[clang] [clang-tools-extra] [clang] Implement CWG2398 provisional TTP matching to class templates (PR #94981)

[via cfe-commits]

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@@ -5327,9 +5332,29 @@ bool Sema::CheckTemplateArgumentList(
   SmallVector<TemplateArgument, 2> CanonicalArgumentPack;
   unsigned ArgIdx = 0, NumArgs = NewArgs.size();
   LocalInstantiationScope InstScope(*this, true);
-  for (TemplateParameterList::iterator Param = Params->begin(),
-                                       ParamEnd = Params->end();
-       Param != ParamEnd; /* increment in loop */) {
+  for (TemplateParameterList::iterator ParamBegin = Params->begin(),
+                                       ParamEnd = Params->end(),
+                                       Param = ParamBegin;
+       Param != ParamEnd;
+       /* increment in loop */) {
+    if (size_t ParamIdx = Param - ParamBegin;
+        DefaultArgs && ParamIdx >= DefaultArgs.StartPos) {
+      // All written arguments should have been consumed by this point.
+      assert(ArgIdx == NumArgs && "bad default argument deduction");
+      // FIXME: Don't ignore parameter packs.
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cor3ntin wrote:

Do we need a test/issue?

https://github.com/llvm/llvm-project/pull/94981