[clang] [Sema] Avoid an undesired pack expansion while transforming PackIndexingType (PR #90195)

2024-04-30 Thread Younan Zhang via cfe-commits 

https://github.com/zyn0217 closed 
https://github.com/llvm/llvm-project/pull/90195
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[clang] [Sema] Avoid an undesired pack expansion while transforming PackIndexingType (PR #90195)

2024-04-30 Thread Younan Zhang via cfe-commits 

https://github.com/zyn0217 updated 
https://github.com/llvm/llvm-project/pull/90195

>From f708694fc2686684589dca7b8f3738a117fc047e Mon Sep 17 00:00:00 2001
From: Younan Zhang 
Date: Fri, 26 Apr 2024 19:06:57 +0800
Subject: [PATCH 1/6] [Sema] Avoid an undesired pack expansion while
 transforming PackIndexingType

---
 clang/lib/Sema/TreeTransform.h |  3 +++
 clang/test/SemaCXX/cxx2c-pack-indexing.cpp | 19 +++
 2 files changed, 22 insertions(+)

diff --git a/clang/lib/Sema/TreeTransform.h b/clang/lib/Sema/TreeTransform.h
index 9404be5a46f3f7..abc4a16c004a9f 100644
--- a/clang/lib/Sema/TreeTransform.h
+++ b/clang/lib/Sema/TreeTransform.h
@@ -6649,6 +6649,9 @@ 
TreeTransform::TransformPackIndexingType(TypeLocBuilder ,
 }
   }
 
+  // We may be doing this in the context of expanding the Pattern. Forget that
+  // because it has been handled above.
+  Sema::ArgumentPackSubstitutionIndexRAII SubstIndex(getSema(), -1);
   QualType Result = getDerived().TransformType(TLB, TL.getPatternLoc());
 
   QualType Out = getDerived().RebuildPackIndexingType(
diff --git a/clang/test/SemaCXX/cxx2c-pack-indexing.cpp 
b/clang/test/SemaCXX/cxx2c-pack-indexing.cpp
index 606715e6aacffd..2fd0dbfed294a5 100644
--- a/clang/test/SemaCXX/cxx2c-pack-indexing.cpp
+++ b/clang/test/SemaCXX/cxx2c-pack-indexing.cpp
@@ -160,3 +160,22 @@ namespace GH88929 {
 using E = P...[0]; // expected-error {{unknown type name 'P'}} \
// expected-error {{expected ';' after alias 
declaration}}
 }
+
+namespace GH88925 {
+template  struct S {};
+
+template  struct sequence {};
+
+template  auto f(sequence) {
+  return S(); // #use
+}
+
+void g() {
+  static_assert(__is_same(decltype(f(sequence<0, 0>())), S));
+  static_assert(__is_same(decltype(f(sequence<0, 0>())), S));
+  static_assert(__is_same(decltype(f(sequence<0, 1>())), S));
+  f(sequence<3>());
+  // expected-error@#use {{invalid index 3 for pack 'Args' of size 2}}}
+  // expected-note-re@-2 {{function template specialization '{{.*}}' requested 
here}}
+}
+}

>From 7b0ae16b6777c6e98df64cd2366434972fc68164 Mon Sep 17 00:00:00 2001
From: Younan Zhang 
Date: Sat, 27 Apr 2024 12:07:30 +0800
Subject: [PATCH 2/6] Clarify comments

---
 clang/lib/Sema/TreeTransform.h | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/clang/lib/Sema/TreeTransform.h b/clang/lib/Sema/TreeTransform.h
index abc4a16c004a9f..b50fdab8bfd05e 100644
--- a/clang/lib/Sema/TreeTransform.h
+++ b/clang/lib/Sema/TreeTransform.h
@@ -6649,8 +6649,9 @@ 
TreeTransform::TransformPackIndexingType(TypeLocBuilder ,
 }
   }
 
-  // We may be doing this in the context of expanding the Pattern. Forget that
-  // because it has been handled above.
+  // A pack indexing type can appear in a larger pack expansion,
+  // e.g. `Pack...[pack_of_indexes]...`
+  // so we need to temporarily disable substitution of pack elements
   Sema::ArgumentPackSubstitutionIndexRAII SubstIndex(getSema(), -1);
   QualType Result = getDerived().TransformType(TLB, TL.getPatternLoc());
 

>From 1dbfe522a1b4efd92023f0ff3c41cc296bf53299 Mon Sep 17 00:00:00 2001
From: Younan Zhang 
Date: Sun, 28 Apr 2024 13:29:37 +0800
Subject: [PATCH 3/6] Make dependent PackIndexingExpr always an LValue

---
 clang/lib/AST/ExprClassification.cpp   |  8 +++-
 clang/test/SemaCXX/cxx2c-pack-indexing.cpp | 23 ++
 2 files changed, 26 insertions(+), 5 deletions(-)

diff --git a/clang/lib/AST/ExprClassification.cpp 
b/clang/lib/AST/ExprClassification.cpp
index 7026fca8554ce9..abfc3a51f95a8d 100644
--- a/clang/lib/AST/ExprClassification.cpp
+++ b/clang/lib/AST/ExprClassification.cpp
@@ -216,8 +216,14 @@ static Cl::Kinds ClassifyInternal(ASTContext , const 
Expr *E) {
 return ClassifyInternal(Ctx,
  cast(E)->getReplacement());
 
-  case Expr::PackIndexingExprClass:
+  case Expr::PackIndexingExprClass: {
+// A dependent pack-index-expression is now supposed to denote a function
+// parameter pack, an NTTP pack, or the pack introduced by a structured
+// binding. Consider it as an LValue expression.
+if (cast(E)->isInstantiationDependent())
+  return Cl::CL_LValue;
 return ClassifyInternal(Ctx, cast(E)->getSelectedExpr());
+  }
 
 // C, C++98 [expr.sub]p1: The result is an lvalue of type "T".
 // C++11 (DR1213): in the case of an array operand, the result is an lvalue
diff --git a/clang/test/SemaCXX/cxx2c-pack-indexing.cpp 
b/clang/test/SemaCXX/cxx2c-pack-indexing.cpp
index 2fd0dbfed294a5..a3e5a0931491b5 100644
--- a/clang/test/SemaCXX/cxx2c-pack-indexing.cpp
+++ b/clang/test/SemaCXX/cxx2c-pack-indexing.cpp
@@ -164,18 +164,33 @@ namespace GH88929 {
 namespace GH88925 {
 template  struct S {};
 
+template  struct W {};
+
 template  struct sequence {};
 
-template  auto f(sequence) {
-  return S(); // #use
+template  auto f(sequence) {
+  return S(); // #use
 }
 
-void g() {
+template  auto g(sequence) {
+  return W(); // #nttp-use
+}
+

[clang] [Sema] Avoid an undesired pack expansion while transforming PackIndexingType (PR #90195)

2024-04-30 Thread via cfe-commits 

github-actions[bot] wrote:




:warning: C/C++ code formatter, clang-format found issues in your code. 
:warning:



You can test this locally with the following command:


``bash
git-clang-format --diff 3ea9ed471c8dca8f703d7f3ce93d274a718b54bb 
7a087f06240ce09a64fe679df14380e2fa538701 -- 
clang/lib/AST/ExprClassification.cpp clang/lib/Sema/TreeTransform.h 
clang/test/SemaCXX/cxx2c-pack-indexing.cpp
``





View the diff from clang-format here.


