https://github.com/alejandro-alvarez-sonarsource updated 
https://github.com/llvm/llvm-project/pull/121768

From da2bbf99b8430d8b6aa6bf7969c9825b4d94219b Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Alejandro=20=C3=81lvarez=20Ayll=C3=B3n?=
 <alejandro.alva...@sonarsource.com>
Date: Mon, 18 Nov 2024 11:36:03 +0100
Subject: [PATCH 1/6] [clang][Sema] Fix initialization of
 `NonTypeTemplateParmDecl`...

...when there are invalid constraints.

When attaching a `TypeConstraint`, in case of error, the trailing
pointer that is supposed to point to the constraint is left
uninitialized.
Sometimes the uninitialized value will be a `nullptr`, but at other
times it will not. If we traverse the AST (for instance, dumping it,
or when writing the BMI), we may get a crash depending on the value
that was left. The serialization may also contain a bogus value.

With this commit, we always initialize this trailing pointer, using a
`RecoveryExpr` in case of failure to parse.
---
 clang/lib/Sema/SemaTemplate.cpp               | 18 ++++++-
 ...constraint-template-non-type-parm-decl.cpp | 54 +++++++++++++++++++
 2 files changed, 70 insertions(+), 2 deletions(-)
 create mode 100644 
clang/test/Modules/malformed-constraint-template-non-type-parm-decl.cpp

diff --git a/clang/lib/Sema/SemaTemplate.cpp b/clang/lib/Sema/SemaTemplate.cpp
index 20ec2fbeaa6a8b..9b51f973fb2bb8 100644
--- a/clang/lib/Sema/SemaTemplate.cpp
+++ b/clang/lib/Sema/SemaTemplate.cpp
@@ -1235,6 +1235,10 @@ bool Sema::AttachTypeConstraint(AutoTypeLoc TL,
         << NewConstrainedParm->getTypeSourceInfo()
                ->getTypeLoc()
                .getSourceRange();
+    NewConstrainedParm->setPlaceholderTypeConstraint(
+        RecoveryExpr::Create(Context, OrigConstrainedParm->getType(),
+                             OrigConstrainedParm->getBeginLoc(),
+                             OrigConstrainedParm->getEndLoc(), {}));
     return true;
   }
   // FIXME: Concepts: This should be the type of the placeholder, but this is
@@ -1242,8 +1246,13 @@ bool Sema::AttachTypeConstraint(AutoTypeLoc TL,
   DeclRefExpr *Ref =
       BuildDeclRefExpr(OrigConstrainedParm, OrigConstrainedParm->getType(),
                        VK_PRValue, OrigConstrainedParm->getLocation());
-  if (!Ref)
+  if (!Ref) {
+    NewConstrainedParm->setPlaceholderTypeConstraint(
+        RecoveryExpr::Create(Context, OrigConstrainedParm->getType(),
+                             OrigConstrainedParm->getBeginLoc(),
+                             OrigConstrainedParm->getEndLoc(), {}));
     return true;
+  }
   ExprResult ImmediatelyDeclaredConstraint = formImmediatelyDeclaredConstraint(
       *this, TL.getNestedNameSpecifierLoc(), TL.getConceptNameInfo(),
       TL.getNamedConcept(), /*FoundDecl=*/TL.getFoundDecl(), TL.getLAngleLoc(),
@@ -1255,8 +1264,13 @@ bool Sema::AttachTypeConstraint(AutoTypeLoc TL,
       },
       EllipsisLoc);
   if (ImmediatelyDeclaredConstraint.isInvalid() ||
-      !ImmediatelyDeclaredConstraint.isUsable())
+      !ImmediatelyDeclaredConstraint.isUsable()) {
+    NewConstrainedParm->setPlaceholderTypeConstraint(
+        RecoveryExpr::Create(Context, OrigConstrainedParm->getType(),
+                             OrigConstrainedParm->getBeginLoc(),
+                             OrigConstrainedParm->getEndLoc(), {}));
     return true;
+  }
 
