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

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

@@ -822,6 +822,7 @@ Bug Fixes to C++ Support
 - Fix a regression introduced in Clang 18 causing incorrect overload resolution in the presence of functions only
   differering by their constraints when only one of these function was variadic.
 - Fix a crash when a variable is captured by a block nested inside a lambda. (Fixes #GH93625).
+- Fixed a type constraint substitution issue involving a generic lambda expression. (#GH93821)
 
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^

clang/lib/Sema/SemaTemplate.cpp

@@ -5660,7 +5660,7 @@ Sema::CheckConceptTemplateId(const CXXScopeSpec &SS,
   LocalInstantiationScope Scope(*this);
 
   EnterExpressionEvaluationContext EECtx{
-      *this, ExpressionEvaluationContext::ConstantEvaluated, CSD};
+      *this, ExpressionEvaluationContext::Unevaluated, CSD};
 
   if (!AreArgsDependent &&
       CheckConstraintSatisfaction(

clang/lib/Sema/SemaTemplateDeduction.cpp

@@ -5134,6 +5134,20 @@ 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::Unevaluated,
+      ImplicitConceptSpecializationDecl::Create(
+          S.getASTContext(), Concept->getDeclContext(), Concept->getLocation(),
+          CanonicalConverted));
   if (S.CheckConstraintSatisfaction(Concept, {Concept->getConstraintExpr()},
                                     MLTAL, TypeLoc.getLocalSourceRange(),
                                     Satisfaction))

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