``diff
diff --git a/clang/lib/AST/ExprClassification.cpp 
b/clang/lib/AST/ExprClassification.cpp
index f943a6ba0d..45e05970df 100644
--- a/clang/lib/AST/ExprClassification.cpp
+++ b/clang/lib/AST/ExprClassification.cpp
@@ -217,7 +217,7 @@ static Cl::Kinds ClassifyInternal(ASTContext , const 
Expr *E) {
  cast(E)->getReplacement());
 
   case Expr::PackIndexingExprClass: {
-// A pack-index-expression always expand to an id-expression. 
+// A pack-index-expression always expand to an id-expression.
 // Consider it as an LValue expression.
 if (cast(E)->isInstantiationDependent())
   return Cl::CL_LValue;

``




https://github.com/llvm/llvm-project/pull/90195
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[clang] [Sema] Avoid an undesired pack expansion while transforming PackIndexingType (PR #90195)

2024-04-30 Thread Younan Zhang via cfe-commits 

https://github.com/zyn0217 updated 
https://github.com/llvm/llvm-project/pull/90195

>From f708694fc2686684589dca7b8f3738a117fc047e Mon Sep 17 00:00:00 2001
From: Younan Zhang 
Date: Fri, 26 Apr 2024 19:06:57 +0800
Subject: [PATCH 1/5] [Sema] Avoid an undesired pack expansion while
 transforming PackIndexingType

---
 clang/lib/Sema/TreeTransform.h |  3 +++
 clang/test/SemaCXX/cxx2c-pack-indexing.cpp | 19 +++
 2 files changed, 22 insertions(+)

diff --git a/clang/lib/Sema/TreeTransform.h b/clang/lib/Sema/TreeTransform.h
index 9404be5a46f3f7..abc4a16c004a9f 100644
--- a/clang/lib/Sema/TreeTransform.h
+++ b/clang/lib/Sema/TreeTransform.h
@@ -6649,6 +6649,9 @@ 
TreeTransform::TransformPackIndexingType(TypeLocBuilder ,
 }
   }
 
+  // We may be doing this in the context of expanding the Pattern. Forget that
+  // because it has been handled above.
+  Sema::ArgumentPackSubstitutionIndexRAII SubstIndex(getSema(), -1);
   QualType Result = getDerived().TransformType(TLB, TL.getPatternLoc());
 
   QualType Out = getDerived().RebuildPackIndexingType(
diff --git a/clang/test/SemaCXX/cxx2c-pack-indexing.cpp 
b/clang/test/SemaCXX/cxx2c-pack-indexing.cpp
index 606715e6aacffd..2fd0dbfed294a5 100644
--- a/clang/test/SemaCXX/cxx2c-pack-indexing.cpp
+++ b/clang/test/SemaCXX/cxx2c-pack-indexing.cpp
@@ -160,3 +160,22 @@ namespace GH88929 {
 using E = P...[0]; // expected-error {{unknown type name 'P'}} \
// expected-error {{expected ';' after alias 
declaration}}
 }
+
+namespace GH88925 {
+template  struct S {};
+
+template  struct sequence {};
+
+template  auto f(sequence) {
+  return S(); // #use
+}
+
+void g() {
+  static_assert(__is_same(decltype(f(sequence<0, 0>())), S));
+  static_assert(__is_same(decltype(f(sequence<0, 0>())), S));
+  static_assert(__is_same(decltype(f(sequence<0, 1>())), S));
+  f(sequence<3>());
+  // expected-error@#use {{invalid index 3 for pack 'Args' of size 2}}}
+  // expected-note-re@-2 {{function template specialization '{{.*}}' requested 
here}}
+}
+}

>From 7b0ae16b6777c6e98df64cd2366434972fc68164 Mon Sep 17 00:00:00 2001
From: Younan Zhang 
Date: Sat, 27 Apr 2024 12:07:30 +0800
Subject: [PATCH 2/5] Clarify comments

---
 clang/lib/Sema/TreeTransform.h | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/clang/lib/Sema/TreeTransform.h b/clang/lib/Sema/TreeTransform.h
index abc4a16c004a9f..b50fdab8bfd05e 100644
--- a/clang/lib/Sema/TreeTransform.h
+++ b/clang/lib/Sema/TreeTransform.h
@@ -6649,8 +6649,9 @@ 
TreeTransform::TransformPackIndexingType(TypeLocBuilder ,
 }
   }
 
-  // We may be doing this in the context of expanding the Pattern. Forget that
-  // because it has been handled above.
+  // A pack indexing type can appear in a larger pack expansion,
+  // e.g. `Pack...[pack_of_indexes]...`
+  // so we need to temporarily disable substitution of pack elements
   Sema::ArgumentPackSubstitutionIndexRAII SubstIndex(getSema(), -1);
   QualType Result = getDerived().TransformType(TLB, TL.getPatternLoc());
 

>From 1dbfe522a1b4efd92023f0ff3c41cc296bf53299 Mon Sep 17 00:00:00 2001
From: Younan Zhang 
Date: Sun, 28 Apr 2024 13:29:37 +0800
Subject: [PATCH 3/5] Make dependent PackIndexingExpr always an LValue

---
 clang/lib/AST/ExprClassification.cpp   |  8 +++-
 clang/test/SemaCXX/cxx2c-pack-indexing.cpp | 23 ++
 2 files changed, 26 insertions(+), 5 deletions(-)

diff --git a/clang/lib/AST/ExprClassification.cpp 
b/clang/lib/AST/ExprClassification.cpp
index 7026fca8554ce9..abfc3a51f95a8d 100644
--- a/clang/lib/AST/ExprClassification.cpp
+++ b/clang/lib/AST/ExprClassification.cpp
@@ -216,8 +216,14 @@ static Cl::Kinds ClassifyInternal(ASTContext , const 
Expr *E) {
 return ClassifyInternal(Ctx,
  cast(E)->getReplacement());
 
