https://gcc.gnu.org/bugzilla/show_bug.cgi?id=92477
Bug ID: 92477
Summary: [[nodiscard]] method in a decltype expression causes
"warning: ignoring return value of"
Product: gcc
Version: 9.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: src at andyf dot de
Target Milestone: ---
Hello,
consider this code:
class Test
{
public:
[[nodiscard]] int Get() const { return 42; }
};
template<typename T>
auto UsedForSFINAE(const T& t) -> decltype(t.Get(), void())
{
}
template<typename T>
auto UsedAsReturnValue(const T& t) -> decltype(t.Get())
{
return t.Get();
}
int main()
{
Test t{};
UsedForSFINAE(t);
UsedAsReturnValue(t);
}
It tries to SFINAE out the UsedForSFINAE based on whether the type passed has a
method Get. The decltype expression in
UsedForSFINAE causes a warning in GCC with -Wall passed:
warning: ignoring return value of 'int Test::Get() const', declared with
attribute nodiscard [-Wunused-result]
8 | auto UsedForSFINAE(const T& t) -> decltype(t.Get(), void())
<source>:4:23: note: declared here
4 | [[nodiscard]] int Get() const { return 42; }
| ^~~
Clang on the other hand is fine with it (see https://godbolt.org/z/9U4ceB). The
warning goes away when t.Get() is casted to void. There is no warning, if the
decltype's resulting type is used as the return type (as for
UsedAsReturnValue).
I talked to Richard Smith about this last week at WG21 Belfast and he told me
that Clang is correct. Unfortunately, I don't recall his exact rational. I
think it was that in [dcl.type.decltype] is says "The operand of the decltype
specifier is an unevaluated operand (7.2)."
Bug 57857 looks a bit similar.
Cheers,
Andreas
