Jonathan pointed me at this issue where
constexpr unsigned f() { constexpr int n = -1; return unsigned{n}; }
is accepted in system headers, despite the narrowing conversion from
a constant. I suspect that whereas narrowing warnings should be
disabled, ill-formed narrowing of constants should be a hard error
(which can still be disabled by -Wno-narrowing).
gcc/cp/ChangeLog:
* typeck2.c (check_narrowing): Don't suppress the pedantic error
in system headers.
libstdc++-v3/ChangeLog:
* testsuite/20_util/ratio/operations/ops_overflow_neg.cc: Add
dg-error.
gcc/testsuite/ChangeLog:
* g++.dg/cpp1y/Wnarrowing2.C: New test.
* g++.dg/cpp1y/Wnarrowing2.h: New test.
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