https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110752
Bug ID: 110752
Summary: decltype in lambda loses const qualifier unless lambda
is mutable
Product: gcc
Version: 13.1.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: jzwinck at gmail dot com
Target Milestone: ---
I believe this code should compile (and with MSVC it does), but with GCC it
does not:
template<typename Arg>
auto & C(Arg && arg) { return arg; }
void DecltypeWithinLambda()
{
int i = 0;
[[maybe_unused]] auto l = [i]() // mutable
{
// does not preserve constness of 'i' within non-mutable lambda
// Seems to be evaluating it as if referencing the outer 'i'
// Though, decltype((i)) does do the right thing.
// alternative solution to mutable is to 'rename' the captured
variable list in the capture spec
using T = decltype(C(i));
[[maybe_unused]] T t(C(i)); // actual use of C() only matches
decltype if mutable
};
}
GCC says the last line is invalid:
error: binding reference of type 'T' {aka 'int&'} to 'const int' discards
qualifiers
| [[maybe_unused]] T t(C(i)); // actual use of C() only matches decltype if
mutable
| ~^~~
If the lambda is marked mutable, it compiles as expected. So does this, which
seems like it should be equivalent to the above:
struct F
{
void DecltypeWithinConstMethod() const
{
using T = decltype(C(i));
[[maybe_unused]] T t(C(i)); // actual use of C() matches decltype
}
int i;
};
Demo: https://gcc.godbolt.org/z/PzEodY1bW
A somewhat similar bug, but not the same:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63192