-  case Expr::PackIndexingExprClass:
+  case Expr::PackIndexingExprClass: {
+// A dependent pack-index-expression is now supposed to denote a function
+// parameter pack, an NTTP pack, or the pack introduced by a structured
+// binding. Consider it as an LValue expression.
+if (cast(E)->isInstantiationDependent())
+  return Cl::CL_LValue;
 return ClassifyInternal(Ctx, cast(E)->getSelectedExpr());
+  }
 
 // C, C++98 [expr.sub]p1: The result is an lvalue of type "T".
 // C++11 (DR1213): in the case of an array operand, the result is an lvalue
diff --git a/clang/test/SemaCXX/cxx2c-pack-indexing.cpp 
b/clang/test/SemaCXX/cxx2c-pack-indexing.cpp
index 2fd0dbfed294a5..a3e5a0931491b5 100644
--- a/clang/test/SemaCXX/cxx2c-pack-indexing.cpp
+++ b/clang/test/SemaCXX/cxx2c-pack-indexing.cpp
@@ -164,18 +164,33 @@ namespace GH88929 {
 namespace GH88925 {
 template  struct S {};
 
+template  struct W {};
+
 template  struct sequence {};
 
-template  auto f(sequence) {
-  return S(); // #use
+template  auto f(sequence) {
+  return S(); // #use
 }
 
-void g() {
+template  auto g(sequence) {
+  return W(); // #nttp-use
+}
+

[clang] [Sema] Avoid an undesired pack expansion while transforming PackIndexingType (PR #90195)

2024-04-30 Thread Younan Zhang via cfe-commits 

https://github.com/zyn0217 updated 
https://github.com/llvm/llvm-project/pull/90195

>From f708694fc2686684589dca7b8f3738a117fc047e Mon Sep 17 00:00:00 2001
From: Younan Zhang 
Date: Fri, 26 Apr 2024 19:06:57 +0800
Subject: [PATCH 1/4] [Sema] Avoid an undesired pack expansion while
 transforming PackIndexingType

---
 clang/lib/Sema/TreeTransform.h |  3 +++
 clang/test/SemaCXX/cxx2c-pack-indexing.cpp | 19 +++
 2 files changed, 22 insertions(+)

diff --git a/clang/lib/Sema/TreeTransform.h b/clang/lib/Sema/TreeTransform.h
index 9404be5a46f3f7..abc4a16c004a9f 100644
--- a/clang/lib/Sema/TreeTransform.h
+++ b/clang/lib/Sema/TreeTransform.h
@@ -6649,6 +6649,9 @@ 
TreeTransform::TransformPackIndexingType(TypeLocBuilder ,
 }
   }
 
+  // We may be doing this in the context of expanding the Pattern. Forget that
+  // because it has been handled above.
+  Sema::ArgumentPackSubstitutionIndexRAII SubstIndex(getSema(), -1);
   QualType Result = getDerived().TransformType(TLB, TL.getPatternLoc());
 
   QualType Out = getDerived().RebuildPackIndexingType(
diff --git a/clang/test/SemaCXX/cxx2c-pack-indexing.cpp 
b/clang/test/SemaCXX/cxx2c-pack-indexing.cpp
index 606715e6aacffd..2fd0dbfed294a5 100644
--- a/clang/test/SemaCXX/cxx2c-pack-indexing.cpp
+++ b/clang/test/SemaCXX/cxx2c-pack-indexing.cpp
@@ -160,3 +160,22 @@ namespace GH88929 {
 using E = P...[0]; // expected-error {{unknown type name 'P'}} \
// expected-error {{expected ';' after alias 
declaration}}
 }
+
+namespace GH88925 {
+template  struct S {};
+
+template  struct sequence {};
+
+template  auto f(sequence) {
+  return S(); // #use
+}
+
+void g() {
+  static_assert(__is_same(decltype(f(sequence<0, 0>())), S));
+  static_assert(__is_same(decltype(f(sequence<0, 0>())), S));
+  static_assert(__is_same(decltype(f(sequence<0, 1>())), S));
+  f(sequence<3>());
+  // expected-error@#use {{invalid index 3 for pack 'Args' of size 2}}}
+  // expected-note-re@-2 {{function template specialization '{{.*}}' requested 
here}}
+}
+}

>From 7b0ae16b6777c6e98df64cd2366434972fc68164 Mon Sep 17 00:00:00 2001
From: Younan Zhang 
Date: Sat, 27 Apr 2024 12:07:30 +0800
Subject: [PATCH 2/4] Clarify comments

---
 clang/lib/Sema/TreeTransform.h | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/clang/lib/Sema/TreeTransform.h b/clang/lib/Sema/TreeTransform.h
index abc4a16c004a9f..b50fdab8bfd05e 100644
--- a/clang/lib/Sema/TreeTransform.h
+++ b/clang/lib/Sema/TreeTransform.h
@@ -6649,8 +6649,9 @@ 
TreeTransform::TransformPackIndexingType(TypeLocBuilder ,
 }
   }
 
-  // We may be doing this in the context of expanding the Pattern. Forget that
-  // because it has been handled above.
+  // A pack indexing type can appear in a larger pack expansion,
+  // e.g. `Pack...[pack_of_indexes]...`
+  // so we need to temporarily disable substitution of pack elements
   Sema::ArgumentPackSubstitutionIndexRAII SubstIndex(getSema(), -1);
   QualType Result = getDerived().TransformType(TLB, TL.getPatternLoc());
 

>From 1dbfe522a1b4efd92023f0ff3c41cc296bf53299 Mon Sep 17 00:00:00 2001
From: Younan Zhang 
Date: Sun, 28 Apr 2024 13:29:37 +0800
Subject: [PATCH 3/4] Make dependent PackIndexingExpr always an LValue

---
 clang/lib/AST/ExprClassification.cpp   |  8 +++-
 clang/test/SemaCXX/cxx2c-pack-indexing.cpp | 23 ++
 2 files changed, 26 insertions(+), 5 deletions(-)

diff --git a/clang/lib/AST/ExprClassification.cpp 
b/clang/lib/AST/ExprClassification.cpp
index 7026fca8554ce9..abfc3a51f95a8d 100644
--- a/clang/lib/AST/ExprClassification.cpp
+++ b/clang/lib/AST/ExprClassification.cpp
@@ -216,8 +216,14 @@ static Cl::Kinds ClassifyInternal(ASTContext , const 
Expr *E) {
 return ClassifyInternal(Ctx,
  cast(E)->getReplacement());
 
