https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89997
--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
In GCC 10+ (with -fconcepts-diagnostics-depth=2), GCC produces:
<source>: In function 'void test()':
<source>:17:16: error: use of function 'void check() requires requires(X x, T
val) {x.X::operator<<()("hello") << val;} [with T = int]' with unsatisfied
constraints
17 | check<int>(); // mangled error
| ^
<source>:13:6: note: declared here
13 | void check() requires requires (X x, T val) { x << "hello" << val; } {}
| ^~~~~
<source>:13:6: note: constraints not satisfied
<source>: In instantiation of 'void check() requires requires(X x, T val)
{x.X::operator<<()("hello") << val;} [with T = int]':
<source>:17:16: required from here
<source>:13:6: required by the constraints of 'template<class T> void check()
requires requires(X x, T val) {x.X::operator<<()("hello") << val;}'
<source>:13:23: in requirements with 'X x', 'T val' [with T = int]
<source>:13:60: note: the required expression '("hello"->x.X::operator<<() <<
val)' is invalid, because
13 | void check() requires requires (X x, T val) { x << "hello" << val; } {}
| ~~~~~~~~~~~~~^~~~~~
<source>:13:63: error: invalid conversion from 'int' to 'void*' [-fpermissive]
13 | void check() requires requires (X x, T val) { x << "hello" << val; } {}
| ^~~
| |
| int
<source>:9:19: note: initializing argument 1 of 'X Y::operator<<(void*)'
9 | X operator<< (void*);
| ^~~~~
Is this good enoug now?
Note clang produces:
<source>:17:5: error: no matching function for call to 'check'
check<int>(); // mangled error
^~~~~~~~~~
<source>:13:6: note: candidate template ignored: constraints not satisfied
[with T = int]
void check() requires requires (X x, T val) { x << "hello" << val; } {}
^
<source>:13:60: note: because 'x << "hello" << val' would be invalid: invalid
operands to binary expression ('Y' and 'int')
void check() requires requires (X x, T val) { x << "hello" << val; } {}
^
