Impossible to fully qualify class-name in out-of-class declarator of function definition

Question

This program results in an undesired parsing greediness dead-end:

struct float4x4 {};
class C
{
    float4x4 M();
};

float4x4 ::C::M()
{
    return float4x4{};
}

:8:1: error: no member named 'C' in 'float4x4'; did you mean simply 'C'?
float4x4 ::C::M()
^~~~~~~~~~~~

Which can be 'fixed' using trailing return type:

auto ::C::M() -> float4x4
{}

now all good.

So I take it we can't fully qualify the class-name when using heading-return-type declarator syntax?

Answer 1

You can put brackets to disambiguate:

float4x4 (::C::M)()
{
    return float4x4{};
}

I cannot really tell you what rule makes this ok, while it is not without the brackets, though I tested with gcc and clang (both -pedantic). I would prefer the trailing return type.