add implicit cast for enum literal to optionals and error unions

[tiehuis]

For example, the following does not compile today:

const Foo = enum {
    A,
    B,
};

pub fn main() void {
    var foo: ?Foo = undefined;
    foo = .A;
}

The error is:

zig run t.zig 
/tmp/t.zig:8:11: error: expected type '?Foo', found '(enum literal)'
    foo = .A;

For this to work today you need to specify the enum type with foo = Foo.A.

This is simply proposing to check the inner type of a nullable in the compiler and support performing enum literal inference based on that.

[andrewrk]

The implementation should also support implicit cast to error unions.

[FireFox317]

@andrewrk, Should it only be allowed for error unions? And not for error sets? Because my current implementation in #2867 does casting for both error unions and error sets.

[andrewrk]

Yeah, not error sets. That's a safeguard against a footgun because if you have .foo casting into E!T where:

const E = error {foo};
const T = enum {foo};

Which one should it cast to? It's ambiguous.

[andrewrk]

Fixed by @FireFox317 in #2867, landed in e540e5b.