Consider contextually typing parenthesized expressions

[RyanCavanaugh]

We've seen some issues where expressions which are non-obviously parenthesized lose contextual typing and cause issues:

var x: SomeType = (n) => ((m) => q)); // 'm' does not get a contextual parameter type
var y: SomeType = t ? (m => m.length) : undefined; // same here

This trips people up because they don't expect parentheses to change the type system, especially when parens are required to make an expression parse.

With the improvements in the type system with union types, we may have crossed the line where we can more aggressively apply contextual types. The 'new' workaround in cases (if there are any) where a contextual type needs to be avoided would be to use a type assertion to any or some other type which lacks a contextual impact.

[danquirk]

Should note that this would be a breaking change. Likely the breaks are bugs but still, we'll have any typed things going to something more specific which is bound to break someone here and there.

[NoelAbrahams]

This trips people up because they don't expect parentheses to change the type system,

Certainly has tripped me up - twice

#862
#1149

[RyanCavanaugh]

This was discussed in passing at the design meeting today and it seemed like there was broad agreement that we should do it.

[DanielRosenwasser]

We now perform the appropriate contextual typing.