fix module test odr violations

[cdacamar]

As of 16.11 Preview 3 the MSVC compiler will no longer be, erroneously, be injecting header include guard macros when importing a module interface. The bugfix exposed some issues related to textually including fmt headers in the test code which causes odr violations with the imported interface.

[DanielaE]

@cdacamar
I'm a bit puzzled regarding the proposed changes. I've upgraded from pre-2 (19.29.30130) to pre-3 (19.29.30132) and can confirm that the include guard macros are no longer injected into the importing TU. To codify this improvement, I changed module-test.cc from

#define FMT_OS_H_  // don't pull in os.h directly or indirectly

import fmt;

// check for macros leaking from BMI
static bool macro_leaked =
#if defined(FMT_CORE_H_) || defined(FMT_FORMAT_H_)
    true;
#else
    false;
#endif

to

#define FMT_OS_H_  // don't pull in os.h directly or indirectly

import fmt;

// check for macros leaking from BMI
static bool macro_leaked =
#if defined(FMT_CORE_H_) || defined(FMT_FORMAT_H_) || !defined(FMT_OS_H_)
    true;
#else
    false;
#endif

Beginning with pre-3, include guard macros no longer become #defined nor #undefed by importing a named module.
By defining the include guard for fmt/os.h, no header becomes #included from {fmt} by the code following the import. Every required definition is rather imported from the module, e.g. no more ODR violations. I've checked that explicitly with both dumping the preprocessed code (/P) and applying my fake consteval emulation.

The only other change to compile module-test.cc successfully with pre-3 is in line 80 where the visibility of the non-exported namespace fmt::detail is checked for. The compiler version needs to be bumped up to 192930132.

No changes to other files were necessary.

[cdacamar]

@DanielaE while your method works it requires knowing the implementation detail of the header include guards by name (which could change in the future). The proposed solution here relies only on the fact that we understand the contents of fmtlib comes from the module alone so we can forget about the mechanics of #include altogether and simply import the module which provides the library.

[vitaut]

Thanks for the fix!