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

[Erich Keane via cfe-commits]

================
@@ -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,
----------------
erichkeane wrote:

I think Corentin is right.  And that link from SemaTemplate might actually be wrong too, but I'm not completely sure.  Though I would have expected us to see a bunch of bugs for evaluating that incorreclty.

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