C89 allowed a type specifier to be elided with the resulting type being
int, aka implicit int behavior. This feature was subsequently removed
in C99 without a deprecation period, so implementations continued to
support the feature. Now, as with implicit function declarations, is a
good time to reevaluate the need for this support.
This patch allows -Wimplicit-int to issue warnings in C89 mode (off by
default), defaults the warning to an error in C99 through C17, and
disables support for the feature entirely in C2x. It also removes a
warning about missing declaration specifiers that really was just an
implicit int warning in disguise and other minor related cleanups.
As a follow-up to D95691, add new diagnostic groups named
pre-c++N-compat to replace the old diagnostic groups with the standards
listed out explicitly. The old group names are retained for backwards
compatibility.
Our diagnostics relating to static assertions were a bit confused. For
instance, when in MS compatibility mode in C (where we accept
static_assert even without including <assert.h>), we would fail
to warn the user that they were using the wrong spelling (even in
pedantic mode), we were missing a compatibility warning about using
_Static_assert in earlier standards modes, diagnostics for the optional
message were not reflected in C as they were in C++, etc.
Aaron Ballman