C2X adds digit separators, as in C++. Enable them accordingly in
libcpp and c-lex.c. Some basic tests are added that digit separators
behave as expected for C2X and are properly disabled for C11; further
test coverage is included in the existing g++.dg/cpp1y/digit-sep*.C
tests.
Bootstrapped with no regressions for x86_64-pc-linux-gnu.
gcc/c-family/
* c-lex.c (interpret_float): Handle digit separators for C2X.
libcpp/
* init.c (lang_defaults): Enable digit separators for GNUC2X and
STDC2X.
gcc/testsuite/
* gcc.dg/c11-digit-separators-1.c,
gcc.dg/c2x-digit-separators-1.c, gcc.dg/c2x-digit-separators-2.c:
New tests.
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