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

[Matheus Izvekov 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())
+      return A;
+    auto *R = TemplateTypeParmDecl::Create(
+        S.Context, A->getDeclContext(), SourceLocation(), SourceLocation(),
+        T->getDepth(), T->getIndex(), T->getIdentifier(),
+        T->wasDeclaredWithTypename(), /*ParameterPack=*/false,
+        T->hasTypeConstraint());
+    R->setDefaultArgument(
+        S.Context.getTrivialTypeSourceInfo(Default.getAsType()));
+    if (R->hasTypeConstraint()) {
+      auto *C = R->getTypeConstraint();
+      R->setTypeConstraint(C->getConceptReference(),
+                           C->getImmediatelyDeclaredConstraint());
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mizvekov wrote:

It's the same tests I linked in the other thread: https://github.com/llvm/llvm-project/blob/15f02723d49be9a828fbf072966a225babd60457/clang/test/CXX/temp/temp.decls/temp.fct/temp.func.order/p6.cpp#L48

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