https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107913
Bug ID: 107913
Summary: Bogus unused variable warning if used in if constexpr
false in lambda with default capture by ref
Product: gcc
Version: unknown
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: blubban at gmail dot com
Target Milestone: ---
template<bool a>
int b()
{
constexpr int c = 123;
constexpr int d = 456;
return [&](){
if constexpr (a)
return c;
else
return d;
}();
}
int g() { return b<true>(); }
int h() { return b<false>(); }
Compile with -Wall.
Expected result: No warnings.
Actual result:
<source>: In instantiation of 'int b() [with bool a = true]':
<source>:15:25: required from here
<source>:5:19: warning: variable 'd' set but not used
[-Wunused-but-set-variable]
5 | constexpr int d = 456;
| ^
<source>: In instantiation of 'int b() [with bool a = false]':
<source>:16:26: required from here
<source>:4:19: warning: variable 'c' set but not used
[-Wunused-but-set-variable]
4 | constexpr int c = 123;
| ^
The warning is very fragile, and goes away if (pick one or more)
- either variable is changed to not constexpr (oddly enough, both warnings
disappear if either variable is not constexpr)
- either variable is changed to static
- the if is changed to not constexpr
- the lambda is removed
- the variables are captured explicitly
- the lambda's default capture is by-value
(The bug does reproduce if the template is removed and changed to if constexpr
(true), but I feel the current form better demonstrates that the warning is
inappropriate.)
https://godbolt.org/z/T9nnzjj91
