Error: func(const(A) a) is not callable using argument types (const(A)

[Andrew Edwards via Digitalmars-d-learn]

What does that even mean?

Scenario:
bool func(const ImVec2 label_size)
{
    return true;
}

void main()
{
    //first attempt:
    const ImVec2 label_size = CalcTextSize(label.ptr, null, true);
    //Error: cannot implicitly convert expression 
(CalcTextSize(cast(immutable(char)*)label, null, true, -1F)) of 
type ImVec2 to const(ImVec2)

    //second attempt
    const ImVec2 label_size = cast(const)CalcTextSize(label.ptr, 
null, true);
    // Error: cannot implicitly convert expression 
(CalcTextSize(cast(immutable(char)*)label, null, true, -1F)) of 
type const(ImVec2) to const(ImVec2)

    //third attempt
    const auto label_size = CalcTextSize(label.ptr, null, true);
    //Okay: don't know why the other two didn't work but I can 
keep going for now

    func(label_size);
    //Error: function imgui_d.func (const(ImVec2) label_size) is 
not callable using argument types (const(ImVec2))
}