[Bug c/101125] New: warn when a construct would become invalid if a function were replaced by a function-like macro

["vincent-gcc at vinc17 dot net" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101125

            Bug ID: 101125
           Summary: warn when a construct would become invalid if a
                    function were replaced by a function-like macro
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: vincent-gcc at vinc17 dot net
  Target Milestone: ---

In the standard C library and some other libraries (like GNU MPFR), functions
may be replaced by function-like macros with the same behavior. So, in order to
detect potential portability issues, GCC should provide a warning to check
whether a construct used with a function call would become invalid if the
function were replaced by a macro.

Such a difference can at least come from the use of the comma in some
expressions. Example:

struct s { int a, b; };
void f (int);
#define F(X) f(X)
void g (void)
{
  f ((struct s){0,1}.a);
  F ((struct s){0,1}.a);
}

The call to f is valid, but the use of the macro yields an error because the
preprocessor assumes that there are 2 arguments in the call: "(struct s){0" and
"1}.a". Here, one should use additional parentheses:

  F (((struct s){0,1}.a));

AFAIK, the library (which provides the function and the macro in a header file)
cannot do anything to avoid such an error; only the user can avoid it, but he
may not be aware of the issue as long as only a function is provided. Hence the
usefulness of the proposed warning.