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

[Aaron Ballman 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 *'))}}
----------------
AaronBallman wrote:

I'm aware of this being preexisting, but that's why I'm pushing back -- that's been the reason to not address the issue previously. We cannot keep kicking the can down the road on this because the diagnostic behavior is too unreasonable. No other implementation implements these DRs, so there's not really anything on fire justifying why we need to land this in a state where the diagnostics are incomprehensible to users.

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