https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90080
Bug ID: 90080
Summary: SFINAE failure with static_cast
Product: gcc
Version: 8.3.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: alex at grundis dot de
Target Milestone: ---
Using `std::enable_if` with a trait checking for `static_castable` fails hard
instead of SFINAE and usage of the alternative signature.
Code (https://godbolt.org/z/2uJc3P):
template<typename T, typename U, typename = U>
struct is_static_castable : std::false_type
{};
template<typename T, typename U>
struct is_static_castable<T, U, decltype(static_cast<U>(std::declval<T>()))> :
std::true_type
{};
template<typename To, typename From, std::enable_if_t<is_static_castable<From*,
To*>::value, int> = 0>
auto safePtrCast(From* from)
{
return static_cast<To*>(from);
}
template<typename To, typename From,
std::enable_if_t<!is_static_castable<From*, To*>::value, int> = 0>
To* safePtrCast(From* from)
{
return dynamic_cast<To*>(from);
}
struct BarBase{ virtual ~BarBase() = default;};
struct Bar : virtual BarBase{};
void foo(){
BarBase* b;
Bar* b2 = safePtrCast<Bar>(b);
}
Error:
source>: In instantiation of 'struct is_static_castable<BarBase*, Bar*, Bar*>':
<source>:17:55: required by substitution of 'template<class To, class From,
typename std::enable_if<(! is_static_castable<From*, To*>::value), int>::type
<anonymous> > To* safePtrCast(From*) [with To = Bar; From = BarBase; typename
std::enable_if<(! is_static_castable<From*, To*>::value), int>::type
<anonymous> = <missing>]'
<source>:28:33: required from here
<source>:8:42: error: cannot convert from pointer to base class 'BarBase' to
pointer to derived class 'Bar' because the base is virtual
struct is_static_castable<T, U, decltype(static_cast<U>(std::declval<T>()))> :
std::true_type
This looks like a regression as a similar error is described here
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=44267
