[clang-format] auto variable initialized with arrow operator can be seen as trailing return type

[rymiel]

Input code:

auto a = (b)->c;

Result from clang-format-17 (main), with default style ({}):

auto a = (b) -> c;

The arrow is annotated as a trailing return type arrow due to an imperfect heuristic in determineTokenType.

This also applies to decltype(auto):

decltype(auto) a = (b) -> c;

This issue isn't encountered without the (redundant) parentheses:

auto a = b->c;

Originally discovered in https://reviews.llvm.org/D141811; I planned to make a fix for it there, but it turned out harder than I expected, so I'm adding an issue for it so I don't forget.

[llvmbot]

@llvm/issue-subscribers-clang-format

[rymiel]

Patch: https://reviews.llvm.org/D147377