-  case Expr::PackIndexingExprClass:
+  case Expr::PackIndexingExprClass: {
+// A dependent pack-index-expression is now supposed to denote a function
+// parameter pack, an NTTP pack, or the pack introduced by a structured
+// binding. Consider it as an LValue expression.
+if (cast(E)->isInstantiationDependent())
+  return Cl::CL_LValue;
 return ClassifyInternal(Ctx, cast(E)->getSelectedExpr());
+  }
 
 // C, C++98 [expr.sub]p1: The result is an lvalue of type "T".
 // C++11 (DR1213): in the case of an array operand, the result is an lvalue
diff --git a/clang/test/SemaCXX/cxx2c-pack-indexing.cpp 
b/clang/test/SemaCXX/cxx2c-pack-indexing.cpp
index 2fd0dbfed294a5..a3e5a0931491b5 100644
--- a/clang/test/SemaCXX/cxx2c-pack-indexing.cpp
+++ b/clang/test/SemaCXX/cxx2c-pack-indexing.cpp
@@ -164,18 +164,33 @@ namespace GH88929 {
 namespace GH88925 {
 template  struct S {};
 
+template  struct W {};
+
 template  struct sequence {};
 
-template  auto f(sequence) {
-  return S(); // #use
+template  auto f(sequence) {
+  return S(); // #use
 }
 
-void g() {
+template  auto g(sequence) {
+  return W(); // #nttp-use
+}
+

[clang] [Sema] Avoid an undesired pack expansion while transforming PackIndexingType (PR #90195)

2024-04-30 Thread via cfe-commits 

https://github.com/cor3ntin edited 
https://github.com/llvm/llvm-project/pull/90195
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[clang] [Sema] Avoid an undesired pack expansion while transforming PackIndexingType (PR #90195)

2024-04-30 Thread via cfe-commits 


@@ -216,8 +216,14 @@ static Cl::Kinds ClassifyInternal(ASTContext , const 
Expr *E) {
 return ClassifyInternal(Ctx,
  cast(E)->getReplacement());
 
-  case Expr::PackIndexingExprClass:
+  case Expr::PackIndexingExprClass: {
+// A dependent pack-index-expression is now supposed to denote a function
+// parameter pack, an NTTP pack, or the pack introduced by a structured
+// binding. Consider it as an LValue expression.
+if (cast(E)->isInstantiationDependent())
+  return Cl::CL_LValue;

cor3ntin wrote:

```suggestion
  case Expr::PackIndexingExprClass: {
// A pack-index-expression always expand to an id-expression. 
Consider it as an LValue expression.
if (cast(E)->isInstantiationDependent())
  return Cl::CL_LValue;
```

https://github.com/llvm/llvm-project/pull/90195
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[clang] [Sema] Avoid an undesired pack expansion while transforming PackIndexingType (PR #90195)

2024-04-30 Thread via cfe-commits 

https://github.com/cor3ntin approved this pull request.

LGTM, thanks!

https://github.com/llvm/llvm-project/pull/90195
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[clang] [Sema] Avoid an undesired pack expansion while transforming PackIndexingType (PR #90195)

2024-04-30 Thread Younan Zhang via cfe-commits 

zyn0217 wrote:

@cor3ntin Mind looking at it again? Thanks so much!

https://github.com/llvm/llvm-project/pull/90195
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[clang] [Sema] Avoid an undesired pack expansion while transforming PackIndexingType (PR #90195)

2024-04-28 Thread Younan Zhang via cfe-commits 

https://github.com/zyn0217 edited 
https://github.com/llvm/llvm-project/pull/90195
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[clang] [Sema] Avoid an undesired pack expansion while transforming PackIndexingType (PR #90195)

2024-04-28 Thread Younan Zhang via cfe-commits 

https://github.com/zyn0217 updated 
https://github.com/llvm/llvm-project/pull/90195

>From f708694fc2686684589dca7b8f3738a117fc047e Mon Sep 17 00:00:00 2001
From: Younan Zhang 
Date: Fri, 26 Apr 2024 19:06:57 +0800
Subject: [PATCH 1/3] [Sema] Avoid an undesired pack expansion while
 transforming PackIndexingType

---
 clang/lib/Sema/TreeTransform.h |  3 +++
 clang/test/SemaCXX/cxx2c-pack-indexing.cpp | 19 +++
 2 files changed, 22 insertions(+)

diff --git a/clang/lib/Sema/TreeTransform.h b/clang/lib/Sema/TreeTransform.h
index 9404be5a46f3f7..abc4a16c004a9f 100644
--- a/clang/lib/Sema/TreeTransform.h
+++ b/clang/lib/Sema/TreeTransform.h
@@ -6649,6 +6649,9 @@ 
TreeTransform::TransformPackIndexingType(TypeLocBuilder ,
 }
   }
 
+  // We may be doing this in the context of expanding the Pattern. Forget that
+  // because it has been handled above.
+  Sema::ArgumentPackSubstitutionIndexRAII SubstIndex(getSema(), -1);
   QualType Result = getDerived().TransformType(TLB, TL.getPatternLoc());
 
   QualType Out = getDerived().RebuildPackIndexingType(
diff --git a/clang/test/SemaCXX/cxx2c-pack-indexing.cpp 
b/clang/test/SemaCXX/cxx2c-pack-indexing.cpp
index 606715e6aacffd..2fd0dbfed294a5 100644
--- a/clang/test/SemaCXX/cxx2c-pack-indexing.cpp
+++ b/clang/test/SemaCXX/cxx2c-pack-indexing.cpp
@@ -160,3 +160,22 @@ namespace GH88929 {
 using E = P...[0]; // expected-error {{unknown type name 'P'}} \
// expected-error {{expected ';' after alias 
declaration}}
 }
+
+namespace GH88925 {
+template  struct S {};
+
+template  struct sequence {};
+
+template  auto f(sequence) {
+  return S(); // #use
+}
+
+void g() {
+  static_assert(__is_same(decltype(f(sequence<0, 0>())), S));
+  static_assert(__is_same(decltype(f(sequence<0, 0>())), S));
+  static_assert(__is_same(decltype(f(sequence<0, 1>())), S));
+  f(sequence<3>());
+  // expected-error@#use {{invalid index 3 for pack 'Args' of size 2}}}
+  // expected-note-re@-2 {{function template specialization '{{.*}}' requested 
here}}
+}
+}

