[clang] [clang][Sema] Improve template argument deduction diagnostic (PR #122754)

[Matheus Izvekov via cfe-commits]

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@@ -3572,10 +3572,17 @@ TemplateDeductionResult Sema::SubstituteExplicitTemplateArguments(
                                 SugaredBuilder, CanonicalBuilder,
                                 /*UpdateArgsWithConversions=*/false) ||
       Trap.hasErrorOccurred()) {
+
     unsigned Index = SugaredBuilder.size();
     if (Index >= TemplateParams->size())
       return TemplateDeductionResult::SubstitutionFailure;
     Info.Param = makeTemplateParameter(TemplateParams->getParam(Index));
+    Info.FirstArg = ExplicitTemplateArgs[Index].getArgument();
+    if (ExplicitTemplateArgs[Index].getArgument().getKind() ==
+        TemplateArgument::Expression)
+      Info.SecondArg =
+          ExplicitTemplateArgs[Index].getSourceExpression()->getType();
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mizvekov wrote:

If you can't, or otherwise this new signature doesn't make sense for the pre-existing callers, you should create a new result kind instead.

https://github.com/llvm/llvm-project/pull/122754