On compiling this:

class Temp
{
  Void doSth(|Str?->| f) { }
  static Void main() { }
}

I get:

/Users/vkuzkokov/tmp/test.fan(3,24): Expected '|', not ')'

Changing signature to |Str? ->| helps. Seems like ?-> is parsed as one token. Though we can enforce using ? -> in type signatures, it would help to see something like

/Users/vkuzkokov/tmp/test.fan(3,18): Expected '|', not '?->'

The signature |Str?->| isn't a legal function type. It needs to be either |Str?| or |Str?->ReturnType|. The ?-> token is handled though (Parser.fan line 2204).

So far |A->| worked exactly as |A->Void| in every context possible.

Renamed from ?-> in closure signatures to Don't allow |A->| function signatures

Promoted to ticket #1204 and assigned to brian

So far |A->| worked exactly as |A->Void| in every context possible.

Ok, that is a bug. The grammar is defined as:

<funcType> :=  "|" [formals] ["->" <type>] "|"

I will take a look.

Ticket resolved in 1.0.55

Tightened up the parser so that function type signatures with parameters must either omit the arrow or include an explicit return type.