>From 7b0ae16b6777c6e98df64cd2366434972fc68164 Mon Sep 17 00:00:00 2001
From: Younan Zhang 
Date: Sat, 27 Apr 2024 12:07:30 +0800
Subject: [PATCH 2/3] Clarify comments

---
 clang/lib/Sema/TreeTransform.h | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/clang/lib/Sema/TreeTransform.h b/clang/lib/Sema/TreeTransform.h
index abc4a16c004a9f..b50fdab8bfd05e 100644
--- a/clang/lib/Sema/TreeTransform.h
+++ b/clang/lib/Sema/TreeTransform.h
@@ -6649,8 +6649,9 @@ 
TreeTransform::TransformPackIndexingType(TypeLocBuilder ,
 }
   }
 
-  // We may be doing this in the context of expanding the Pattern. Forget that
-  // because it has been handled above.
+  // A pack indexing type can appear in a larger pack expansion,
+  // e.g. `Pack...[pack_of_indexes]...`
+  // so we need to temporarily disable substitution of pack elements
   Sema::ArgumentPackSubstitutionIndexRAII SubstIndex(getSema(), -1);
   QualType Result = getDerived().TransformType(TLB, TL.getPatternLoc());
 

>From 1dbfe522a1b4efd92023f0ff3c41cc296bf53299 Mon Sep 17 00:00:00 2001
From: Younan Zhang 
Date: Sun, 28 Apr 2024 13:29:37 +0800
Subject: [PATCH 3/3] Make dependent PackIndexingExpr always an LValue

---
 clang/lib/AST/ExprClassification.cpp   |  8 +++-
 clang/test/SemaCXX/cxx2c-pack-indexing.cpp | 23 ++
 2 files changed, 26 insertions(+), 5 deletions(-)

diff --git a/clang/lib/AST/ExprClassification.cpp 
b/clang/lib/AST/ExprClassification.cpp
index 7026fca8554ce9..abfc3a51f95a8d 100644
--- a/clang/lib/AST/ExprClassification.cpp
+++ b/clang/lib/AST/ExprClassification.cpp
@@ -216,8 +216,14 @@ static Cl::Kinds ClassifyInternal(ASTContext , const 
Expr *E) {
 return ClassifyInternal(Ctx,
  cast(E)->getReplacement());
 
-  case Expr::PackIndexingExprClass:
+  case Expr::PackIndexingExprClass: {
+// A dependent pack-index-expression is now supposed to denote a function
+// parameter pack, an NTTP pack, or the pack introduced by a structured
+// binding. Consider it as an LValue expression.
+if (cast(E)->isInstantiationDependent())
+  return Cl::CL_LValue;
 return ClassifyInternal(Ctx, cast(E)->getSelectedExpr());
+  }
 
 // C, C++98 [expr.sub]p1: The result is an lvalue of type "T".
 // C++11 (DR1213): in the case of an array operand, the result is an lvalue
diff --git a/clang/test/SemaCXX/cxx2c-pack-indexing.cpp 
b/clang/test/SemaCXX/cxx2c-pack-indexing.cpp
index 2fd0dbfed294a5..a3e5a0931491b5 100644
--- a/clang/test/SemaCXX/cxx2c-pack-indexing.cpp
+++ b/clang/test/SemaCXX/cxx2c-pack-indexing.cpp
@@ -164,18 +164,33 @@ namespace GH88929 {
 namespace GH88925 {
 template  struct S {};
 
+template  struct W {};
+
 template  struct sequence {};
 
-template  auto f(sequence) {
-  return S(); // #use
+template  auto f(sequence) {
+  return S(); // #use
 }
 
-void g() {
+template  auto g(sequence) {
+  return W(); // #nttp-use
+}
+

[clang] [Sema] Avoid an undesired pack expansion while transforming PackIndexingType (PR #90195)

2024-04-28 Thread Younan Zhang via cfe-commits 


@@ -160,3 +160,22 @@ namespace GH88929 {
 using E = P...[0]; // expected-error {{unknown type name 'P'}} \
// expected-error {{expected ';' after alias 
declaration}}
 }
+
+namespace GH88925 {
+template  struct S {};
+
+template  struct sequence {};
+
+template  auto f(sequence) {
+  return S(); // #use
+}
+

zyn0217 wrote:

I realized those expressions could be PRValues after an instantiation because 
in `PackIndexingExpr` we have:
```cpp
if (!isInstantiationDependent())
  setValueKind(getSelectedExpr()->getValueKind());
```

So, I'm going to make it always an LValue iff it's dependent.

https://github.com/llvm/llvm-project/pull/90195
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[clang] [Sema] Avoid an undesired pack expansion while transforming PackIndexingType (PR #90195)

2024-04-27 Thread Younan Zhang via cfe-commits 

https://github.com/zyn0217 updated 
https://github.com/llvm/llvm-project/pull/90195

>From f708694fc2686684589dca7b8f3738a117fc047e Mon Sep 17 00:00:00 2001
From: Younan Zhang 
Date: Fri, 26 Apr 2024 19:06:57 +0800
Subject: [PATCH 1/3] [Sema] Avoid an undesired pack expansion while
 transforming PackIndexingType

---
 clang/lib/Sema/TreeTransform.h |  3 +++
 clang/test/SemaCXX/cxx2c-pack-indexing.cpp | 19 +++
 2 files changed, 22 insertions(+)

diff --git a/clang/lib/Sema/TreeTransform.h b/clang/lib/Sema/TreeTransform.h
index 9404be5a46f3f7..abc4a16c004a9f 100644
--- a/clang/lib/Sema/TreeTransform.h
+++ b/clang/lib/Sema/TreeTransform.h
@@ -6649,6 +6649,9 @@ 
TreeTransform::TransformPackIndexingType(TypeLocBuilder ,
 }
   }
 
+  // We may be doing this in the context of expanding the Pattern. Forget that
+  // because it has been handled above.
+  Sema::ArgumentPackSubstitutionIndexRAII SubstIndex(getSema(), -1);
   QualType Result = getDerived().TransformType(TLB, TL.getPatternLoc());
 
