const enum keyof typeof

[styfle]

TypeScript Version: 2.7.0-dev.20180105

Code index.ts

const enum FileType { png, jpg }
type FileTypeStr = keyof typeof FileType; // fails, not expected!

function f1(f: FileType) { }
function f2(f: FileTypeStr) { }

f1(FileType.png); // works as expected
f1(FileType.nope); // fails as expected
f2('png'); // works as expected
f2('nope'); // fails as expected

Expected behavior:

Expected the second line to work without compilation errors just like a non-const enum.

Actual behavior:

Emits an unexpected error on line 2:

index.ts(2,33): error TS2475: 'const' enums can only be used in property or index access expressions or the right hand side of an import declaration or export assignment.

Oddly enough, it looks like the type checking below the type declaration is working as expected. So I think this might be bug...line 2 should not be a compiler error.

Related issue: #14106

[sylvanaar]

Seems like it should be allowed in a TypeQuery.

[mhegazy]

PRs welcomed.

[styfle]

@mhegazy I can try taking this on but I don't know where to start.
I found error code 2475 diagnosticMessages.json but where do I go from here?

[Kingwl]

@styfle
I'm working on this
(and in fact it's almost done

[mhegazy]

thanks @Kingwl !

[styfle]

Great job @Kingwl and @mhegazy! 🎉

When can I expect 2.7 to be released?