type inference error in multi-olverloads function body

[dardino]

TypeScript Version: 2.9

Search Terms:
method overloads when pipe

Code

class C1 { P1: string; }
class C2 { P2: string; }
class C3 { P3: string; }

function a(value: C1 | null): string;
function a(value: C2 | null): string;
function a(value: C1 | C2 | null): string {
    return "";
}

var b = (value?: C1 | C2 | C3 | null | undefined): string => {
    if (value instanceof C1) return a(value);  // C1 : no error OK!
    if (value instanceof C2) return a(value);  // C2 : no error OK!
    return value ? value.P3 : ""; // : no error OK!
}

var c = (value?: C1 | C2 | C3 | null | undefined): string => {
    if (value instanceof C1 || value instanceof C2)        
        return a(value); // C1 | C2  --> not null or undefined, there is no reasons to give an error...
        // is a(C1) a valid call? YES
        // is a(C2) a valid call? YES
        // why error? !!KO!!
    return value ? value.P3 : ""; // no error OK!
}

Expected behavior:
no errors in the function "c" because is exactly the same as function "b"

Actual behavior:
error in function "c":

Argument of type 'C1 | C2' is not assignable to parameter of type 'C2'.
  Type 'C1' is not assignable to type 'C2'.
    Property 'P2' is missing in type 'C1'.
(parameter) value: C1 | C2

Playground Link:
PlayGround

Related Issues:
#25637
#25791
#25826

[j-oliveras]

From FAQs: A function or a method implementation signature is not part of the overloads.

Add another overload to function a:

function a(value: C1 | null): string;
function a(value: C2 | null): string;
function a(value: C1 | C2 | null): string;
function a(value: C1 | C2 | null): string {
    return "";
}

[AlCalzone]

@j-oliveras that was going to be my first response too, but then I thought the error is a bit illogical.

In the line where the error occurs, we know that value is either a C1 or a C2. For both options, an overload of a exists. There's probably some design limitation or intent behind this, but why should ((value: C1) => string) | ((value: C2) => string) not be callable with a C1 | C2?

[j-oliveras]

@AlCalzone In this case, it is duplicate of #14107 and #1805.

[AlCalzone]

And possibly related: #7294

type Foo = ((val: number) => any) | ((val: string) => any);
let f: Foo;
f(1); // error: Type Foo lacks a call signature

[mhegazy]

indeed duplicate of #14107

[dardino]

@j-oliveras I am not the owner of ".d.ts" file that describes the overloads. I can't add the overloads as you suggest.