Reducing union types when inferring type parameters in function invocations

[osdm]

Spec 3.4 says:

Union types are reduced to the smallest possible set of constituent types using these rules

But in compiler they are not reduced in case of inferring type parameters in function invocations. For example:

var o = {};

// According to spec, type of p should be reduced to {}
function f<T>(p: T | typeof o): T 
{
    var t: T;
    var temp = t || o; // here union type is reduced, and it is really {}
    return null;
}

// Another function where type of p is really {}. According to spec, it should be equal to f, shouldn't it?
function f2<T>(p: typeof o): T 
{
    return null;
}

// But really they are different
var e = f(5); // number
var e2 = f2(5); // {}

Could you please clarify, when union type are really reduced and when they are not.

[JsonFreeman]

I imagine you are using the latest from master. Constituents in a union type are no longer reduced, at least not by the subtype relation. I believe this change is reflected in spec update #4033.

[JsonFreeman]

And the code change for removing subtype reduction is #3823.

[ahejlsberg]

Union types are reduced to the smallest possible set of constituent types using these rules

That paragraph has been removed from the spec.

I have yet to add a description of the new deduplication algorithm, but basically a type S is considered a duplicate of a type T if:

• S and T are identical,
• S is undefined,
• S is null and T is not undefined,
• S is an object literal type, T is an object type, and S has a property set that is a duplicate of T's,
• S and T are array types, and S has an element type that is a duplicate of T's.
• S and T are tuple types, and S has element types that are duplicates of T's.

In other words, we get rid of types that are identical to other types in the set, except they may have nulls or undefineds where the other types don't.

The implementation of this algorithm is in the removeDuplicateTypes function in checker.ts.

[osdm]

@JsonFreeman @ahejlsberg Thanks for updates!