Potential regression in 4.2 - Calling `.map` on `T[] | Y[]` is now callable but leads to an implicit any error

Bug Report

🔎 Search Terms

expression not callable union arrays

🕗 Version & Regression Information

This changed between versions 4.1.3 and 4.2 beta

⏯ Playground Link

https://www.typescriptlang.org/play?ts=4.1.3#code/CYUwxgNghgTiAEYD2A7AzgF3gDwFzxQFcBbAIxBgG0BdeAH3kxgEsUBzGgKE+wDpioABwAUAM3gBeAHzxRASiA

https://www.typescriptlang.org/play?ts=4.2.0-beta#code/CYUwxgNghgTiAEYD2A7AzgF3gDwFzxQFcBbAIxBgG0BdeAH3kxgEsUBzGgKE+wDpioABwAUAM3gBeAHzxRASiA

💻 Code

declare const x: number[] | string[]

x.map(f => f)

🙁 Actual behavior

.map is now callable but f as an implicit type of any, leading to an implicit any error.

🙂 Expected behavior

The current behaviour in 4.1.3 is to mark the map() call as not callable as well as f should have an implicit any type.

I don’t know if this is a regression but I’m raising it because this change in behaviour is not documented in the release notes.

[weswigham]

every and filter have multiple overloads (with differing numbers of type parameters) which makes the union call difficult to resolve, while map only has one overload, so combining the signatures is much more straightforward.

[thorn0]

An isolated real-world case to demo how this issue is a pain: playground

import * as Babel from "@babel/types";
import { TSESTree } from "@typescript-eslint/typescript-estree";
type Node = Babel.Node | TSESTree.Node;

declare const node: Node;

if (
    (node.type === "GenericTypeAnnotation" || node.type === "TSTypeReference") &&
    (!node.typeParameters ||
        node.typeParameters.params.length <= 2 && node.typeParameters.params.every(n => true))
) { }

every causes the following error:

This expression is not callable.
  Each member of the union type '{ <S extends TSType>(predicate: (value: TSType, index: number, array: TSType[]) => value is S, thisArg?: any): this is S[]; (predicate: (value: TSType, index: number, array: TSType[]) => unknown, thisArg?: any): boolean; <S extends TSType>(predicate: (value: TSType, index: number, array: TSType[]) => value is S, thi...' has signatures, but none of those signatures are compatible with each other.(2349)