https://gcc.gnu.org/bugzilla/show_bug.cgi?id=92232
Bug ID: 92232
Summary: [C++17] Unable to deduce template function parameter
Product: gcc
Version: unknown
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: daniel at hebirobotics dot com
Target Milestone: ---
When trying to use a function pointer (which has a non-void return type and one
or more parameters) as a template argument parameter - whose return type is
specified as `auto` and one of its arguments is dependent on another template
typename - GCC fails to deduce the function template parameter.
A simplified use case is as followed:
---
enum MyEnumT {};
struct MyStructDecl;
typedef MyStructDecl* MyStructDeclPtr;
MyEnumT myFunc(MyStructDeclPtr, int someArg);
template<typename FirstArgT, auto(*functor)(FirstArgT, int)>
void deducer(FirstArgT arg, int i) {
auto ret = functor(arg, i);
// Something
}
int main() {
deducer<MyStructDeclPtr, myFunc>(nullptr, 12);
}
---
Some interesting and potentially relevant notes:
* If you change the problematic template function parameter
(`auto(*functor)(FirstArgT, int)`) to have a `MyEnumT` return type
(`MyEnumT(*functor)(FirstArgT, int)`), the code will compile
* If you change the problematic template function parameter to accept a
non-template typename as the first parameter (`auto(*functor)(MyStructDeclPtr,
int)`), the code will compile.
I've seen this as an issue on 9.1 and 9.2, but I assume it to be an issue for
any version which can utilize C++17. This code compiles without any issues on
both Clang and MSVC ( https://gcc.godbolt.org/z/EIcOJc ).