   QualType Out = getDerived().RebuildPackIndexingType(
diff --git a/clang/test/SemaCXX/cxx2c-pack-indexing.cpp 
b/clang/test/SemaCXX/cxx2c-pack-indexing.cpp
index 606715e6aacffd..2fd0dbfed294a5 100644
--- a/clang/test/SemaCXX/cxx2c-pack-indexing.cpp
+++ b/clang/test/SemaCXX/cxx2c-pack-indexing.cpp
@@ -160,3 +160,22 @@ namespace GH88929 {
 using E = P...[0]; // expected-error {{unknown type name 'P'}} \
// expected-error {{expected ';' after alias 
declaration}}
 }
+
+namespace GH88925 {
+template  struct S {};
+
+template  struct sequence {};
+
+template  auto f(sequence) {
+  return S(); // #use
+}
+
+void g() {
+  static_assert(__is_same(decltype(f(sequence<0, 0>())), S));
+  static_assert(__is_same(decltype(f(sequence<0, 0>())), S));
+  static_assert(__is_same(decltype(f(sequence<0, 1>())), S));
+  f(sequence<3>());
+  // expected-error@#use {{invalid index 3 for pack 'Args' of size 2}}}
+  // expected-note-re@-2 {{function template specialization '{{.*}}' requested 
here}}
+}
+}

>From 7b0ae16b6777c6e98df64cd2366434972fc68164 Mon Sep 17 00:00:00 2001
From: Younan Zhang 
Date: Sat, 27 Apr 2024 12:07:30 +0800
Subject: [PATCH 2/3] Clarify comments

---
 clang/lib/Sema/TreeTransform.h | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/clang/lib/Sema/TreeTransform.h b/clang/lib/Sema/TreeTransform.h
index abc4a16c004a9f..b50fdab8bfd05e 100644
--- a/clang/lib/Sema/TreeTransform.h
+++ b/clang/lib/Sema/TreeTransform.h
@@ -6649,8 +6649,9 @@ 
TreeTransform::TransformPackIndexingType(TypeLocBuilder ,
 }
   }
 
-  // We may be doing this in the context of expanding the Pattern. Forget that
-  // because it has been handled above.
+  // A pack indexing type can appear in a larger pack expansion,
+  // e.g. `Pack...[pack_of_indexes]...`
+  // so we need to temporarily disable substitution of pack elements
   Sema::ArgumentPackSubstitutionIndexRAII SubstIndex(getSema(), -1);
   QualType Result = getDerived().TransformType(TLB, TL.getPatternLoc());
 

>From d4a2f1266114b2bc96c17f4d0065338bb0040fb1 Mon Sep 17 00:00:00 2001
From: Younan Zhang 
Date: Sun, 28 Apr 2024 13:29:37 +0800
Subject: [PATCH 3/3] Make dependnt PackIndexingExpr always an LValue

---
 clang/lib/AST/ExprClassification.cpp   |  8 +++-
 clang/test/SemaCXX/cxx2c-pack-indexing.cpp | 23 ++
 2 files changed, 26 insertions(+), 5 deletions(-)

diff --git a/clang/lib/AST/ExprClassification.cpp 
b/clang/lib/AST/ExprClassification.cpp
index 7026fca8554ce9..1c4be91b0ce80f 100644
--- a/clang/lib/AST/ExprClassification.cpp
+++ b/clang/lib/AST/ExprClassification.cpp
@@ -216,8 +216,14 @@ static Cl::Kinds ClassifyInternal(ASTContext , const 
Expr *E) {
 return ClassifyInternal(Ctx,
  cast(E)->getReplacement());
 
-  case Expr::PackIndexingExprClass:
+  case Expr::PackIndexingExprClass: {
+// A dependent pack-index-expression now is supposed to denote a function
+// parameter pack, an NTTP pack, or the pack introduced by a structured
+// binding. Consider it as an LValue expression.
+if (cast(E)->isInstantiationDependent())
+  return Cl::CL_LValue;
 return ClassifyInternal(Ctx, cast(E)->getSelectedExpr());
+  }
 
 // C, C++98 [expr.sub]p1: The result is an lvalue of type "T".
 // C++11 (DR1213): in the case of an array operand, the result is an lvalue
diff --git a/clang/test/SemaCXX/cxx2c-pack-indexing.cpp 
b/clang/test/SemaCXX/cxx2c-pack-indexing.cpp
index 2fd0dbfed294a5..a3e5a0931491b5 100644
--- a/clang/test/SemaCXX/cxx2c-pack-indexing.cpp
+++ b/clang/test/SemaCXX/cxx2c-pack-indexing.cpp
@@ -164,18 +164,33 @@ namespace GH88929 {
 namespace GH88925 {
 template  struct S {};
 
+template  struct W {};
+
 template  struct sequence {};
 
-template  auto f(sequence) {
-  return S(); // #use
+template  auto f(sequence) {
+  return S(); // #use
 }
 
-void g() {
+template  auto g(sequence) {
+  return W(); // #nttp-use
+}
+

[clang] [Sema] Avoid an undesired pack expansion while transforming PackIndexingType (PR #90195)

2024-04-27 Thread via cfe-commits 


@@ -160,3 +160,22 @@ namespace GH88929 {
 using E = P...[0]; // expected-error {{unknown type name 'P'}} \
// expected-error {{expected ';' after alias 
declaration}}
 }
+
+namespace GH88925 {
+template  struct S {};
+
+template  struct sequence {};
+
+template  auto f(sequence) {
+  return S(); // #use
+}
+

cor3ntin wrote:

> I guess we can perceive dependent PackIndexingExprs as LValues in such cases, 
> like what we did for UnresolvedLookupExpr. Additionally, P2662 now limits 
> them to id-expressions only, and hence we can probably always treat them as 
> LValues?

Yes, they are always LValues for now.  So we might want to do that indeed

https://github.com/llvm/llvm-project/pull/90195
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[clang] [Sema] Avoid an undesired pack expansion while transforming PackIndexingType (PR #90195)

2024-04-26 Thread Younan Zhang via cfe-commits 

https://github.com/zyn0217 updated 
https://github.com/llvm/llvm-project/pull/90195

>From f708694fc2686684589dca7b8f3738a117fc047e Mon Sep 17 00:00:00 2001
From: Younan Zhang 
Date: Fri, 26 Apr 2024 19:06:57 +0800
Subject: [PATCH 1/2] [Sema] Avoid an undesired pack expansion while
 transforming PackIndexingType

---
 clang/lib/Sema/TreeTransform.h |  3 +++
 clang/test/SemaCXX/cxx2c-pack-indexing.cpp | 19 +++
 2 files changed, 22 insertions(+)

diff --git a/clang/lib/Sema/TreeTransform.h b/clang/lib/Sema/TreeTransform.h
index 9404be5a46f3f7..abc4a16c004a9f 100644
--- a/clang/lib/Sema/TreeTransform.h
+++ b/clang/lib/Sema/TreeTransform.h
@@ -6649,6 +6649,9 @@ 
TreeTransform::TransformPackIndexingType(TypeLocBuilder ,
 }
   }
 
