Def, macro, and block parameters could end with `?` or `!`

[HertzDevil]

The following code is valid in Crystal but not in Ruby:

def foo(x?, y!)
  x? + y!
end

foo 1, 2

{"foo" => "bar"}.each do |key?, value!|
  puts key? + value!
end

I would expect ? and ! to always denote a call rather than a variable name.

[vlazar]

Interestingly enough I sometimes wish I can use ? suffix in variable names in Ruby, so that I'm not forced to use prefix like is_valid to show it's boolean and use valid? instead just like with method names.

[straight-shoota]

I agree on the expectation that ? and ! suffixes denote a call. They can't be used for regular local variables, so I think it makes much sense to restrict parameters the same.

This can technically be seen as a parser bug (it unintentionally allows these names). But at this point, it's also a compiler feature (even if undocumented) and changing it could potentially break existing code. I don't think it's urgent to disallow these parameter names, so this should probably wait for 2.0.

[lbguilherme]

If the parser were updated to show a deprecated warning on these, but still parse, it wouldn't be a breaking change, right? This could be the way forward for compiler changes waiting for 2.0.

[HertzDevil]

See also: https://github.com/crystal-lang/crystal/issues/2617#issuecomment-220323655

[potomak]

If this issue is still relevant can I claim it?

I found in the parser the piece of code that parses functions. My plan was to add a test and update the code to output a warning in case input code uses ‘?’ or ‘!’ in function parameters identifiers as suggested in the comments to keep the backward compatibility.

Does it sound good?

[potomak]

I've published a #12197 to fix this issue. I decided to raise an exception because I didn't find how to log a warning + test warning message. In parser_spec.cr I just found it_parses and assert_syntax_error. See discussion in the PR.