parsing of braceless for-comprehensions: unify error message for unintended code in various edge cases

[unkarjedy]

Compiler version

3.0.0-RC1

Minimized code

@main
def Main23() = {
    for x <- 0 to 1;
    x <- 0 to 1;
    do println(1)
  
    for 
        x <- 0 to 1
    y <- 0 to 1
    do println(2)
    
    for 
    x <- 0 to 1
  y <- 0 to 1
    do println(3)
}

Output

Note that compilation error "yield or do expected" is only shown for the 2nd for and not for the 1st and 3rd

Expectation

An error should be reported in all 3 cases or in none

[odersky]

That's because (1) or (3) don't see an indented region.

[unkarjedy]

Still looks awkward that this code is accepted by the compiler:

while the example with indented first enumerator is not.

Taking into account that braceless for-expressions require do/yield keyword maybe it's worth not treating indented region inside enumerators at all?

[bishabosha]

I refer to this comment chain - where indentation is not checked when the context it appears in is unambiguous: #8659 (review)