Type inference problems when using string literal types

[jvitkauskas]

TypeScript Version: 2.3.4

Code

function example(parameter: { value: "a" }) { // Function requires argument with string literal
    return;
}

var myObject = {
    value: "a" // TS infers value to be string here, causing problems passing this object to function
};

example(myObject); // Error: Argument of type '{ value: string; }' is not assignable to parameter of type 'ObjectType'. Types of property 'value' are incompatible. Type 'string' is not assignable to type '"a"'.

Expected behavior:
No compiler error

Actual behavior:
Compiler error (see self-contained example)

[jvitkauskas]

I realize workaround exists for this (just specify type explcitly for myObject), but one of my colleagues ran into this issue while using 3rd party library and was puzzled by this behavior.

Since workarounds are possible, this probably is not a high priority issue, but maybe there's some possibility to explore more flexible type inference for TypeScript project.

[ahejlsberg]

This is working as intended. Literal types are widened when they are inferred for mutable locations (such as the value property in your example) unless you have a type annotation or type assertion indicating otherwise.

[ahejlsberg]

See #10676 for more details.