[llvm-bugs] [Bug 36413] New: MSVC: implement /Zc:externConstexpr- to make constexpr globals internal by default

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=36413

            Bug ID: 36413
           Summary: MSVC: implement /Zc:externConstexpr- to make constexpr
                    globals internal by default
           Product: clang
           Version: unspecified
          Hardware: PC
                OS: Windows NT
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: LLVM Codegen
          Assignee: unassignedclangbugs at nondot.org
          Reporter: rnk at google.com
                CC: llvm-bugs at lists.llvm.org

MSVC has a bug where some constexpr globals are given internal (static) linkage
when the standard requires external linkage. Here is an example:

$ cat t.cpp
extern int some_int;
constexpr const int &int_ref = some_int;
int useit() { return int_ref; }

$ cl -nologo -c t.cpp  && dumpbin -symbols t.obj | grep int_ref
t.cpp
009 00000000 SECT3  notype       Static       | ?int_ref@@3AEBHEB (int const &
const int_ref)

$ clang-cl -c t.cpp  && dumpbin -symbols t.obj | grep int_ref
00C 00000000 SECT4  notype       External     | ?int_ref@@3AEBHEB (int const &
const int_ref)

See 'Static' vs. 'External'.

This lead to link errors when using certain parts of the 10.0.16299.0 Windows
SDK in Chromium, originally reported here: https://crbug.com/780056 See the
comments relating to MIDL_CONST_ID and windows.ui.viewmanagement.h.

MIDL appears to contain (or generate) this macro:

#pragma push_macro("MIDL_CONST_ID")
#if !defined(_MSC_VER) || (_MSC_VER >= 1910)
#define MIDL_CONST_ID constexpr const
#else
#define MIDL_CONST_ID const __declspec(selectany)
#endif

It seems that the authors expected 'constexpr' to imply __declspec(selectany),
which is odd, because MSVC doesn't do that at all, it just makes the thing
static, not comdat.

MSVC has a flag that disables this behavior, /Zc:externConstexpr, but I have to
update VS before I can experiment with it.

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