[clang] [clang] Enable C++17 relaxed template template argument matching by default (PR #89807)

[via cfe-commits]

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@@ -507,10 +507,62 @@ static TemplateDeductionResult DeduceNonTypeTemplateArgument(
       S, TemplateParams, NTTP, DeducedTemplateArgument(New), T, Info, Deduced);
 }
 
+static NamedDecl *DeduceTemplateArguments(Sema &S, NamedDecl *A,
+                                          TemplateArgument Default) {
+  switch (A->getKind()) {
+  case Decl::TemplateTypeParm: {
+    auto *T = cast<TemplateTypeParmDecl>(A);
+    // FIXME: DefaultArgument can't represent a pack.
+    if (T->isParameterPack())
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cor3ntin wrote:

My question was more whether we want to deduce packs at all.

I think the new behavior make sense, in that it is consistent with https://eel.is/c++draft/temp.deduct#partial-11 (
I suspect because both candidates are valid, this tie breaker gets triggered)

We might change that in the future, but now packs can't be defaulted, so I struggle to come up to think of a case where considering the pack would change the order of partial ordering (and not trailing packs can't be deduced)

here are more tests which I think are consistent https://godbolt.org/z/5Ke3TGfK6


https://github.com/llvm/llvm-project/pull/89807