[Bug preprocessor/93677] New: Create a warning for duplicate macro definition

[vincent-gcc at vinc17 dot net]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93677

            Bug ID: 93677
           Summary: Create a warning for duplicate macro definition
           Product: gcc
           Version: 10.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: preprocessor
          Assignee: unassigned at gcc dot gnu.org
          Reporter: vincent-gcc at vinc17 dot net
  Target Milestone: ---

There should be a warning for duplicate macro definition (i.e. with the same
value), such as

#define FOO 1
#define FOO 1

Such code is valid, but a duplicate macro definition may occur inadvertently in
the code, and it does not ease code maintenance and readability. For instance,
one definition may occur in some header file, and another one in a .c file for
some completely different meaning, while the value in both definitions is the
same only by chance. Such code introduces confusion and makes code evolution
more error prone: if for some reason one of the values needs to be changed, the
macro name needs to be changed too (this is what should have been done earlier,
but it would have been better to detect the issue as soon as the duplicate
macro definition was added).

For function-like macros, issues may also occur when one wants to change the
implementation, keeping the same behavior: the change in one of the definitions
becomes invalid. Thus it is better to define such macros in only one place, and
the warning would detect a failure to do that.

The switch could be combined with the request in PR83773 (which is about macro
redefinition with a different value).