[clang] [clang] Distinguish NTTPs with deduced types in variable template partial specializations (PR #152864)

[via cfe-commits]

================
@@ -4126,7 +4126,11 @@ static bool isTemplateArgumentTemplateParameter(const TemplateArgument &Arg,
       return false;
     const NonTypeTemplateParmDecl *NTTP =
         dyn_cast<NonTypeTemplateParmDecl>(DRE->getDecl());
-    return NTTP && NTTP->getDepth() == Depth && NTTP->getIndex() == Index;
+    if (!NTTP || NTTP->getDepth() != Depth || NTTP->getIndex() != Index)
+      return false;
+    QualType ParamType = cast<NonTypeTemplateParmDecl>(Param)->getType();
+    QualType NTTPType = NTTP->getType();
+    return ParamType.getCanonicalType() == NTTPType.getCanonicalType();
----------------
keinflue wrote:

(Assuming my understanding is correct) `Param` is here supposed to be the (non-type) template parameter of the primary template and `NTTP` the non-type template parameter of the partial specialization, i.e. they are not the same template parameter.

The intent is to test whether the template arguments/parameters of the primary template and specialization are "identical" for the purpose of https://timsong-cpp.github.io/cppwp/n4140/temp.class.spec#8.3 (which since has been removed since from the standard because https://timsong-cpp.github.io/cppwp/n4140/temp.class.spec#8.4 which was added later covers it).

This is done only to give a more user-friendly error message in case the user intended to declare a primary template instead of a specialization, the relevant check for p8.4 is done elsewhere. So I think it is ok if the function incorrectly returns `true`, but it must not return `false` incorrectly.

https://github.com/llvm/llvm-project/pull/152864