[clang] [Clang][Sema] Push an evaluation context for type constraints (PR #93945)

[Younan Zhang via cfe-commits]

https://github.com/zyn0217 created https://github.com/llvm/llvm-project/pull/93945

This helps getTemplateInstantiationArgs to properly recover template arguments of an enclosing concept Decl.

Fixes https://github.com/llvm/llvm-project/issues/93821

>From 088c4199dd37172a57d965fe1b22f782084e127e Mon Sep 17 00:00:00 2001
From: Younan Zhang <zyn7109 at gmail.com>
Date: Fri, 31 May 2024 18:15:54 +0800
Subject: [PATCH] [Clang][Sema] Push an evaluation context for type constraints

This helps getTemplateInstantiationArgs to properly recover template
arguments of an enclosing concept Decl.

Fixes https://github.com/llvm/llvm-project/issues/93821
---
 clang/docs/ReleaseNotes.rst                 |  1 +
 clang/lib/Sema/SemaTemplateDeduction.cpp    | 14 ++++++++++++++
 clang/test/SemaTemplate/concepts-lambda.cpp | 12 ++++++++++++
 3 files changed, 27 insertions(+)

diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index 49ab222bec405..83eb285e9ca48 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -710,6 +710,7 @@ Bug Fixes to C++ Support
 - Correctly treat the compound statement of an ``if consteval`` as an immediate context. Fixes (#GH91509).
 - When partial ordering alias templates against template template parameters,
   allow pack expansions when the alias has a fixed-size parameter list. Fixes (#GH62529).
+- Fixed a type constraint substitution issue involving a generic lambda expression. (#GH93821)
 
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^
diff --git a/clang/lib/Sema/SemaTemplateDeduction.cpp b/clang/lib/Sema/SemaTemplateDeduction.cpp
index 853c0e1b50619..56529b4d852e7 100644
--- a/clang/lib/Sema/SemaTemplateDeduction.cpp
+++ b/clang/lib/Sema/SemaTemplateDeduction.cpp
@@ -5004,6 +5004,20 @@ static bool CheckDeducedPlaceholderConstraints(Sema &S, const AutoType &Type,
     return true;
   MultiLevelTemplateArgumentList MLTAL(Concept, CanonicalConverted,
                                        /*Final=*/false);
+  // Build up an EvaluationContext with an ImplicitConceptSpecializationDecl so
+  // that the template arguments of the constraint can be preserved. For
+  // example:
+  //
+  //  template <class T>
+  //  concept C = []<D U = void>() { return true; }();
+  //
+  // We need the argument for T while evaluating type constraint D in
+  // building the CallExpr to the lambda.
+  EnterExpressionEvaluationContext EECtx(
+      S, Sema::ExpressionEvaluationContext::ConstantEvaluated,
+      ImplicitConceptSpecializationDecl::Create(
+          S.getASTContext(), Concept->getDeclContext(), Concept->getLocation(),
+          CanonicalConverted));
   if (S.CheckConstraintSatisfaction(Concept, {Concept->getConstraintExpr()},
                                     MLTAL, TypeLoc.getLocalSourceRange(),
                                     Satisfaction))
diff --git a/clang/test/SemaTemplate/concepts-lambda.cpp b/clang/test/SemaTemplate/concepts-lambda.cpp
index fac790d09f9cf..280be71284f97 100644
--- a/clang/test/SemaTemplate/concepts-lambda.cpp
+++ b/clang/test/SemaTemplate/concepts-lambda.cpp
@@ -225,3 +225,15 @@ void foo() {
   }(x);
 }
 } // namespace GH73418
+
+namespace GH93821 {
+
+template <class>
+concept C = true;
+
+template <class...>
+concept D = []<C T = int>() { return true; }();
+
+D auto x = 0;
+
+} // namespace GH93821