   NewConstrainedParm->setPlaceholderTypeConstraint(
       ImmediatelyDeclaredConstraint.get());
diff --git 
a/clang/test/Modules/malformed-constraint-template-non-type-parm-decl.cpp 
b/clang/test/Modules/malformed-constraint-template-non-type-parm-decl.cpp
new file mode 100644
index 00000000000000..cea6404bbebd28
--- /dev/null
+++ b/clang/test/Modules/malformed-constraint-template-non-type-parm-decl.cpp
@@ -0,0 +1,54 @@
+// RUN: rm -rf %t
+// RUN: split-file %s %t
+// RUN: cd %t
+
+// RUN: %clang_cc1 -std=c++20 mod.cppm -emit-module-interface -o mod.pcm 
-fallow-pcm-with-compiler-errors -verify
+// RUN: %clang_cc1 -std=c++20 main.cpp -fmodule-file=mod=mod.pcm -verify 
-fallow-pcm-with-compiler-errors -fsyntax-only -ast-dump-all | FileCheck %s
+
+//--- mod.cppm
+export module mod;
+
+template <typename T, auto Q>
+concept ReferenceOf = Q;
+
+// expected-error@+2 {{unknown type name 'AngleIsInvalidNow'}}
+// expected-error@+1 {{constexpr variable 'angle' must be initialized by a 
constant expression}}
+constexpr struct angle {AngleIsInvalidNow e;} angle;
+
+// expected-error@+1 {{non-type template argument is not a constant 
expression}}
+template<ReferenceOf<angle> auto R, typename Rep> requires requires(Rep v) 
{cos(v);}
+auto cos(const Rep& q);
+
+// expected-error@+1 {{non-type template argument is not a constant 
expression}}
+template<ReferenceOf<angle> auto R, typename Rep> requires requires(Rep v) 
{tan(v);}
+auto tan(const Rep& q);
+
+//--- main.cpp
+// expected-no-diagnostics
+import mod;
+
+// CHECK:      |-FunctionTemplateDecl {{.*}} <line:11:1, line:12:22> col:6 
imported in mod hidden invalid cos
+// CHECK-NEXT: | |-NonTypeTemplateParmDecl {{.*}} <line:11:10, col:34> col:34 
imported in mod hidden referenced invalid 'ReferenceOf<angle> auto' depth 0 
index 0 R
+// CHECK-NEXT: | | `-RecoveryExpr {{.*}} <col:10, col:34> 'ReferenceOf<angle> 
auto' contains-errors lvalue
+// CHECK-NEXT: | |-TemplateTypeParmDecl {{.*}} <col:37, col:46> col:46 
imported in mod hidden referenced typename depth 0 index 1 Rep
+// CHECK-NEXT: | |-RequiresExpr {{.*}} <col:60, col:84> 'bool'
+// CHECK-NEXT: | | |-ParmVarDecl {{.*}} <col:69, col:73> col:73 imported in 
mod hidden referenced v 'Rep'
+// CHECK-NEXT: | | `-SimpleRequirement {{.*}} dependent
+// CHECK-NEXT: | |   `-CallExpr {{.*}} <col:77, col:82> '<dependent type>'
+// CHECK-NEXT: | |     |-UnresolvedLookupExpr {{.*}} <col:77> '<overloaded 
function type>' lvalue (ADL) = 'cos' empty
+// CHECK-NEXT: | |     `-DeclRefExpr {{.*}} <col:81> 'Rep' lvalue ParmVar 
{{.*}} 'v' 'Rep' non_odr_use_unevaluated
+// CHECK-NEXT: | `-FunctionDecl {{.*}} <line:12:1, col:22> col:6 imported in 
mod hidden cos 'auto (const Rep &)'
+// CHECK-NEXT: |   `-ParmVarDecl {{.*}} <col:10, col:21> col:21 imported in 
mod hidden q 'const Rep &'
+
+// CHECK:      |-FunctionTemplateDecl {{.*}} <line:15:1, line:16:22> col:6 
imported in mod hidden invalid tan
+// CHECK-NEXT: | |-NonTypeTemplateParmDecl {{.*}} <line:15:10, col:34> col:34 
imported in mod hidden referenced invalid 'ReferenceOf<angle> auto' depth 0 
index 0 R
+// CHECK-NEXT: | | `-RecoveryExpr {{.*}} <col:10, col:34> 'ReferenceOf<angle> 
auto' contains-errors lvalue
+// CHECK-NEXT: | |-TemplateTypeParmDecl {{.*}} <col:37, col:46> col:46 
imported in mod hidden referenced typename depth 0 index 1 Rep
+// CHECK-NEXT: | |-RequiresExpr {{.*}} <col:60, col:84> 'bool'
+// CHECK-NEXT: | | |-ParmVarDecl {{.*}} <col:69, col:73> col:73 imported in 
mod hidden referenced v 'Rep'
+// CHECK-NEXT: | | `-SimpleRequirement {{.*}} dependent
+// CHECK-NEXT: | |   `-CallExpr {{.*}} <col:77, col:82> '<dependent type>'
+// CHECK-NEXT: | |     |-UnresolvedLookupExpr {{.*}} <col:77> '<overloaded 
function type>' lvalue (ADL) = 'tan' empty
+// CHECK-NEXT: | |     `-DeclRefExpr {{.*}} <col:81> 'Rep' lvalue ParmVar 
{{.*}} 'v' 'Rep' non_odr_use_unevaluated
+// CHECK-NEXT: | `-FunctionDecl {{.*}} <line:16:1, col:22> col:6 imported in 
mod hidden tan 'auto (const Rep &)'
+// CHECK-NEXT: |   `-ParmVarDecl {{.*}} <col:10, col:21> col:21 imported in 
mod hidden q 'const Rep &'

