[llvm-bugs] [Bug 25436] New: clang -Wunknown-pragmas shouldn't warn about a '#pragma GCC diagnostic' it doesn't know

[via llvm-bugs]

https://llvm.org/bugs/show_bug.cgi?id=25436

            Bug ID: 25436
           Summary: clang -Wunknown-pragmas shouldn't warn about a
                    '#pragma GCC diagnostic' it doesn't know
           Product: new-bugs
           Version: trunk
          Hardware: Macintosh
                OS: MacOS X
            Status: NEW
          Severity: normal
          Priority: P
         Component: new bugs
          Assignee: unassignedbugs at nondot.org
          Reporter: sean at rogue-research.com
                CC: llvm-bugs at lists.llvm.org
    Classification: Unclassified

If clang r252202 encounters this:

#pragma GCC diagnostic ignored "-Wlarger-than="                   

it warns:

test.c:1:32: warning: unknown warning group '-Wlarger-than=', ignored
[-Wunknown-pragmas]
#pragma GCC diagnostic ignored "-Wlarger-than="                   
                               ^

GCC has a -Wlarger-than= warning, clang does not (itself bug #10623).

I realize I'm sorta asking for such warnings by specifying -Wunknown-pragmas,
but I think this is a bit different.  Ordinarily, I would guard
complier/platform-specific pragmas with something like:

#if __GNUC__

but that won't help because clang (incompletely) pretends to be GCC.  (And
-Wlarger-than= is in gcc 4.2, so adding version checks won't help.)

I guess the only workaround for portable code is something like:

#if __GNUC__ && !__clang__

Basically, I see little value in -Wunknown-pragmas warning that clang's GCC
emulation is not 100%.  Perhaps a special case for "#pragma GCC"?  Or whitelist
known GCC warning flags that clang doesn't support?

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