silence warning C4100 on MSVC 2019 when exceptions are disabled

[mattiasljungstrom]

Removes the following warnings under MSVC 2019 (16.10.2), with W4 enabled and when exceptions are disabled. (/W4 -D_HAS_EXCEPTIONS=0)

fmt\include\fmt\format.h(810,44): warning C4100: 'message': unreferenced formal parameter
fmt\include\fmt/format-inl.h(2553,59): warning C4100: 'message': unreferenced formal parameter

[vitaut]

This is strange because FMT_THROW always uses the argument:

fmt/include/fmt/format.h

Lines 91 to 100 in a3f762c

	# define FMT_THROW(x) detail::do_throw(x)
	# else
	# define FMT_THROW(x) throw x
	# endif
	# else
	# define FMT_THROW(x) \
	do { \
	FMT_ASSERT(false, (x).what()); \
	} while (false)
	# endif

Do you have a godbolt repro?

[mattiasljungstrom]

I believe that in release mode the FMT_ASSERT is declared ((void)0), which means that the compiler removes everything in the FMT_THROW(x) macro. (Given that exceptions are disabled.)

[vitaut]

Right. In this case it should be fixed in the FMT_ASSERT macro, not in individual call sites.

[phprus]

@vitaut, maybe replace

fmt/include/fmt/core.h

Line 364 in a3f762c

# define FMT_ASSERT(condition, message) ((void)0)

to:

FMT_INLINE void assert_ignore(bool, const char*) {}
#    define FMT_ASSERT(condition, message) ::fmt::detail::assert_ignore((condition), (message))

?

@mattiasljungstrom, please check this solution.

[mattiasljungstrom]

@vitaut makes sense!

This works for my case:

#    define FMT_ASSERT(condition, message) ((void)message) 

Or this works for MSVC (note constexpr needed), haven't tried other compilers:

FMT_INLINE constexpr void assert_ignore(bool, const char*) {}
#    define FMT_ASSERT(condition, message) ::fmt::detail::assert_ignore((condition), (message))

[mattiasljungstrom]

I've updated the PR to use the assert_ignore() solution for FMT_ASSERT.

[vitaut]

Can we use the newly added ignore_unused

fmt/include/fmt/core.h

Line 340 in 889bbf2

template <class T> void ignore_unused(const T&) {}

instead of introducing a new function?

[mattiasljungstrom]

Sure! I've changed the code to use the ::fmt::ignore_unused(), but I had to add FMT_CONSTEXPR since the assert is sometimes used in constexpr functions.

[phprus]

Maybe rewrite ignore_unused function as:

template <class ...Ts> FMT_CONSTEXPR void ignore_unused(const Ts& ...) {}

and remove do...while block?

[vitaut]

Maybe rewrite ignore_unused function as

Was about to suggest the same =)

[mattiasljungstrom]

updated code with @phprus suggestions.

[vitaut]

Thank you!