It doesn't make sense to warn about a switch case not being an enumerator if the enum type has no enumerators; such a type is being used as an opaque typedef rather than a true enumerated type. This usage makes more sense with C++11 enums with explicitly specified underlying type, but I don't think it's necessary to limit the patch to that case.
Tested x86_64-pc-linux-gnu, applying to trunk.
commit 673265c3d4cd77c7b0e927cc1547444ef869ad3a Author: Jason Merrill <ja...@redhat.com> Date: Thu May 18 22:45:24 2017 -0400 * c-warn.c (match_case_to_enum_1): Don't warn about enums with no enumerators. diff --git a/gcc/c-family/c-warn.c b/gcc/c-family/c-warn.c index e67ffb7..8971156 100644 --- a/gcc/c-family/c-warn.c +++ b/gcc/c-family/c-warn.c @@ -1069,6 +1069,10 @@ warnings_for_convert_and_check (location_t loc, tree type, tree expr, static void match_case_to_enum_1 (tree key, tree type, tree label) { + /* Avoid warning about enums that have no enumerators. */ + if (TYPE_VALUES (type) == NULL_TREE) + return; + char buf[WIDE_INT_PRINT_BUFFER_SIZE]; if (tree_fits_uhwi_p (key)) diff --git a/gcc/testsuite/g++.dg/cpp1z/byte2.C b/gcc/testsuite/g++.dg/cpp1z/byte2.C new file mode 100644 index 0000000..6a395c1 --- /dev/null +++ b/gcc/testsuite/g++.dg/cpp1z/byte2.C @@ -0,0 +1,13 @@ +// { dg-options "-std=c++17 -Wall" } + +#include <cstddef> + +bool white_space(std::byte x) { + switch (x) { + case std::byte{' '}: case std::byte{'\t'}: case std::byte{'\v'}: + case std::byte{'\f'}: case std::byte{'\n'}: + return true; + default: + return false; + } +}