Mixed-valued literal enums assignability issue

[zhuravlik]

TypeScript Version: 2.6 (just checked out)

Code

enum E {
  A = 1,
  B = "2"
}

var q: E.A = 1; // okay
var z: E.B = "2"; // error TS2322: Type '"2"' is not assignable to type 'E.B'

Expected behavior:
In both cases assignability should work the same way.

Actual behavior:
Assignability passes with numeric literal enum values, but doesn't pass with string literal enum values

[zhuravlik]

By the way, looks like typechecking for numeric items in such mixed enums doesn't work at all
var q: E.A = 5; // still okay

[ikatyang]

var z: E.B = "2"; // error TS2322: Type '"2"' is not assignable to type 'E.B'

This one seems duplicate of #16851.

[mhegazy]

The numeric values allowing numbers is there to support bit flags. it is a common use case of enums, and changing that would be a massive break. see similar discussion in #17734.

The string values behave as i would expect. enums are "tagged" versions of the string literals. the enum member type is assignable to the string literal type but not vice versa, other wise rename operations are not guaranteed to work.

[zhuravlik]

@mhegazy Thanks a lot for the explanation.