[BUG] Contract after unnamed declaration fails

[JohelEGP]

Title: Contract after unnamed declaration fails.

Description: May be related to #356.

Minimal reproducer (https://cpp2.godbolt.org/z/5s7Kb53EG):

main: () = {
  _ = :i32 = 0;
  [[assert: true]]
}

Commands:

cppfront main.cpp2
clang++17 -std=c++23 -stdlib=libc++ -lc++abi -pedantic-errors -Wall -Wextra -Wconversion -I . main.cpp

Expected result: Same as using i32(0) instead of :i32 = 0.

Actual result and error:

<source>...

example.cpp2(3,4): error: subscript expression [ ] must not be empty (if you were trying to name a C-style array type, use 'std::array' instead) (at '[')

Compiler returned: 1

[JohelEGP]

Doesn't reproduce since commit a812dcc.