https://gcc.gnu.org/bugzilla/show_bug.cgi?id=87629
Bug ID: 87629
Summary: function template parameter deduction succeeds but
parameter and deduced arg does not match.
Product: gcc
Version: 8.2.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: okannen at gmail dot com
Target Milestone: ---
This bug is present in all gcc versions.
Consider the following code:
template<class...Args>
struct x{
x(void(*)(int));
};
void foo(int);
template<class T>
void Tfoo(T);
template<class...Args>
void x_func(x<Args...> a);
void test(){
x_func(foo); //does not compile => standard compliant
x_func(Tfoo); //compile! template deduction + conversion => not standard
compliant
}
`x_func(Tfoo)` compiles, but this violate this standard rule
[\[temp.deduct.call\]/4](http://eel.is/c++draft/temp.deduct#call-4) which
states that the deduced function argument type shall match (almost) the type of
the argument:
In general, the deduction process attempts to find template argument values
that will make the deduced A identical to A (after the type A is transformed as
described above).
However, there are three cases that allow a difference:
> - If the original P is a reference type, the deduced A (i.e., the type
> referred to by the reference) can be more cv-qualified than the transformed A.
> - The transformed A can be another pointer or pointer-to-member type that
> can be converted to the deduced A via a function pointer conversion and/or
> qualification conversion.
> - If P is a class and P has the form simple-template-id, then the
> transformed A can be a derived class of the deduced A. Likewise, if P is a
> pointer to a class of the form simple-template-id, the transformed A can be a
> pointer to a derived class pointed to by the deduced A.
See also this stackoverflow
question:https://stackoverflow.com/q/52845621/5632316
