 
   /// \brief Emit diagnostics explaining why a constraint expression was deemed
   /// unsatisfied.
@@ -8778,7 +8830,8 @@
 
   MultiLevelTemplateArgumentList getTemplateInstantiationArgs(
       const NamedDecl *D, const TemplateArgumentList *Innermost = nullptr,
-      bool RelativeToPrimary = false, const FunctionDecl *Pattern = nullptr);
+      bool RelativeToPrimary = false, const FunctionDecl *Pattern = nullptr,
+      bool LookBeyondLambda = false, bool IncludeContainingStruct = false);
 
   /// A context in which code is being synthesized (where a source location
   /// alone is not sufficient to identify the context). This covers template
@@ -9499,6 +9552,11 @@
                       ExtParameterInfoBuilder &ParamInfos);
   ExprResult SubstExpr(Expr *E,
                        const MultiLevelTemplateArgumentList &TemplateArgs);
+  // Unlike the above, this evaluates constraints, which should only happen at
+  // 'constraint checking' time.
+  ExprResult
+  SubstConstraintExpr(Expr *E,
+                      const MultiLevelTemplateArgumentList &TemplateArgs);
 
   /// Substitute the given template arguments into a list of
   /// expressions, expanding pack expansions if required.
Index: clang/docs/ReleaseNotes.rst
===================================================================
--- clang/docs/ReleaseNotes.rst
+++ clang/docs/ReleaseNotes.rst
@@ -195,6 +195,10 @@
   `Issue 54578 <https://github.com/llvm/llvm-project/issues/54578>`_.
 
 - Implemented `__builtin_source_location()` which enables library support for std::source_location.
+- Clang now correctly delays the instantiation of function constraints until
+  the time of checking, which should now allow the libstdc++ ranges implementation
+  to work for at least trivial examples.  This fixes
+  `Issue 44178 <https://github.com/llvm/llvm-project/issues/44178>`_.
 
 C++2b Feature Support
 ^^^^^^^^^^^^^^^^^^^^^
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