Function types: overload/merging behavior is unclear

[layershifter]

TypeScript Version: 3.9.2

Search Terms: interface, function types

Expected behavior:

1. Compiler error if overloads are not supported
2. foo is resolved to number, string, null type

Actual behavior: foo becomes any on a & b

Parameter 'foo' implicitly has an 'any' type.

Related Issues: N/A

Code

interface A {
    (foo: string): string
    (foo: number): number
}

interface B extends A {
    (foo: null): null
}

const a: A = (foo) => foo
const b: B = (foo) => foo

console.log(a, b)

Output

"use strict";
const a = (foo) => foo;
const b = (foo) => foo;
console.log(a, b);

Compiler Options

{
  "compilerOptions": {
    "noImplicitAny": true,
    "strictNullChecks": true,
    "strictFunctionTypes": true,
    "strictPropertyInitialization": true,
    "strictBindCallApply": true,
    "noImplicitThis": true,
    "noImplicitReturns": true,
    "alwaysStrict": true,
    "esModuleInterop": true,
    "declaration": true,
    "experimentalDecorators": true,
    "emitDecoratorMetadata": true,
    "moduleResolution": 2,
    "target": "ES2017",
    "jsx": "React",
    "module": "ESNext"
  }
}

Playground Link: Provided

[RyanCavanaugh]

I don't see a described defect here

[layershifter]

I updated a playground, can you please check? It fails for a & b, but passes for c if there is a single definition without extends, Playground Link

[RyanCavanaugh]

Contextual typing can only apply a type to a parameter if there is a single overload to source the type from.

[typescript-bot]

This issue has been marked as 'Question' and has seen no recent activity. It has been automatically closed for house-keeping purposes. If you're still waiting on a response, questions are usually better suited to stackoverflow.