https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64467
--- Comment #6 from Jonathan Wakely <redi at gcc dot gnu.org> --- Like some of the BSDs, newlib provides a _B bitmask, but that does not correspond to isblank so can't be used. The _B mask doesn't match the TAB character, but isblank must match that. The newlib isblank macro handles '\t' manually, but libstdc++ can't do that and only works with simple bitmasks, not bitmasks plus extra checks. https://gcc.gnu.org/ml/gcc-patches/2015-02/msg00059.html just adjusts the test to account for this.
