str.fomat index_string too restricted

Bug Report

The following code is flagged by mypy-0.800:

"{0[k-y]}".format({"k-y": "val"})

The error is

Invalid index expression in format field accessor "[k-y]"

According to Python Format String Syntax:

replacement_field ::= "{" [field_name] …
field_name ::= arg_name ( … | "[" element_index "]")*
element_index ::= … | index_string
index_string ::= <any source character except "]"> +

the - should be okay. Similar when using + or /. Looks like only [0-9A-Z_a-z] are allowed with mypy.

To Reproduce

mypy -c '"{0[k-y]}".format({"k-y": ""})'
<string>:1: error: Invalid index expression in format field accessor "[k-y]"

Expected Behavior

No error should be detected.

Actual Behavior

mypy prints an error.

Your Environment

Debian-10-Buster with self-built mypy-0.800

Start by reproducing the report with the provided mypy command, then trace the format-string index validation that rejects [k-y]. Done means the command accepts string keys containing -, +, or / without an error, with regression coverage for the reported case.