I’ve come across a strange syntax error when I made a call to a templated function in a struct that use a concept.

main.h

struct A {
    template<typename T>
    T get();
};

struct B : A {
    template<typename T>
    T get();
};

template<> int A::get() {
    return 1;
}

template<> float A::get() {
    return 1.0;
}

template<> int B::get() {
    return 1;
}

template<> float B::get() {
    return 1.0;
}

main.cpp

#include "main.h"
#include <iostream>

template <typename T> 
concept DerivedFromA = std::is_base_of<A, T>::value;

struct MyStruct1 {
    B _b;

    MyStruct1(B b) { _b = b; }

    int test() {
        // Everything fine here !
        return _b.get<int>();
    }
};

// Note the use of concept 
template<DerivedFromA D>
struct MyStruct2 {

    D _d;

    MyStruct2(D d) { _d = d; }

    int test() {
        // Here Syntax error !
        return _d.get<int>();
    }
};

int main() {

    B b;

    // // Here OK !
    // std::cout << b.get<int>() << std::endl;

    MyStruct1 myStruct1(b);
    auto result1 = myStruct1.test();

    MyStruct2 myStruct2(b);
    auto result2 = myStruct2.test();

    return 0;
}

This code produce the following syntax errors:

• error: expected primary-expression before ‘int’ return _d.get<int>();
• error: expected ‘;’ before ‘int’

Only, and only on the struct that is templated by a concept. Is there any reason for that ?