https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95116
Bug ID: 95116
Summary: [C++ 20] Accepts invalid code with decltype dependent
type
Product: gcc
Version: 9.3.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: ojman101 at protonmail dot com
Target Milestone: ---
The code below is invalid. The keyword "typename" needs to be inserted before
the decltype expression as "bar" is a dependent type (dependent on template
type T).
template <typename T>
struct Bar {
using bar_type = int;
};
template <typename T>
struct Foo {
Bar<T> bar;
using foo_type = decltype(bar)::bar_type;
};
int main() {
// Instantiate Foo template
Foo<int> foo;
}
The code successfully compiles with GCC 9.3.0 when running the command "g++
-std=c++2a main.cpp". Note that the "-std=c++2a" must be passed. When passing
"-std=c++17", GCC successfully detects the error and prints:
main.cpp:10:22: error: need 'typename' before 'decltype
(((Foo<T>*)(void)0)->Foo<T>::bar)::bar_type' because 'decltype
(((Foo<T>*)(void)0)->Foo<T>::bar)' is a dependent scope
10 | using foo_type = decltype(bar)::bar_type;
| ^~~~~~~~
| typename
Running clang 10 with the command "clang++ -std=c++2a main.cpp" also
successfully detects the error and prints:
main.cpp:10:22: error: missing 'typename' prior to dependent type name
'decltype(bar)::bar_type'
using foo_type = decltype(bar)::bar_type;
^~~~~~~~~~~~~~~~~~~~~~~
typename
1 error generated.
