[clang] [Clang] SubstituteConstraintExpressionWithoutSatisfaction needs an unevaluated context (PR #123883)

[Younan Zhang via cfe-commits]

https://github.com/zyn0217 updated https://github.com/llvm/llvm-project/pull/123883

>From 5427c748811041d0878dc4aec8b7b37c4241af37 Mon Sep 17 00:00:00 2001
From: Younan Zhang <zyn7109 at gmail.com>
Date: Tue, 21 Jan 2025 20:21:20 +0800
Subject: [PATCH 1/3] [Clang] SubstituteConstraintExpressionWithoutSatisfaction
 needs an unevaluated context

It turns out that the substitution for expression comparing also needs
an unevaluated context, otherwise any reference to immediate functions
might not be properly handled.
---
 clang/docs/ReleaseNotes.rst                        |  2 +-
 clang/lib/Sema/SemaConcept.cpp                     |  3 +++
 .../test/SemaTemplate/concepts-out-of-line-def.cpp | 14 ++++++++++++++
 3 files changed, 18 insertions(+), 1 deletion(-)

diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index f75c726e2751cf..5a5a97b341881a 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -966,7 +966,7 @@ Bug Fixes to C++ Support
   constraints are applied. (#GH122134)
 - Fixed canonicalization of pack indexing types - Clang did not always recognized identical pack indexing. (#GH123033)
 - Fixed a nested lambda substitution issue for constraint evaluation. (#GH123441)
-
+- Fixed various false diagnostics related to the use of immediate functions. (#GH123472)
 
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^
diff --git a/clang/lib/Sema/SemaConcept.cpp b/clang/lib/Sema/SemaConcept.cpp
index 6a40a59c977d78..8a77cbf8c9477b 100644
--- a/clang/lib/Sema/SemaConcept.cpp
+++ b/clang/lib/Sema/SemaConcept.cpp
@@ -1027,6 +1027,9 @@ static const Expr *SubstituteConstraintExpressionWithoutSatisfaction(
     ContextScope.emplace(S, const_cast<DeclContext *>(cast<DeclContext>(RD)),
                          /*NewThisContext=*/false);
   }
+  EnterExpressionEvaluationContext UnevaluatedContext(
+      S, Sema::ExpressionEvaluationContext::Unevaluated,
+      Sema::ReuseLambdaContextDecl);
   ExprResult SubstConstr = S.SubstConstraintExprWithoutSatisfaction(
       const_cast<clang::Expr *>(ConstrExpr), MLTAL);
   if (SFINAE.hasErrorOccurred() || !SubstConstr.isUsable())
diff --git a/clang/test/SemaTemplate/concepts-out-of-line-def.cpp b/clang/test/SemaTemplate/concepts-out-of-line-def.cpp
index 7cb5cfc10b9a7b..6c1a229a9fddab 100644
--- a/clang/test/SemaTemplate/concepts-out-of-line-def.cpp
+++ b/clang/test/SemaTemplate/concepts-out-of-line-def.cpp
@@ -737,3 +737,17 @@ ptr<U> make_item(auto &&args)
 ptr<char> p;
 
 } // namespace GH114685
+
+namespace GH123472 {
+
+consteval bool fn() { return true; }
+
+struct S {
+  template <typename T>
+  static consteval void mfn() requires (bool(&fn));
+};
+
+template <typename T>
+consteval void S::mfn() requires (bool(&fn)) {}
+
+}

>From 8dba2df946a367706605572f1f2935f70d6c16a6 Mon Sep 17 00:00:00 2001
From: Younan Zhang <zyn7109 at gmail.com>
Date: Wed, 22 Jan 2025 11:51:56 +0800
Subject: [PATCH 2/3] Fix a VLA diagnostic regression - BuildArrayType() is
 doubly called under unevaluated context

