Generator comprehension in call with trailing comma spuriously fails

[msullivan]

Trying to reformat list(i for i in range(10), ) fails with "Black produced code that is not equivalent to the source."

Passing a generator comprehension to a function as its sole argument with a trailing comma is valid in Python 2.7 but not in Python 3.7. (It is valid in Python 3.6 as well but typed_ast rejects it in 3.6 mode because it is based on the 3.7 parser... I expected the Python 3.8 parser rejects it as well.)

Black fails on this because the original code parses only in 2.7, so it gets a 2.7 AST. Once the trailing comma is removed, it parses in 3.7 as well, so gets a 3.7 AST, which will never match the 2.7 one.

I think the solution is probably to ensure that the same parser is used for before and after?

[JelleZijlstra]

This still reproduces, but only with --skip-magic-trailing-comma.

[JelleZijlstra]

This no longer crashes because we dropped support for 2.7.

Technically there's a bug here as this syntax works at runtime in 3.6 and Black rejects it even though we still support formatting Python 3.6, but I don't care.