[SOLVED] Ambiguous template instantiation in GCC

Issue

Why does the code below compile with MSVC2022, but not with GCC 12?

#include <iostream>

template <auto value>
class getter;

template <class T, class ReturnType, ReturnType(T::* func_ptr)() const>
class getter<func_ptr>
{
public:

    using object_type = T;
    using value_type = ReturnType;

    constexpr ReturnType operator() (const T& val) const
    {
        return (val.*func_ptr)();
    }
};

template <class T, class Field, Field T::* field_ptr>
class getter<field_ptr>
{
public:

    using object_type = T;
    using value_type = Field;

    constexpr const Field& operator() (const T& val) const
    {
        return val.*field_ptr;
    }
};

struct A
{
    A() = default;

    explicit A(size_t k) : key(k), attribute(k + 1)
    {
    }

    size_t key;
    size_t attribute;

    size_t GetKey() const
    {
        return key;
    }

    const size_t & GetKeyRef() const
    {
        return key;
    }
};

int main()
{
    auto g1 = getter<&A::key>();
    auto g2 = getter<&A::GetKey>();

    A a(5);

    std::cout << g1(a) << " " << g2(a) << std::endl;

    return 0;
}

GCC errors:

  MyTest.cpp: In function ‘int {anonymous}::main()’:
  MyTest.cpp:66:38: error: ambiguous template instantiation for ‘class {anonymous}::getter<&{anonymous}::A::GetKey>’
     66 |         auto g2 = getter<&A::GetKey>();
        |                                      ^
  MyTest.cpp:14:11: note: candidates are: ‘template<class T, class ReturnType, ReturnType (T::* func_ptr)() const> class {anonymous}::getter<func_ptr> [with T = {anonymous}::A; ReturnType = long unsigned int; ReturnType (T::* func_ptr)() const = &{anonymous}::A::GetKey]’
     14 |     class getter<func_ptr>
        |           ^~~~~~~~~~~~~~~~
  MyTest.cpp:28:11: note:                 ‘template<class T, class Field, Field T::* field_ptr> class {anonymous}::getter<field_ptr> [with T = {anonymous}::A; Field = long unsigned int() const; Field T::* field_ptr = &{anonymous}::A::GetKey]’
     28 |     class getter<field_ptr>
        |           ^~~~~~~~~~~~~~~~~
  MyTest.cpp:66:38: error: invalid use of incomplete type ‘class {anonymous}::getter<&{anonymous}::A::GetKey>’
     66 |         auto g2 = getter<&A::GetKey>();
        |                                      ^
  MyTest.cpp:11:11: note: declaration of ‘class {anonymous}::getter<&{anonymous}::A::GetKey>’
     11 |     class getter;
        |           ^~~~~~
  MyTest.cpp: At global scope:
  MyTest.cpp:63:9: warning: ‘int {anonymous}::main()’ defined but not used [-Wunused-function]
     63 |     int main()
        |         ^~~~
  ninja: build stopped: subcommand failed.

Solution

This seems to be a gcc bug as the specialization getter<func_ptr> is more specialized than the other specialization getter<field_ptr> for &A::GetKey.

Answered By - user12002570

Answer Checked By - Dawn Plyler (WPSolving Volunteer)