From fc140834694f45faf9ba72653fe76f05974dd89f Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Alejandro=20=C3=81lvarez=20Ayll=C3=B3n?=
 <alejandro.alva...@sonarsource.com>
Date: Mon, 6 Jan 2025 16:18:42 +0100
Subject: [PATCH 2/6] `BuildDeclRefExpr` should not fail

---
 clang/lib/Sema/SemaTemplate.cpp | 9 ++-------
 1 file changed, 2 insertions(+), 7 deletions(-)

diff --git a/clang/lib/Sema/SemaTemplate.cpp b/clang/lib/Sema/SemaTemplate.cpp
index 9b51f973fb2bb8..a6e701141cb092 100644
--- a/clang/lib/Sema/SemaTemplate.cpp
+++ b/clang/lib/Sema/SemaTemplate.cpp
@@ -1246,13 +1246,8 @@ bool Sema::AttachTypeConstraint(AutoTypeLoc TL,
   DeclRefExpr *Ref =
       BuildDeclRefExpr(OrigConstrainedParm, OrigConstrainedParm->getType(),
                        VK_PRValue, OrigConstrainedParm->getLocation());
-  if (!Ref) {
-    NewConstrainedParm->setPlaceholderTypeConstraint(
-        RecoveryExpr::Create(Context, OrigConstrainedParm->getType(),
-                             OrigConstrainedParm->getBeginLoc(),
-                             OrigConstrainedParm->getEndLoc(), {}));
-    return true;
-  }
+  assert(Ref != nullptr && "Unexpected nullptr!");
+
   ExprResult ImmediatelyDeclaredConstraint = formImmediatelyDeclaredConstraint(
       *this, TL.getNestedNameSpecifierLoc(), TL.getConceptNameInfo(),
       TL.getNamedConcept(), /*FoundDecl=*/TL.getFoundDecl(), TL.getLAngleLoc(),

From ab1616f4238852d5efa44b4aab67e72048af9738 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Alejandro=20=C3=81lvarez=20Ayll=C3=B3n?=
 <alejandro.alva...@sonarsource.com>
Date: Mon, 6 Jan 2025 17:02:54 +0100
Subject: [PATCH 3/6] Refactor to avoid duplication

---
 clang/lib/Sema/SemaTemplate.cpp | 59 +++++++++++++++++----------------
 1 file changed, 30 insertions(+), 29 deletions(-)

