https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101677
Bug ID: 101677
Summary: [11 Regression] Concept with use of incomplete type
succeeds on GCC 10.3.0, fails on GCC 11 onward
Product: gcc
Version: 11.1.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: joeloser93 at gmail dot com
Target Milestone: ---
A concept that uses an incomplete type in testing the well-formedness of
calling a function template is well-formed in GCC 10.3.0, but fails in GCC 11
onward (including trunk at the time of this writing).
For example, consider the following:
template<class T>
concept C_bug_with_forward_decl = requires(T& t){
t.template f<class S>();
};
struct good {
template<class T> auto f() -> void {}
};
static_assert(C_bug_with_forward_decl<good>); // works in GCC 10.3.0, fails on
GCC 11 onward
This bug can be worked around by using a complete type instead when defining
the concept. For example:
struct U{};
template<class T>
concept C_workaround = requires(T& t){
t.template f<U>();
};
static_assert(C_workaround<good>); // works on GCC 10.3.0 and GCC 11 onward
See it live at https://godbolt.org/z/h7GzqGhYn