+  // We may be doing this in the context of expanding the Pattern. Forget that
+  // because it has been handled above.
+  Sema::ArgumentPackSubstitutionIndexRAII SubstIndex(getSema(), -1);
   QualType Result = getDerived().TransformType(TLB, TL.getPatternLoc());
 
   QualType Out = getDerived().RebuildPackIndexingType(
diff --git a/clang/test/SemaCXX/cxx2c-pack-indexing.cpp 
b/clang/test/SemaCXX/cxx2c-pack-indexing.cpp
index 606715e6aacffd..2fd0dbfed294a5 100644
--- a/clang/test/SemaCXX/cxx2c-pack-indexing.cpp
+++ b/clang/test/SemaCXX/cxx2c-pack-indexing.cpp
@@ -160,3 +160,22 @@ namespace GH88929 {
 using E = P...[0]; // expected-error {{unknown type name 'P'}} \
// expected-error {{expected ';' after alias 
declaration}}
 }
+
+namespace GH88925 {
+template  struct S {};
+
+template  struct sequence {};
+
+template  auto f(sequence) {
+  return S(); // #use
+}
+
+void g() {
+  static_assert(__is_same(decltype(f(sequence<0, 0>())), S));
+  static_assert(__is_same(decltype(f(sequence<0, 0>())), S));
+  static_assert(__is_same(decltype(f(sequence<0, 1>())), S));
+  f(sequence<3>());
+  // expected-error@#use {{invalid index 3 for pack 'Args' of size 2}}}
+  // expected-note-re@-2 {{function template specialization '{{.*}}' requested 
here}}
+}
+}

>From 7b0ae16b6777c6e98df64cd2366434972fc68164 Mon Sep 17 00:00:00 2001
From: Younan Zhang 
Date: Sat, 27 Apr 2024 12:07:30 +0800
Subject: [PATCH 2/2] Clarify comments

---
 clang/lib/Sema/TreeTransform.h | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/clang/lib/Sema/TreeTransform.h b/clang/lib/Sema/TreeTransform.h
index abc4a16c004a9f..b50fdab8bfd05e 100644
--- a/clang/lib/Sema/TreeTransform.h
+++ b/clang/lib/Sema/TreeTransform.h
@@ -6649,8 +6649,9 @@ 
TreeTransform::TransformPackIndexingType(TypeLocBuilder ,
 }
   }
 
-  // We may be doing this in the context of expanding the Pattern. Forget that
-  // because it has been handled above.
+  // A pack indexing type can appear in a larger pack expansion,
+  // e.g. `Pack...[pack_of_indexes]...`
+  // so we need to temporarily disable substitution of pack elements
   Sema::ArgumentPackSubstitutionIndexRAII SubstIndex(getSema(), -1);
   QualType Result = getDerived().TransformType(TLB, TL.getPatternLoc());
 

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[clang] [Sema] Avoid an undesired pack expansion while transforming PackIndexingType (PR #90195)

2024-04-26 Thread Younan Zhang via cfe-commits 


@@ -160,3 +160,22 @@ namespace GH88929 {
 using E = P...[0]; // expected-error {{unknown type name 'P'}} \
// expected-error {{expected ';' after alias 
declaration}}
 }
+
+namespace GH88925 {
+template  struct S {};
+
+template  struct sequence {};
+
+template  auto f(sequence) {
+  return S(); // #use
+}
+

zyn0217 wrote:

> ```cpp
> template  struct S2 {};
> template  auto f(sequence) {
>  return S2(); // #use
> }
> ```

OK, so the problem here is *not* the same: we end up [classifying the 
`PackIndexingExpr`'s 
category](https://github.com/llvm/llvm-project/blob/338561657685c1831a53563b1bc36ffc7470239e/clang/lib/AST/ExprClassification.cpp#L219-L220)
 while deducing against the NTTP of `S2`. This happens before the instantiation 
of `f`, and therefore `Args...[indices]...` is still dependent.

I *guess* we can perceive dependent `PackIndexingExprs` as LValues in such 
cases, like what we did for `UnresolvedLookupExpr`. Additionally, P2662 now 
limits them to id-expressions only, and hence we can probably always treat them 
as LValues?

https://github.com/llvm/llvm-project/pull/90195
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[clang] [Sema] Avoid an undesired pack expansion while transforming PackIndexingType (PR #90195)

2024-04-26 Thread Younan Zhang via cfe-commits 


@@ -160,3 +160,22 @@ namespace GH88929 {
 using E = P...[0]; // expected-error {{unknown type name 'P'}} \
// expected-error {{expected ';' after alias 
declaration}}
 }
+
+namespace GH88925 {
+template  struct S {};
+
+template  struct sequence {};
+
+template  auto f(sequence) {
+  return S(); // #use
+}
+

zyn0217 wrote:

Oops, yeah, good catch! Honestly, I have tried one similar example (with a fold 
expression) but had no luck:
```cpp
template  auto f(sequence) {
  return (Args...[indices] + ...);
}
```

Let me see what's happening here.

https://github.com/llvm/llvm-project/pull/90195
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[clang] [Sema] Avoid an undesired pack expansion while transforming PackIndexingType (PR #90195)

2024-04-26 Thread via cfe-commits 

https://github.com/cor3ntin commented:

Thanks for working on that!
I suspect there is the same problem for pack indexing expressions, could you 
try to add a test?

