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

[Matheus Izvekov via cfe-commits]

================
@@ -406,39 +406,41 @@ namespace PR42362 {
 namespace QualConv {
   int *X;
   template<const int *const *P> void f() {
-    using T = decltype(P);
-    using T = const int* const*;
+    using T = decltype(P);       // expected-note  {{previous definition}}
+    using T = const int* const*; // expected-error {{redefinition with different types ('const int *const *' vs 'decltype(P)' (aka 'const int *const *'))}}
   }
   template void f<&X>();
 
   template<const int *const &R> void g() {
-    using T = decltype(R);
-    using T = const int *const &;
+    using T = decltype(R);        // expected-note  {{previous definition}}
+    using T = const int *const &; // expected-error {{redefinition with different types ('const int *const &' vs 'decltype(R)' (aka 'const int *const &'))}}
   }
   template void g<(const int *const&)X>();
 }
 
 namespace FunctionConversion {
   struct a { void c(char *) noexcept; };
   template<void (a::*f)(char*)> void g() {
-    using T = decltype(f);
+    using T = decltype(f);        // expected-note  {{previous definition}}
     using T = void (a::*)(char*); // (not 'noexcept')
+    // expected-error at -1 {{redefinition with different types ('void (a::*)(char *)' vs 'decltype(f)' (aka 'void (a::*)(char *)'))}}
   }
   template void g<&a::c>();
 
   void c() noexcept;
   template<void (*p)()> void h() {
-    using T = decltype(p);
+    using T = decltype(p);// expected-note  {{previous definition}}
     using T = void (*)(); // (not 'noexcept')
+    // expected-error at -1 {{redefinition with different types ('void (*)()' vs 'decltype(p)' (aka 'void (*)()'))}}
   }
   template void h<&c>();
 }
 
 namespace VoidPtr {
   // Note, this is an extension in C++17 but valid in C++20.
   template<void *P> void f() {
-    using T = decltype(P);
-    using T = void*;
+    using T = decltype(P); // expected-note  {{previous definition}}
+    using T = void*;       // expected-error {{redefinition with different types ('void *' vs 'decltype(P)' (aka 'void *'))}}
----------------
mizvekov wrote:

Another example: https://compiler-explorer.com/z/nGbjYsbEs
```C++
template<int V> struct A {
    static int f;
};
template<int V> decltype(V) A<V>::f = 0;
```

That's not a bug, in this context, 'decltype(P)' shouldn't be desugared to int.

I don't think the quote means this is "a syntactic element" necessarily, and that even if that were true, it would imply your conclusion.

The quote is used to disambiguate whatever we are trying to quote from the rest of the prose, as in some cases we can try to refer to something that looks like english text, and that would make it hard to read. 

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