diff --git a/clang/lib/Sema/SemaTemplate.cpp b/clang/lib/Sema/SemaTemplate.cpp
index a6e701141cb092..336a71b151b575 100644
--- a/clang/lib/Sema/SemaTemplate.cpp
+++ b/clang/lib/Sema/SemaTemplate.cpp
@@ -1228,36 +1228,37 @@ bool Sema::AttachTypeConstraint(AutoTypeLoc TL,
                                 NonTypeTemplateParmDecl *NewConstrainedParm,
                                 NonTypeTemplateParmDecl *OrigConstrainedParm,
                                 SourceLocation EllipsisLoc) {
-  if (NewConstrainedParm->getType().getNonPackExpansionType() != TL.getType() 
||
-      TL.getAutoKeyword() != AutoTypeKeyword::Auto) {
-    Diag(NewConstrainedParm->getTypeSourceInfo()->getTypeLoc().getBeginLoc(),
-         diag::err_unsupported_placeholder_constraint)
-        << NewConstrainedParm->getTypeSourceInfo()
-               ->getTypeLoc()
-               .getSourceRange();
-    NewConstrainedParm->setPlaceholderTypeConstraint(
-        RecoveryExpr::Create(Context, OrigConstrainedParm->getType(),
-                             OrigConstrainedParm->getBeginLoc(),
-                             OrigConstrainedParm->getEndLoc(), {}));
-    return true;
-  }
-  // FIXME: Concepts: This should be the type of the placeholder, but this is
-  // unclear in the wording right now.
-  DeclRefExpr *Ref =
-      BuildDeclRefExpr(OrigConstrainedParm, OrigConstrainedParm->getType(),
-                       VK_PRValue, OrigConstrainedParm->getLocation());
-  assert(Ref != nullptr && "Unexpected nullptr!");
+  ExprResult ImmediatelyDeclaredConstraint = [&] {
+    if (NewConstrainedParm->getType().getNonPackExpansionType() !=
+            TL.getType() ||
+        TL.getAutoKeyword() != AutoTypeKeyword::Auto) {
+      Diag(NewConstrainedParm->getTypeSourceInfo()->getTypeLoc().getBeginLoc(),
+           diag::err_unsupported_placeholder_constraint)
+          << NewConstrainedParm->getTypeSourceInfo()
+                 ->getTypeLoc()
+                 .getSourceRange();
+      return ExprResult();
+    }
+
+    // FIXME: Concepts: This should be the type of the placeholder, but this is
+    // unclear in the wording right now.
+    DeclRefExpr *Ref =
+        BuildDeclRefExpr(OrigConstrainedParm, OrigConstrainedParm->getType(),
+                         VK_PRValue, OrigConstrainedParm->getLocation());
+    assert(Ref != nullptr && "Unexpected nullptr!");
+
+    return formImmediatelyDeclaredConstraint(
+        *this, TL.getNestedNameSpecifierLoc(), TL.getConceptNameInfo(),
+        TL.getNamedConcept(), /*FoundDecl=*/TL.getFoundDecl(),
+        TL.getLAngleLoc(), TL.getRAngleLoc(), BuildDecltypeType(Ref),
+        OrigConstrainedParm->getLocation(),
+        [&](TemplateArgumentListInfo &ConstraintArgs) {
+          for (unsigned I = 0, C = TL.getNumArgs(); I != C; ++I)
+            ConstraintArgs.addArgument(TL.getArgLoc(I));
+        },
+        EllipsisLoc);
+  }();
 
-  ExprResult ImmediatelyDeclaredConstraint = formImmediatelyDeclaredConstraint(
-      *this, TL.getNestedNameSpecifierLoc(), TL.getConceptNameInfo(),
-      TL.getNamedConcept(), /*FoundDecl=*/TL.getFoundDecl(), TL.getLAngleLoc(),
-      TL.getRAngleLoc(), BuildDecltypeType(Ref),
-      OrigConstrainedParm->getLocation(),
-      [&](TemplateArgumentListInfo &ConstraintArgs) {
-        for (unsigned I = 0, C = TL.getNumArgs(); I != C; ++I)
-          ConstraintArgs.addArgument(TL.getArgLoc(I));
-      },
-      EllipsisLoc);
   if (ImmediatelyDeclaredConstraint.isInvalid() ||
       !ImmediatelyDeclaredConstraint.isUsable()) {
     NewConstrainedParm->setPlaceholderTypeConstraint(

From 12d918ff45689ed286230fa28b456fea24af713d Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Alejandro=20=C3=81lvarez=20Ayll=C3=B3n?=
 <alejandro.alva...@sonarsource.com>
Date: Thu, 9 Jan 2025 11:12:30 +0100
Subject: [PATCH 4/6] Revert `RecoverExpr`