---
 clang/lib/Parse/ParseExpr.cpp | 19 +++++++++++++++++++
 clang/lib/Sema/SemaExpr.cpp   |  5 +++--
 2 files changed, 22 insertions(+), 2 deletions(-)

diff --git a/clang/lib/Parse/ParseExpr.cpp b/clang/lib/Parse/ParseExpr.cpp
index 8dd72db8f5b4a2..bf017a3131b4a7 100644
--- a/clang/lib/Parse/ParseExpr.cpp
+++ b/clang/lib/Parse/ParseExpr.cpp
@@ -248,6 +248,25 @@ ExprResult Parser::ParseArrayBoundExpression() {
   // If we parse the bound of a VLA... we parse a non-constant
   // constant-expression!
   Actions.ExprEvalContexts.back().InConditionallyConstantEvaluateContext = true;
+  // For a VLA type inside an unevaluated operator like:
+  //
+  //   sizeof(typeof(*(int (*)[N])array))
+  //
+  // in which the expression N is supposed to be ODR-used, as is the `array`.
+  // Initially when encountering `array`, it is deemed unevaluated and non-ODR
+  // used because that occurs before parsing the type cast. Therefore we use
+  // Sema::TransformToPotentiallyEvaluated() to rebuild the expression to ensure
+  // it's actually ODR-used.
+  //
+  // However, in other unevaluated contexts as in constraint substitution, it
+  // would end up rebuilding the type twice which is unnecessary. So we push up
+  // a flag to help distinguish these cases.
+  for (auto Iter = Actions.ExprEvalContexts.rbegin() + 1;
+       Iter != Actions.ExprEvalContexts.rend(); ++Iter) {
+    if (!Iter->isUnevaluated())
+      break;
+    Iter->InConditionallyConstantEvaluateContext = true;
+  }
   return ParseConstantExpressionInExprEvalContext(NotTypeCast);
 }
 
diff --git a/clang/lib/Sema/SemaExpr.cpp b/clang/lib/Sema/SemaExpr.cpp
index ae40895980d90a..444640b27de1fe 100644
--- a/clang/lib/Sema/SemaExpr.cpp
+++ b/clang/lib/Sema/SemaExpr.cpp
@@ -4630,8 +4630,9 @@ ExprResult Sema::CreateUnaryExprOrTypeTraitExpr(TypeSourceInfo *TInfo,
 
   // Adds overload of TransformToPotentiallyEvaluated for TypeSourceInfo to
   // properly deal with VLAs in nested calls of sizeof and typeof.
-  if (isUnevaluatedContext() && ExprKind == UETT_SizeOf &&
-      TInfo->getType()->isVariablyModifiedType())
+  if (currentEvaluationContext().isUnevaluated() &&
+      currentEvaluationContext().InConditionallyConstantEvaluateContext &&
+      ExprKind == UETT_SizeOf && TInfo->getType()->isVariablyModifiedType())
     TInfo = TransformToPotentiallyEvaluated(TInfo);
 
   // C99 6.5.3.4p4: the type (an unsigned integer type) is size_t.

>From a217b5fdc47ba87a21370c5cb5f70dcfe9d7957f Mon Sep 17 00:00:00 2001
From: Younan Zhang <zyn7109 at gmail.com>
Date: Wed, 22 Jan 2025 15:06:24 +0800
Subject: [PATCH 3/3] Apply comment

Co-authored-by: cor3ntin <corentinjabot at gmail.com>
---
 clang/lib/Parse/ParseExpr.cpp | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/clang/lib/Parse/ParseExpr.cpp b/clang/lib/Parse/ParseExpr.cpp
index bf017a3131b4a7..aa8b3870a188c6 100644
--- a/clang/lib/Parse/ParseExpr.cpp
+++ b/clang/lib/Parse/ParseExpr.cpp
@@ -252,7 +252,7 @@ ExprResult Parser::ParseArrayBoundExpression() {
   //
   //   sizeof(typeof(*(int (*)[N])array))
   //
-  // in which the expression N is supposed to be ODR-used, as is the `array`.
+  // N and array are supposed to be ODR-used.
   // Initially when encountering `array`, it is deemed unevaluated and non-ODR
   // used because that occurs before parsing the type cast. Therefore we use
   // Sema::TransformToPotentiallyEvaluated() to rebuild the expression to ensure