http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56566
Bug #: 56566
Summary: bogus "is narrower than values of its type" warning
Classification: Unclassified
Product: gcc
Version: unknown
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
AssignedTo: unassig...@gcc.gnu.org
ReportedBy: ppluzhni...@google.com
Test:
struct S {
enum E {
N = -1,
Z = 0
} e : 1;
};
int main() { return 0; }
Using current trunk (r196526):
gcc -c tt.c
tt.c:5:3: warning: ‘e’ is narrower than values of its type [enabled by default]
} e : 1;
^
g++ -c tt.c
tt.c:5:9: warning: ‘S::e’ is too small to hold all values of ‘enum S::E’
[enabled by default]
} e : 1;
^
7.2/7 of the C++ standard says
for an enumeration where emin is the smallest enumerator and emax is the
largest, the values of the enumeration are the values in the range bmin to
bmax, defined as follows: Let K be 1 for a two’s complement representation
and 0 for a one’s complement or sign-magnitude representation. bmax is the
smallest value greater than or equal to max(|emin| − K, |emax|) and equal to
2^M − 1, where M is a non-negative integer. bmin is zero if emin is non-
negative and −(bmax + K) otherwise. The size of the smallest bit-field large
enough to hold all the values of the enumeration type is max(M, 1) if bmin
is
zero and M + 1 otherwise.
given emin = -1, emax = 0, K = 1
then max(|emin|-K,|emax|) = max(0,0) = 0
therefore bmax = 0, M = 0, bmin = -(bmax+K) = -1, size = M+1 = 1
