https://gcc.gnu.org/bugzilla/show_bug.cgi?id=57170
--- Comment #6 from Jonathan Wakely <redi at gcc dot gnu.org> ---
It should be an error, not a warning. For other narrowing conversions involving
constants we do actually use -Wnarrowing (but still defaulting to an error) so
I guess my last comment was wrong.
Or it could just be an unconditional error. Or a permerror, so -fpermissive
would allow it.
The code that handles this is in gcc/c-family/c-common.c so isn't C++-specific.
I'm not sure if that code has enough context to know if the conversion happens
in a constant expression where narrowing conversions are disallowed.
#0 unsafe_conversion_p (loc=<optimized out>, loc@entry=267203,
type=type@entry=0x7ffff013c690, expr=expr@entry=0x7ffff01401f8,
result=result@entry=0x7ffff0140048, produce_warns=produce_warns@entry=true) at
/home/jwakely/src/gcc/gcc/gcc/c-family/c-common.c:1261
#1 0x00000000008cdb27 in conversion_warning (loc=267203,
type=type@entry=0x7ffff013c690, expr=expr@entry=0x7ffff01401f8,
result=result@entry=0x7ffff0140048) at
/home/jwakely/src/gcc/gcc/gcc/c-family/c-warn.c:1127
#2 0x00000000008d11d1 in warnings_for_convert_and_check (loc=<optimized out>,
loc@entry=267203, type=type@entry=0x7ffff013c690,
expr=expr@entry=0x7ffff01401f8, result=result@entry=0x7ffff0140048) at
/home/jwakely/src/gcc/gcc/gcc/c-family/c-warn.c:1278
#3 0x000000000085df41 in convert_and_check (loc=loc@entry=267203,
type=type@entry=0x7ffff013c690, expr=0x7ffff01401f8) at
/home/jwakely/src/gcc/gcc/gcc/c-family/c-common.c:1548
#4 0x0000000000886a55 in c_add_case_label (loc=loc@entry=267203,
cases=0x24ad5e0, cond=cond@entry=0x7ffff7ff6cf0,
orig_type=orig_type@entry=0x7ffff013c690, low_value=<optimized out>,
low_value@entry=0x7ffff01401f8, high_value=<optimized out>,
high_value@entry=0x0, outside_range_p=0x24abff0)
at /home/jwakely/src/gcc/gcc/gcc/c-family/c-common.c:4780
#5 0x000000000066b1fa in finish_case_label (loc=267203, low_value=<optimized
out>, low_value@entry=0x7ffff01401f8, high_value=<optimized out>) at
/home/jwakely/src/gcc/gcc/gcc/cp/decl.c:3669
#6 0x00000000007309e9 in cp_parser_label_for_labeled_statement
(parser=parser@entry=0x7ffff7ff6ab0, attributes=attributes@entry=0x0) at
/home/jwakely/src/gcc/gcc/gcc/cp/parser.c:11072
