[clang] [lldb] [clang] implement CWG1980 and CWG2064: instantiation-dependency improvements (PR #190495)

[Matheus Izvekov via cfe-commits]

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@@ -89,23 +89,20 @@ static bool SubstQualifier(Sema &SemaRef, const DeclT *OldDecl, DeclT *NewDecl,
     NestedNameSpecifierLoc NewQualifierLoc =
         SemaRef.SubstNestedNameSpecifierLoc(OldDecl->getQualifierLoc(),
                                             TemplateArgs);
-
-    if (!NewQualifierLoc)
-      return true;
-
+    assert(NewQualifierLoc &&
+           "substitution for the declaration qualifiers should never fail");
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mizvekov wrote:

Not in a declarative nested name specifier, that has extra restrictions on top of regular nested name specifiers.

With this patch, we enforce the declarative nested specifiers are well behaved, so it's not possible to perform void_t SFINAE tricks that only affect out-of-line definitions for example.

https://github.com/llvm/llvm-project/pull/190495