Which GCC feature compiles (const T){}.f();?

Question

In my attempt to learn C++'s const correctness in combination with move semantics, I ended up with the following code.

MRE:

#include <iostream>
class ThisReference
{
    public:
    void f() const && { std::cout << "f() const &&\n"; }
};

int main()
{
    (const ThisReference){}.f();
}

Godbolt [a little bit longer with all method candidates]

That code compiles with GCC and I thought I understood what it means, i.e.:

1. (const ThisReference) defines the type. I wanted it to be const, so I put that into parens.
2. {} does the initialization of that type; creates an object
3. .f() calls the method
4. ; ends the statement

Eventually I put that code into Visual Studio and it didn't compile. The error message is

error C4576: a parenthesized type followed by an initializer list is a non-standard explicit type conversion syntax

I have 2 questions about my code.

1. Which non-standard GCC feature am I using here and how would I turn it off?
2. Something in my thinking is probably wrong. What is wrong?

Answer 1

(const ThisReference) {}

This is a compound literal, and it's a C99 feature, not a C++ feature.

GCC compiles this code because it supports compound literals as a non-standard extension. You can disallow this by adding -Wpedantic -Werror to your compiler flags, or just -Werror=pedantic:

<source>: In function 'int main()':
<source>:10:27: error: ISO C++ forbids compound-literals [-Werror=pedantic]
   10 |     (const ThisReference){}.f();
      | 

See live demo

The reason why MSVC (Microsoft Visual Studio Compiler) gives you that error message is that it thinks {} is list initialization, and (const ThisReference) is a C-style cast, and these two can't appear together in C++ normally.

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See also: Are compound literals Standard C++?