Thanks!

https://github.com/llvm/llvm-project/pull/90195
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[clang] [Sema] Avoid an undesired pack expansion while transforming PackIndexingType (PR #90195)

2024-04-26 Thread via cfe-commits 


@@ -160,3 +160,22 @@ namespace GH88929 {
 using E = P...[0]; // expected-error {{unknown type name 'P'}} \
// expected-error {{expected ';' after alias 
declaration}}
 }
+
+namespace GH88925 {
+template  struct S {};
+
+template  struct sequence {};
+
+template  auto f(sequence) {
+  return S(); // #use
+}
+

cor3ntin wrote:

I suspect there is the same problem for expressions

```cpp
template  struct S2 {};
template  auto f(sequence) {
  return S2(); // #use
}
```


https://github.com/llvm/llvm-project/pull/90195
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[clang] [Sema] Avoid an undesired pack expansion while transforming PackIndexingType (PR #90195)

2024-04-26 Thread via cfe-commits 

https://github.com/cor3ntin edited 
https://github.com/llvm/llvm-project/pull/90195
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[clang] [Sema] Avoid an undesired pack expansion while transforming PackIndexingType (PR #90195)

2024-04-26 Thread via cfe-commits 


@@ -6649,6 +6649,9 @@ 
TreeTransform::TransformPackIndexingType(TypeLocBuilder ,
 }
   }
 
+  // We may be doing this in the context of expanding the Pattern. Forget that
+  // because it has been handled above.
+  Sema::ArgumentPackSubstitutionIndexRAII SubstIndex(getSema(), -1);

cor3ntin wrote:

```suggestion
  // A pack indexing type can appear in a larger pack expansion, 
  // e.g `Pack...[pack_of_indexes]...`
  // so we need to temporarily disable substitution of pack elements
  Sema::ArgumentPackSubstitutionIndexRAII SubstIndex(getSema(), -1);
```

https://github.com/llvm/llvm-project/pull/90195
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[clang] [Sema] Avoid an undesired pack expansion while transforming PackIndexingType (PR #90195)

2024-04-26 Thread via cfe-commits 

llvmbot wrote:




@llvm/pr-subscribers-clang

Author: Younan Zhang (zyn0217)


Changes

Fixes https://github.com/llvm/llvm-project/issues/88925

---
Full diff: https://github.com/llvm/llvm-project/pull/90195.diff


2 Files Affected:

- (modified) clang/lib/Sema/TreeTransform.h (+3) 
- (modified) clang/test/SemaCXX/cxx2c-pack-indexing.cpp (+19) 


``diff
diff --git a/clang/lib/Sema/TreeTransform.h b/clang/lib/Sema/TreeTransform.h
index 9404be5a46f3f7..abc4a16c004a9f 100644
--- a/clang/lib/Sema/TreeTransform.h
+++ b/clang/lib/Sema/TreeTransform.h
@@ -6649,6 +6649,9 @@ 
TreeTransform::TransformPackIndexingType(TypeLocBuilder ,
 }
   }
 
+  // We may be doing this in the context of expanding the Pattern. Forget that
+  // because it has been handled above.
+  Sema::ArgumentPackSubstitutionIndexRAII SubstIndex(getSema(), -1);
   QualType Result = getDerived().TransformType(TLB, TL.getPatternLoc());
 
   QualType Out = getDerived().RebuildPackIndexingType(
diff --git a/clang/test/SemaCXX/cxx2c-pack-indexing.cpp 
b/clang/test/SemaCXX/cxx2c-pack-indexing.cpp
index 606715e6aacffd..2fd0dbfed294a5 100644
--- a/clang/test/SemaCXX/cxx2c-pack-indexing.cpp
+++ b/clang/test/SemaCXX/cxx2c-pack-indexing.cpp
@@ -160,3 +160,22 @@ namespace GH88929 {
 using E = P...[0]; // expected-error {{unknown type name 'P'}} \
// expected-error {{expected ';' after alias 
declaration}}
 }
+
+namespace GH88925 {
+template  struct S {};
+
+template  struct sequence {};
+
+template  auto f(sequence) {
+  return S(); // #use
+}
+
+void g() {
+  static_assert(__is_same(decltype(f(sequence<0, 0>())), S));
+  static_assert(__is_same(decltype(f(sequence<0, 0>())), S));
+  static_assert(__is_same(decltype(f(sequence<0, 1>())), S));
+  f(sequence<3>());
+  // expected-error@#use {{invalid index 3 for pack 'Args' of size 2}}}
+  // expected-note-re@-2 {{function template specialization '{{.*}}' requested 
here}}
+}
+}

``




https://github.com/llvm/llvm-project/pull/90195
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[clang] [Sema] Avoid an undesired pack expansion while transforming PackIndexingType (PR #90195)

2024-04-26 Thread Younan Zhang via cfe-commits 

https://github.com/zyn0217 created 
https://github.com/llvm/llvm-project/pull/90195

Fixes https://github.com/llvm/llvm-project/issues/88925

>From f708694fc2686684589dca7b8f3738a117fc047e Mon Sep 17 00:00:00 2001
From: Younan Zhang 
Date: Fri, 26 Apr 2024 19:06:57 +0800
Subject: [PATCH] [Sema] Avoid an undesired pack expansion while transforming
 PackIndexingType

---
 clang/lib/Sema/TreeTransform.h |  3 +++
 clang/test/SemaCXX/cxx2c-pack-indexing.cpp | 19 +++
 2 files changed, 22 insertions(+)

diff --git a/clang/lib/Sema/TreeTransform.h b/clang/lib/Sema/TreeTransform.h
index 9404be5a46f3f7..abc4a16c004a9f 100644
--- a/clang/lib/Sema/TreeTransform.h
+++ b/clang/lib/Sema/TreeTransform.h
@@ -6649,6 +6649,9 @@ 
TreeTransform::TransformPackIndexingType(TypeLocBuilder ,
 }
   }
 
+  // We may be doing this in the context of expanding the Pattern. Forget that
+  // because it has been handled above.
+  Sema::ArgumentPackSubstitutionIndexRAII SubstIndex(getSema(), -1);
   QualType Result = getDerived().TransformType(TLB, TL.getPatternLoc());
 
   QualType Out = getDerived().RebuildPackIndexingType(
diff --git a/clang/test/SemaCXX/cxx2c-pack-indexing.cpp 
b/clang/test/SemaCXX/cxx2c-pack-indexing.cpp
index 606715e6aacffd..2fd0dbfed294a5 100644
--- a/clang/test/SemaCXX/cxx2c-pack-indexing.cpp
+++ b/clang/test/SemaCXX/cxx2c-pack-indexing.cpp
@@ -160,3 +160,22 @@ namespace GH88929 {
 using E = P...[0]; // expected-error {{unknown type name 'P'}} \
// expected-error {{expected ';' after alias 
declaration}}
 }
+
+namespace GH88925 {
+template  struct S {};
+
+template  struct sequence {};
+
+template  auto f(sequence) {
+  return S(); // #use
+}
+
+void g() {
+  static_assert(__is_same(decltype(f(sequence<0, 0>())), S));
+  static_assert(__is_same(decltype(f(sequence<0, 0>())), S));
+  static_assert(__is_same(decltype(f(sequence<0, 1>())), S));
+  f(sequence<3>());
+  // expected-error@#use {{invalid index 3 for pack 'Args' of size 2}}}
+  // expected-note-re@-2 {{function template specialization '{{.*}}' requested 
here}}
+}
+}

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