Defining/undefining builtin macros

[llvmbot]

Bugzilla Link	3943
Version	unspecified
OS	NetBSD
Reporter	LLVM Bugzilla Contributor

Extended Description

The standard forbids this, but it is not a constraint. Clang diagnoses some but not others, e.g.

#undef __LINE__   // diagnosed
#undef __STDC_HOSTED__ // undiagnosed

[llvmbot]

#undef STDC // undiagnosed too

[llvmbot]

#undef STDC // undiagnosed too

[arsenm]

Currently a warning:

/tmp/macro.c:1:8: warning: undefining builtin macro [-Wbuiltin-macro-redefined]
    1 | #undef __LINE__   // diagnosed
      |        ^
/tmp/macro.c:2:8: warning: undefining builtin macro [-Wbuiltin-macro-redefined]
    2 | #undef __STDC_HOSTED__ // undiagnosed
      |        ^
/tmp/macro.c:3:8: warning: undefining builtin macro [-Wbuiltin-macro-redefined]
    3 | #undef __STDC__ // undiagnosed