No-op mapped type not assignable; but original type is assignable

[AnyhowStep]

TypeScript Version: 3.8.3

Search Terms:

no-op mapped type, assignable

Code

type Foo<IdentifierT extends Record<PropertyKey, PropertyKey>> =
    IdentifierT
;

type Bar<IdentifierT extends Record<PropertyKey, PropertyKey>, T> =
    {
        [k in keyof T] : Foo<IdentifierT & { k : k }>
    }
;

type Merge2<T> = { [k in keyof T] : T[k] }
type Bar2<IdentifierT extends Record<PropertyKey, PropertyKey>, T> =
    {
        [k in keyof T]: Foo<Merge2<IdentifierT & { k: k }>>
        //                  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
        //                  Works in 3.5.1
        //                  Works in 3.6.3
        //                  Works in 3.7.5
        //                  Fails in 3.8.3
    }
;

type Identity<T> = T;
type Merge3<T> = Identity<{ [k in keyof T] : T[k] }>
type Bar3<IdentifierT extends Record<PropertyKey, PropertyKey>, T> =
    {
        [k in keyof T]: Foo<Merge3<IdentifierT & { k: k }>>
        //                  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
        //                  Works in 3.5.1
        //                  Works in 3.6.3
        //                  Works in 3.7.5
        //                  Fails in 3.8.3
    }
;

Expected behavior:

Passes type checking

Actual behavior:

Fails in TS 3.8.3

Playground Link:

Playground

Related Issues:

Not that I could find

---

Well, it isn't exactly a "no-op" mapped type.

It will strip call and constructor signatures, but "no-op mapped type" was the best name I had for it.

[phiresky]

I think I have the same issue with https://github.com/phiresky/ts-typed-sql, which destroys type inference everywhere, means our whole project is stuck on 3.7 for now :(

[AnyhowStep]

I've been stuck at 3.5.1 forever now =(

[phiresky]

I just bisected the ts compiler to find the cause using

build.sh

#!/bin/bash
set -e -x
bisect_error=125

yarn install --silent || exit $bisect_error
gulp -L local || exit $bisect_error

node built/local/tsc.js --noEmit ../example.ts

git bisect start v3.8.3 v3.7.5
git bisect run ../build.sh

bisect reports:

first bad commit: [357f715] Check combined intersection properties against target index signatures (#35143) by @ahejlsberg

[ahejlsberg]

A simpler repro for this issue:

type Same<T> = { [P in keyof T]: T[P] };

type Foo<T extends Record<PropertyKey, number>> = T;

type Bar<T extends Record<PropertyKey, number>> = Foo<Same<T>>;  // Error

We shouldn't error above and strangely we don't if the declaration of Foo is changed to

type Foo<T extends Record<string, number>> = T;

That's definitely a bug.

The example above worked before #35143 because we were very lax in our checking of relations involving intersections on the source side.