[clang] [libcxx] [Clang] Implement CWG2369 "Ordering between constraints and substitution" (PR #102857)

[via cfe-commits]

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@@ -4008,6 +3996,38 @@ TemplateDeductionResult Sema::FinishTemplateArgumentDeduction(
     Owner = FunctionTemplate->getLexicalDeclContext();
   FunctionDecl *FD = FunctionTemplate->getTemplatedDecl();
 
+  // C++20 [temp.deduct.general]p5: (CWG2369)
+  // If the function template has associated constraints, those constraints are
+  // checked for satisfaction. If the constraints are not satisfied, type
+  // deduction fails.
+  // FIXME: We haven't implemented CWG2369 for lambdas yet, because we need
+  // to figure out how to instantiate lambda captures to the scope without
+  // first instantiating the lambda.
+  bool IsLambda = isLambdaCallOperator(FD) || isLambdaConversionOperator(FD);
+  if (!IsLambda && !IsIncomplete) {
+    if (CheckFunctionTemplateConstraints(
+            Info.getLocation(),
+            FunctionTemplate->getCanonicalDecl()->getTemplatedDecl(),
+            CanonicalBuilder, Info.AssociatedConstraintsSatisfaction))
+      return TemplateDeductionResult::MiscellaneousDeductionFailure;
+    if (!Info.AssociatedConstraintsSatisfaction.IsSatisfied) {
+      Info.reset(Info.takeSugared(),
+                 TemplateArgumentList::CreateCopy(Context, CanonicalBuilder));
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cor3ntin wrote:

Why is that necessary?

https://github.com/llvm/llvm-project/pull/102857