Constexpr template member function call is not taken as a constant expression

[WorldRobertProject]

    class X
    {
    public:
        constexpr X()
        {
            Foo(1);
        }
    
        template <typename T>
        constexpr void Foo(T value)
        {
        }
    };
    
    constexpr X x;

The above code produces the following compilation error:

    error: constexpr variable 'x' must be initialized by a constant expression
    | constexpr X x;
    |             ^
    note: undefined function 'Foo<int>' cannot be used in a constant expression
    |         Foo(1);
    |         ^
    note: in call to 'X()'
    | constexpr X x;
    |             ^
    note: declared here
    |     constexpr void Foo(T value)
    |                    ^

When Foo is a global function, no error occurs.

clang version: 18.1.8 (Red Hat, Inc. 18.1.8-3.el9)

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Robert (WorldRobertProject)

class X { public: constexpr X() { Foo(1); }

    template <typename T>
    constexpr void Foo(T value)
    {
    }
};

constexpr X x;

The above code produces the following compilation error:

error: constexpr variable 'x' must be initialized by a constant expression
| constexpr X x;
|             ^
note: undefined function 'Foo<int>' cannot be used in a constant expression
|         Foo(1);
|         ^
note: in call to 'X()'
| constexpr X x;
|             ^
note: declared here
|     constexpr void Foo(T value)
|                    ^

When Foo is a global function, no error occurs.

clang version: 18.1.8 (Red Hat, Inc. 18.1.8-3.el9)

[tbaederr]

This is probably #73232

[WorldRobertProject]

Thank you so much, @tbaederr!

I understand that this is a problem with the language specification itself
regarding the timing of template instantiation.

The following code is successfully compiled.

class X
{
public:
    constexpr X();

    template <typename T>
    constexpr void Foo(T value)
    {
    }
};

constexpr X::X()
{
    Foo(1);
}

constexpr X x;

I'm thinking of writing codes like this to avoid this problem.