https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71843
Bug ID: 71843
Summary: [concepts] Diagnostics issued for constraint
satisfaction failure fail to elucidate unsatisfied
constraints
Product: gcc
Version: unknown
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: tom at honermann dot net
CC: andrew.n.sutton at gmail dot com, asutton at gcc dot gnu.org
Target Milestone: ---
The diagnostics emitted when overload resolution fails due to constraint
satisfaction failure do not elucidate the constraints that were not satisfied.
For non-trivial concepts, it can be difficult and tedious for a user to
determine exactly why a given type did not satisfy a set of constraints.
For example, using gcc 6.1.0:
$ cat t.cpp
template<typename T>
concept bool C = requires(T t) { t.i; };
template<C T>
auto f(T t) {
return t.i;
}
struct S {
};
auto x = f(S{});
$ g++ --version
g++ (GCC) 6.1.0
...
$ g++ -c -std=c++1z -fconcepts t.cpp
t.cpp:9:15: error: cannot call function ‘auto f(T) [with T = S]’
auto x = f(S{});
^
t.cpp:4:6: note: constraints not satisfied
auto f(T t) {
^
t.cpp:4:6: note: concept ‘C<S>’ was not satisfied
The error message clearly indicates that S does not satisfy the constraints
associated with concept C, but does not explain that the reason constraint
satisfaction failed is because the expression t.i is ill-formed for an object t
of type S.
When designing types intended to model a particular concept, it is often useful
to statically assert that the type models the intended concept. Here too the
error messages are less helpful then desired:
$ cat t2.cpp
template<typename T>
concept bool C = requires(T t) { t.i; };
struct S {
};
static_assert(C<S>);
$ g++ -c -std=c++1z -fconcepts t2.cpp
t2.cpp:5:1: error: static assertion failed
static_assert(C<S>);
^~~~~~~~~~~~~
