[PATCH] tighten up -Wbuiltin-declaration-mismatch (PR 86125)

[Martin Sebor]

The C implementation of the -Wbuiltin-declaration-mismatch
warning relies on TYPE_MODE to detect incompatibilities
between return and argument types in user declarations of
built-in functions.  That prevents mistakes like

   char* strlen (const char*);

from being detected (where sizeof (char*) == sizeof (size_t)),
while at the same diagnosing similar bugs such as

   char* strcmp (const char*, const char*);

where sizeof (char*) != sizeof (int), and always diagnosing
benign declarations like:

   void strcpy (char*, const char*)

The attached patch tightens up the detection of incompatible
types so that when -Wextra is set it diagnoses more of these
kinds of problems, including mismatches in qualifiers.  (I
added this under -Wextra mainly to avoid the warning with
just -Wall for some of the more benign incompatibilities
like those in const-qualifiers).

This enhancement was prompted by bug 86114.  As it is, it
would not have prevented the ICE in that bug because it does
not reject the incompatible redeclaration (I did that to keep
compatibility with prior GCC versions).  If there is support
for it, though, I think rejecting all incompatible declarations
would be a better solution.  Partly because the middle-end
doesn't seem to fully consider incompatible return types and
so might end up introducing transformations that don't make
sense.  And partly because I think at least the C and POSIX
standard built-in functions should be declared with
the types they are specified.

Martin
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