https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96675
Bug ID: 96675
Summary: tautological-compare warning emitted for NTTP bitwise
comparison
Product: gcc
Version: 10.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: ed at catmur dot uk
Target Milestone: ---
template<char c>
constexpr bool f(char d) {
return 'a' <= c && c <= 'z' ? (d | 0x20) == c :
'A' <= c && c <= 'Z' ? (d & ~0x20) == c :
d == c;
}
static_assert(f<'p'>('P'));
In instantiation of 'constexpr bool f(char) [with char c = 'p']':
<source>:7:21: required from here
<source>:4:44: warning: bitwise comparison always evaluates to false
[-Wtautological-compare]
4 | 'A' <= c && c <= 'Z' ? (d & ~0x20) == c :
| ~~~~~~~~~~~~^~~~
Per godbolt, this happened somewhere between 10.1.0 and 10.2.0.
Motivation is
https://github.com/capnproto/capnproto/commit/c38629e5a4b8ce9561b81cb23ea5fc39cbd93eb7
