[BUG] fix(parse): diagnose function expression without initializer

[JohelEGP]

Title: fix(parse): diagnose function expression without initializer

Description:

I tried writing a function type,
but accidentally put a : in front.

Minimal reproducer (https://cpp2.godbolt.org/z/fdfvs9o1a):

require_fn: type == std::function<:(copy b: bool, copy s: std::string) -> void>;
main: () = { }

Commands:

cppfront main.cpp2
clang++-18 -std=c++26 -stdlib=libc++ -lc++abi -pedantic-errors -Wall -Wextra -Wconversion -Werror=unused-result -Werror=unused-value -Werror=unused-parameter -Werror=unused-variable -I . main.cpp

Expected result: A diagnostic to drop the : or add an = initializer.

Actual result and error:

using require_fn = std::function<>;

Cpp2 lowered to Cpp1:

//=== Cpp2 type declarations ====================================================


#include "cpp2util.h"



//=== Cpp2 type definitions and function declarations ===========================

using require_fn = std::function<>;
auto main() -> int;

//=== Cpp2 function definitions =================================================


auto main() -> int{}

Output:

//=== Cpp2 type declarations ====================================================


#include "cpp2util.h"



//=== Cpp2 type definitions and function declarations ===========================

using require_fn = std::function<>;
auto main() -> int;

//=== Cpp2 function definitions =================================================


auto main() -> int{}