Extended Description

My diagnosis is:
template struct basic_string {
static T _S_empty_rep_storage[]; // #​1
};
template T basic_string::_S_empty_rep_storage[sizeof(T)]; // #​2

#​1 and #​2 end up on the same redecl chain. #​2 gets registered in the lookup table and when isSameEntity compares the type of the imported VarDecl (essentially #​1 coming from another module) it finds a mismatch.

A fix to this issue is just dialing Existing->getCanonicalDecl. There are two questions I cannot answer (yet?):

1. Should #​2 really end up in the redecl chain, it looks like a different decl (at least when it comes to types, true that both are IncompleteArrayTypes]?
2. Why #​2 ended up in the lookup table when #​1 was seen before?
   3. Where should the fix go:
   a) should it be isSameEntity(Existing->getCanonicalDecl(), D) - semantically #​2 cannot end up as a canonical decl.
   b) should I extend the logic (somehow) of isSameEntity (VarDecl section)?

2872 DeclContext::lookup_result R = MergeDC->noload_lookup(Name);
2873 for (DeclContext::lookup_iterator I = R.begin(), E = R.end(); I != E; ++I) {
2874 if (NamedDecl *Existing = getDeclForMerging(*I, TypedefNameForLinkage))
-> 2875 if (isSameEntity(Existing, D))
2876 return FindExistingResult(Reader, D, Existing, AnonymousDeclNumber,
2877 TypedefNameForLinkage);
2878 }
(lldb) p Existing
(clang::VarDecl *) $186 = 0x000000010c831390
(lldb) p Existing->dump()
VarDecl 0x10c831390 parent 0x10c051530 prev 0x10c830fa8 </Users/vvassilev/workspace/llvm-git/src/tools/clang/test/Modules/Inputs/PRN/basic_string.h:9:1, line:10:50> col:20 _S_empty_rep_storage 'T [sizeof(T)]'
(lldb) p Existing->getPreviousDecl()
(clang::VarDecl *) $187 = 0x000000010c830fa8
(lldb) p Existing->getPreviousDecl()->dump()
VarDecl 0x10c830fa8 </Users/vvassilev/workspace/llvm-git/src/tools/clang/test/Modules/Inputs/PRN/basic_string.h:6:3, col:33> col:12 _S_empty_rep_storage 'T []' static
(lldb) p Existing->getPreviousDecl()->getPreviousDecl()
(clang::Decl *) $188 = 0x0000000000000000
(lldb) p Existing->getMostRecentDecl()
(clang::VarDecl *) $189 = 0x000000010c831390
(lldb) p Existing->getCanonicalDecl()
(clang::VarDecl *) $190 = 0x000000010c830fa8
(lldb) p Existing->getCanonicalDecl()->dump()
VarDecl 0x10c830fa8 </Users/vvassilev/workspace/llvm-git/src/tools/clang/test/Modules/Inputs/PRN/basic_string.h:6:3, col:33> col:12 _S_empty_rep_storage 'T []' static

The error message is:

In module 'B' imported from /Users/vvassilev/workspace/llvm-git/src/tools/clang/test/Modules/Inputs/PRN/A.h:5:
/Users/vvassilev/workspace/llvm-git/src/tools/clang/test/Modules/Inputs/PRN/basic_string.h:6:12: error: 'basic_string::_S_empty_rep_storage' from module 'B' is not present in definition of 'basic_string' provided earlier
static T _S_empty_rep_storage[];
^
/Users/vvassilev/workspace/llvm-git/src/tools/clang/test/Modules/Inputs/PRN/basic_string.h:6:12: note: declaration of '_S_empty_rep_storage' does not match
static T _S_empty_rep_storage[];
^
/Users/vvassilev/workspace/llvm-git/src/tools/clang/test/Modules/prN.cpp:5:10: fatal error: could not build module 'A'
#include "A.h"

2 errors generated.