---
 clang/lib/Sema/SemaTemplate.cpp | 64 ++++++++++++++-------------------
 1 file changed, 27 insertions(+), 37 deletions(-)

diff --git a/clang/lib/Sema/SemaTemplate.cpp b/clang/lib/Sema/SemaTemplate.cpp
index 336a71b151b575..20ec2fbeaa6a8b 100644
--- a/clang/lib/Sema/SemaTemplate.cpp
+++ b/clang/lib/Sema/SemaTemplate.cpp
@@ -1228,45 +1228,35 @@ bool Sema::AttachTypeConstraint(AutoTypeLoc TL,
                                 NonTypeTemplateParmDecl *NewConstrainedParm,
                                 NonTypeTemplateParmDecl *OrigConstrainedParm,
                                 SourceLocation EllipsisLoc) {
-  ExprResult ImmediatelyDeclaredConstraint = [&] {
-    if (NewConstrainedParm->getType().getNonPackExpansionType() !=
-            TL.getType() ||
-        TL.getAutoKeyword() != AutoTypeKeyword::Auto) {
-      Diag(NewConstrainedParm->getTypeSourceInfo()->getTypeLoc().getBeginLoc(),
-           diag::err_unsupported_placeholder_constraint)
-          << NewConstrainedParm->getTypeSourceInfo()
-                 ->getTypeLoc()
-                 .getSourceRange();
-      return ExprResult();
-    }
-
-    // FIXME: Concepts: This should be the type of the placeholder, but this is
-    // unclear in the wording right now.
-    DeclRefExpr *Ref =
-        BuildDeclRefExpr(OrigConstrainedParm, OrigConstrainedParm->getType(),
-                         VK_PRValue, OrigConstrainedParm->getLocation());
-    assert(Ref != nullptr && "Unexpected nullptr!");
-
-    return formImmediatelyDeclaredConstraint(
-        *this, TL.getNestedNameSpecifierLoc(), TL.getConceptNameInfo(),
-        TL.getNamedConcept(), /*FoundDecl=*/TL.getFoundDecl(),
-        TL.getLAngleLoc(), TL.getRAngleLoc(), BuildDecltypeType(Ref),
-        OrigConstrainedParm->getLocation(),
-        [&](TemplateArgumentListInfo &ConstraintArgs) {
-          for (unsigned I = 0, C = TL.getNumArgs(); I != C; ++I)
-            ConstraintArgs.addArgument(TL.getArgLoc(I));
-        },
-        EllipsisLoc);
-  }();
-
-  if (ImmediatelyDeclaredConstraint.isInvalid() ||
-      !ImmediatelyDeclaredConstraint.isUsable()) {
-    NewConstrainedParm->setPlaceholderTypeConstraint(
-        RecoveryExpr::Create(Context, OrigConstrainedParm->getType(),
-                             OrigConstrainedParm->getBeginLoc(),
-                             OrigConstrainedParm->getEndLoc(), {}));
+  if (NewConstrainedParm->getType().getNonPackExpansionType() != TL.getType() 
||
+      TL.getAutoKeyword() != AutoTypeKeyword::Auto) {
+    Diag(NewConstrainedParm->getTypeSourceInfo()->getTypeLoc().getBeginLoc(),
+         diag::err_unsupported_placeholder_constraint)
+        << NewConstrainedParm->getTypeSourceInfo()
+               ->getTypeLoc()
+               .getSourceRange();
     return true;
   }
+  // FIXME: Concepts: This should be the type of the placeholder, but this is
+  // unclear in the wording right now.
+  DeclRefExpr *Ref =
+      BuildDeclRefExpr(OrigConstrainedParm, OrigConstrainedParm->getType(),
+                       VK_PRValue, OrigConstrainedParm->getLocation());
+  if (!Ref)
+    return true;
+  ExprResult ImmediatelyDeclaredConstraint = formImmediatelyDeclaredConstraint(
+      *this, TL.getNestedNameSpecifierLoc(), TL.getConceptNameInfo(),
+      TL.getNamedConcept(), /*FoundDecl=*/TL.getFoundDecl(), TL.getLAngleLoc(),
+      TL.getRAngleLoc(), BuildDecltypeType(Ref),
+      OrigConstrainedParm->getLocation(),
+      [&](TemplateArgumentListInfo &ConstraintArgs) {
+        for (unsigned I = 0, C = TL.getNumArgs(); I != C; ++I)
+          ConstraintArgs.addArgument(TL.getArgLoc(I));
+      },
+      EllipsisLoc);
+  if (ImmediatelyDeclaredConstraint.isInvalid() ||
+      !ImmediatelyDeclaredConstraint.isUsable())
+    return true;
 
