Unary string literal types don't seem to work when applied to function arguments

[malibuzios]

TypeScript Version:

1.8.7

Code

function func(val: "Test") {
    console.log(val);
}

func("Test");

Expected behavior:
Should work normally and print "Test".

Actual behavior:
A compile-time error is reported (highlighting func):

A signature with an implementation cannot use a string literal type.

---

In my code I had a function that accepted an argument with a type including only two possible string literal values: say "Value1" and "Value2", e.g function func(val: "Value1" | "Value2"), which worked OK. At some point I decided that "Value2" should not yet be supported so I removed it as an option. This left me with a unary string literal type, e.g. function func(val: "Value1"), but the compiler doesn't seem to like it?

[malibuzios]

OK, just found out this is a duplicate of #6143 and #6991. Closing.