What does ^{ <stmts..> }() mean in C?

Question

While reading one of LLVM static analyzer documents, I stumbled upon a strange operator.

^{ int y = x; }();

I know I can define a nested block inside a function like { ... }, but can we even call it? Also, I've never seen any usage placing ^ in front of a curly bracket block. I thought this is kind of a language extension supported by GCC and googled this with keywords like anonymous function or lambda but was of no avail. Is there anyone who has any idea about this?

Answer 1

From Clang 9 Documentation Language Specification for Blocks it's a Block Literal Expression. It has the form of (from wiki):

^return_type ( parameters ) { function_body }

But:

If the return type is omitted and the argument list is ( void ), the ( void ) argument list may also be omitted.

The following:

^{ int y = x; }();

is equal to:

( ^void (void) { int y = x; } )();

is equal to:

void (^f)(void) = ^void (void) { int y = x; };
f();

It declares a block literal that does int y = x and immediately after declaring executes is.