   NewConstrainedParm->setPlaceholderTypeConstraint(
       ImmediatelyDeclaredConstraint.get());

From 792fb35e538abfde1f50257b55dc02059a8644c9 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Alejandro=20=C3=81lvarez=20Ayll=C3=B3n?=
 <alejandro.alva...@sonarsource.com>
Date: Thu, 9 Jan 2025 11:48:09 +0100
Subject: [PATCH 5/6] Test only reduced BMI

---
 clang/include/clang/AST/DeclTemplate.h                 | 10 +++++-----
 clang/lib/AST/DeclTemplate.cpp                         | 10 ++++++++++
 clang/lib/Serialization/ASTReaderDecl.cpp              |  2 +-
 clang/lib/Serialization/ASTWriterDecl.cpp              |  6 ++++--
 ...alformed-constraint-template-non-type-parm-decl.cpp |  5 +++--
 5 files changed, 23 insertions(+), 10 deletions(-)

diff --git a/clang/include/clang/AST/DeclTemplate.h 
b/clang/include/clang/AST/DeclTemplate.h
index d3a466a8617bb1..2778d362fd03ca 100644
--- a/clang/include/clang/AST/DeclTemplate.h
+++ b/clang/include/clang/AST/DeclTemplate.h
@@ -1366,6 +1366,8 @@ class NonTypeTemplateParmDecl final
       DefaultArgStorage<NonTypeTemplateParmDecl, TemplateArgumentLoc *>;
   DefArgStorage DefaultArgument;
 
+  bool PlaceholderTypeConstraintInitialized = false;
+
   // FIXME: Collapse this into TemplateParamPosition; or, just move depth/index
   // down here to save memory.
 
@@ -1534,13 +1536,11 @@ class NonTypeTemplateParmDecl final
   /// Return the constraint introduced by the placeholder type of this non-type
   /// template parameter (if any).
   Expr *getPlaceholderTypeConstraint() const {
-    return hasPlaceholderTypeConstraint() ? *getTrailingObjects<Expr *>() :
-        nullptr;
+    return PlaceholderTypeConstraintInitialized ? *getTrailingObjects<Expr *>()
+                                                : nullptr;
   }
 
-  void setPlaceholderTypeConstraint(Expr *E) {
-    *getTrailingObjects<Expr *>() = E;
-  }
+  void setPlaceholderTypeConstraint(Expr *E);
 
   /// Determine whether this non-type template parameter's type has a
   /// placeholder with a type-constraint.
diff --git a/clang/lib/AST/DeclTemplate.cpp b/clang/lib/AST/DeclTemplate.cpp
index 40ee3753c24227..4027764daabfa2 100644
--- a/clang/lib/AST/DeclTemplate.cpp
+++ b/clang/lib/AST/DeclTemplate.cpp
@@ -853,6 +853,16 @@ void NonTypeTemplateParmDecl::setDefaultArgument(
     DefaultArgument.set(new (C) TemplateArgumentLoc(DefArg));
 }
 
+void NonTypeTemplateParmDecl::setPlaceholderTypeConstraint(Expr *E) {
+  assert(hasPlaceholderTypeConstraint() &&
+         "setPlaceholderTypeConstraint called on a NonTypeTemplateParmDecl "
+         "without constraint");
+  assert(!PlaceholderTypeConstraintInitialized && "PlaceholderTypeConstraint "
+                                                  "was already initialized!");
+  *getTrailingObjects<Expr *>() = E;
+  PlaceholderTypeConstraintInitialized = true;
+}
+
 
//===----------------------------------------------------------------------===//
 // TemplateTemplateParmDecl Method Implementations
 
