Function type ascriptions discrepancy between parser and syntax.

[odersky]

Compiler version

3.2

Minimized example

def f[T](x: T): T = x
val x1 = (f: Int => Int)      // ok, but should be error
val x2 = f: Int => Int       // error

Output

3 |val x2 = f: Int => Int
  |                ^
  |parentheses are required around the parameter of a lambda
  |This construct can be rewritten automatically under -rewrite -source 3.0-migration.```

Expectation

According to the syntax, both lines 2 and 3 should give an error since type ascriptions require an InfixType. Line 3 gives an error because the compiler thinks it's a lambda. Line 2 compiles without error, but should ideally give an error like this:

2 |val x1 = (f: Int => Int)
  |             ^^^^^^^^^^
  |          function type in type ascription must be enclosed in parentheses

[odersky]

We need to fix this if we want fewerBraces with : in front of single-line lambda arguments. Without a fix, (that version of) the fewerBraces syntax would be ambiguous.

[odersky]

We don't need this to work for fewerBraces after all. Fixing this would break quite a bit of existsing code, so we might also consider legalizing this pattern by changing the grammar.