[clang] [clang] disallow narrowing when matching template template parameters (PR #124313)

[Erich Keane via cfe-commits]

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@@ -6930,17 +6934,20 @@ ExprResult Sema::CheckTemplateArgument(NonTypeTemplateParmDecl *Param,
     IsConvertedConstantExpression = false;
   }
 
-  if (getLangOpts().CPlusPlus17) {
+  if (getLangOpts().CPlusPlus17 || PartialOrderingTTP) {
     // C++17 [temp.arg.nontype]p1:
     //   A template-argument for a non-type template parameter shall be
     //   a converted constant expression of the type of the template-parameter.
     APValue Value;
     ExprResult ArgResult;
     if (IsConvertedConstantExpression) {
-      ArgResult = BuildConvertedConstantExpression(Arg, ParamType,
-                                                   CCEK_TemplateArg, Param);
-      if (ArgResult.isInvalid())
+      ArgResult = BuildConvertedConstantExpression(
+          Arg, ParamType,
+          PartialOrderingTTP ? CCEK_InjectedTTP : CCEK_TemplateArg, Param);
+      if (ArgResult.isInvalid()) {
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erichkeane wrote:

In every singe case I've seen this come up, it has meant either 'error' or 'there is nothing here'.  And the question we usually ask is, "Can I do something with this".  Which `isUsable` means.  `isInvalid` means, "WAS something here, but there was a problem with it".

In this case, you're translating "There is nothing I can do with this" to "error".  Which makes more sense as `isUsable` to me.  

https://github.com/llvm/llvm-project/pull/124313