//===----------------------------------------------------------------------===//
diff --git a/clang/lib/Serialization/ASTReaderDecl.cpp 
b/clang/lib/Serialization/ASTReaderDecl.cpp
index 719bc0d06f5b11..0154c97ccb59dd 100644
--- a/clang/lib/Serialization/ASTReaderDecl.cpp
+++ b/clang/lib/Serialization/ASTReaderDecl.cpp
@@ -2684,7 +2684,7 @@ void 
ASTDeclReader::VisitNonTypeTemplateParmDecl(NonTypeTemplateParmDecl *D) {
   // TemplateParmPosition.
   D->setDepth(Record.readInt());
   D->setPosition(Record.readInt());
-  if (D->hasPlaceholderTypeConstraint())
+  if (Record.readBool()) // PlaceholderTypeConstraintInitialized
     D->setPlaceholderTypeConstraint(Record.readExpr());
   if (D->isExpandedParameterPack()) {
     auto TypesAndInfos =
diff --git a/clang/lib/Serialization/ASTWriterDecl.cpp 
b/clang/lib/Serialization/ASTWriterDecl.cpp
index 75c1d9a6d438ce..d9fae52c17f075 100644
--- a/clang/lib/Serialization/ASTWriterDecl.cpp
+++ b/clang/lib/Serialization/ASTWriterDecl.cpp
@@ -1983,8 +1983,7 @@ void 
ASTDeclWriter::VisitNonTypeTemplateParmDecl(NonTypeTemplateParmDecl *D) {
   // For an expanded parameter pack, record the number of expansion types here
   // so that it's easier for deserialization to allocate the right amount of
   // memory.
-  Expr *TypeConstraint = D->getPlaceholderTypeConstraint();
-  Record.push_back(!!TypeConstraint);
+  Record.push_back(D->hasPlaceholderTypeConstraint());
   if (D->isExpandedParameterPack())
     Record.push_back(D->getNumExpansionTypes());
 
@@ -1992,6 +1991,9 @@ void 
ASTDeclWriter::VisitNonTypeTemplateParmDecl(NonTypeTemplateParmDecl *D) {
   // TemplateParmPosition.
   Record.push_back(D->getDepth());
   Record.push_back(D->getPosition());
+  Expr *TypeConstraint = D->getPlaceholderTypeConstraint();
+  Record.push_back(/*PlaceholderTypeConstraintInitialized=*/TypeConstraint !=
+                   nullptr);
   if (TypeConstraint)
     Record.AddStmt(TypeConstraint);
 
diff --git 
a/clang/test/Modules/malformed-constraint-template-non-type-parm-decl.cpp 
b/clang/test/Modules/malformed-constraint-template-non-type-parm-decl.cpp
index cea6404bbebd28..73dff88e506b4d 100644
--- a/clang/test/Modules/malformed-constraint-template-non-type-parm-decl.cpp
+++ b/clang/test/Modules/malformed-constraint-template-non-type-parm-decl.cpp
@@ -5,6 +5,9 @@
 // RUN: %clang_cc1 -std=c++20 mod.cppm -emit-module-interface -o mod.pcm 
-fallow-pcm-with-compiler-errors -verify
 // RUN: %clang_cc1 -std=c++20 main.cpp -fmodule-file=mod=mod.pcm -verify 
-fallow-pcm-with-compiler-errors -fsyntax-only -ast-dump-all | FileCheck %s
 
+// RUN: %clang_cc1 -std=c++20 mod.cppm -emit-reduced-module-interface -o 
mod.pcm -fallow-pcm-with-compiler-errors -verify
+// RUN: %clang_cc1 -std=c++20 main.cpp -fmodule-file=mod=mod.pcm -verify 
-fallow-pcm-with-compiler-errors -fsyntax-only -ast-dump-all | FileCheck %s
+
 //--- mod.cppm
 export module mod;
 
@@ -29,7 +32,6 @@ import mod;
 
 // CHECK:      |-FunctionTemplateDecl {{.*}} <line:11:1, line:12:22> col:6 
imported in mod hidden invalid cos
 // CHECK-NEXT: | |-NonTypeTemplateParmDecl {{.*}} <line:11:10, col:34> col:34 
imported in mod hidden referenced invalid 'ReferenceOf<angle> auto' depth 0 
index 0 R
-// CHECK-NEXT: | | `-RecoveryExpr {{.*}} <col:10, col:34> 'ReferenceOf<angle> 
auto' contains-errors lvalue
 // CHECK-NEXT: | |-TemplateTypeParmDecl {{.*}} <col:37, col:46> col:46 
imported in mod hidden referenced typename depth 0 index 1 Rep
 // CHECK-NEXT: | |-RequiresExpr {{.*}} <col:60, col:84> 'bool'
 // CHECK-NEXT: | | |-ParmVarDecl {{.*}} <col:69, col:73> col:73 imported in 
mod hidden referenced v 'Rep'
@@ -42,7 +44,6 @@ import mod;
 
