https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91388
Bug ID: 91388
Summary: -Wreturn-type "no return statement" warning in
function that is already ill-formed
Product: gcc
Version: 9.1.1
Status: UNCONFIRMED
Keywords: diagnostic
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: redi at gcc dot gnu.org
Target Milestone: ---
template<typename T> struct copyable;
template<typename T>
T func(const T& t)
{
static_assert(copyable<T>::value);
return t;
}
struct NoCopy {
NoCopy() { }
NoCopy(const NoCopy&) = delete;
};
template<> struct copyable<NoCopy> { static constexpr bool value = false; };
int main()
{
NoCopy nc;
func(nc);
}
This produces the following diagnostics:
nc.cc:20:10: required from here
nc.cc:6:30: error: static assertion failed
6 | static_assert(copyable<T>::value);
| ^~~~~
nc.cc:7:10: error: use of deleted function 'NoCopy::NoCopy(const NoCopy&)'
7 | return t;
| ^
nc.cc:12:3: note: declared here
12 | NoCopy(const NoCopy&) = delete;
| ^~~~~~
As the author of func<T> I have already added a static assertion to check if
the return statement will be well-formed, so the second error is redundant (and
I'd prefer for my users to not see it, the static assertion is for their
benefit).
So I try writing the function like this instead:
template<typename T>
T func(const T& t)
{
static_assert(copyable<T>::value);
if constexpr (copyable<T>::value)
return t;
}
Now the compiler says:
nc.cc: In instantiation of 'T func(const T&) [with T = NoCopy]':
nc.cc:22:10: required from here
nc.cc:6:30: error: static assertion failed
6 | static_assert(copyable<T>::value);
| ^~~~~
nc.cc:10:1: warning: no return statement in function returning non-void
[-Wreturn-type]
10 | }
| ^
To suppress the warning I need to do:
template<typename T>
T func(const T& t)
{
static_assert(copyable<T>::value);
if constexpr (copyable<T>::value)
return t;
__builtin_unreachable();
}
I don't think it should be necessary to mark it unreachable. The fact the
static assertion failed means it's already guaranteed to be unreachable,
because it's not even compilable!
Could the -Wreturn-type warning be suppressed when the function is already
ill-formed?
I note that clang's -Wreturn-type diagnostic is phrased "warning: control
reaches end of non-void function" and isn't given for ill-formed functions.
That makes sense because control cannot reach the end of a function that cannot
possibly execute.