 // CHECK:      |-FunctionTemplateDecl {{.*}} <line:15:1, line:16:22> col:6 
imported in mod hidden invalid tan
 // CHECK-NEXT: | |-NonTypeTemplateParmDecl {{.*}} <line:15:10, col:34> col:34 
imported in mod hidden referenced invalid 'ReferenceOf<angle> auto' depth 0 
index 0 R
-// CHECK-NEXT: | | `-RecoveryExpr {{.*}} <col:10, col:34> 'ReferenceOf<angle> 
auto' contains-errors lvalue
 // CHECK-NEXT: | |-TemplateTypeParmDecl {{.*}} <col:37, col:46> col:46 
imported in mod hidden referenced typename depth 0 index 1 Rep
 // CHECK-NEXT: | |-RequiresExpr {{.*}} <col:60, col:84> 'bool'
 // CHECK-NEXT: | | |-ParmVarDecl {{.*}} <col:69, col:73> col:73 imported in 
mod hidden referenced v 'Rep'

From 38d144e38bee202ac4f1c237b0e5c77958820bb9 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Alejandro=20=C3=81lvarez=20Ayll=C3=B3n?=
 <alejandro.alva...@sonarsource.com>
Date: Mon, 20 Jan 2025 17:34:29 +0100
Subject: [PATCH 6/6] Do not serialize PlaceholderTypeConstraintInitialized if

hasPlaceholderTypeConstraint is false
---
 clang/lib/Serialization/ASTReaderDecl.cpp |  6 ++++--
 clang/lib/Serialization/ASTWriterDecl.cpp | 13 ++++++++-----
 2 files changed, 12 insertions(+), 7 deletions(-)

diff --git a/clang/lib/Serialization/ASTReaderDecl.cpp 
b/clang/lib/Serialization/ASTReaderDecl.cpp
index c4f9cb12ad12f2..a031bb18f0044e 100644
--- a/clang/lib/Serialization/ASTReaderDecl.cpp
+++ b/clang/lib/Serialization/ASTReaderDecl.cpp
@@ -2703,8 +2703,10 @@ void 
ASTDeclReader::VisitNonTypeTemplateParmDecl(NonTypeTemplateParmDecl *D) {
   // TemplateParmPosition.
   D->setDepth(Record.readInt());
   D->setPosition(Record.readInt());
-  if (Record.readBool()) // PlaceholderTypeConstraintInitialized
-    D->setPlaceholderTypeConstraint(Record.readExpr());
+  if (D->hasPlaceholderTypeConstraint()) {
+    if (Record.readBool()) // PlaceholderTypeConstraintInitialized
+      D->setPlaceholderTypeConstraint(Record.readExpr());
+  }
   if (D->isExpandedParameterPack()) {
     auto TypesAndInfos =
         D->getTrailingObjects<std::pair<QualType, TypeSourceInfo *>>();
diff --git a/clang/lib/Serialization/ASTWriterDecl.cpp 
b/clang/lib/Serialization/ASTWriterDecl.cpp
index 77f49af375f563..ec27a197919b06 100644
--- a/clang/lib/Serialization/ASTWriterDecl.cpp
+++ b/clang/lib/Serialization/ASTWriterDecl.cpp
@@ -1993,11 +1993,14 @@ void 
ASTDeclWriter::VisitNonTypeTemplateParmDecl(NonTypeTemplateParmDecl *D) {
   // TemplateParmPosition.
   Record.push_back(D->getDepth());
   Record.push_back(D->getPosition());
-  Expr *TypeConstraint = D->getPlaceholderTypeConstraint();
-  Record.push_back(/*PlaceholderTypeConstraintInitialized=*/TypeConstraint !=
-                   nullptr);
-  if (TypeConstraint)
-    Record.AddStmt(TypeConstraint);
+
+  if (D->hasPlaceholderTypeConstraint()) {
+    Expr *TypeConstraint = D->getPlaceholderTypeConstraint();
+    Record.push_back(/*PlaceholderTypeConstraintInitialized=*/TypeConstraint !=
+                     nullptr);
+    if (TypeConstraint)
+      Record.AddStmt(TypeConstraint);
+  }
 
   if (D->isExpandedParameterPack()) {
     for (unsigned I = 0, N = D->getNumExpansionTypes(); I != N; ++I) {

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