Contravariance broke in 3.8 when combining spreading parameter list with conditional type

[nicojs]

TypeScript Version: 3.8.3

Search Terms: variance contravariance

Description: An Injector in typed-inject is a dependency injection container that is type safe. It can only provide values for tokens it knows. A small example:

declare const fooInjector: Injector<{foo: string}>; // this injector can provide string values for the token 'foo'
const foo = fooInjector.resolve('foo'); 
// => typeof foo === 'string'
fooInjector.resolve('bar');
// => Error ts(2345) Argument of type '"bar"' is not assignable to parameter of type '"foo"'

It makes sense that an injector Injector<{}> is not assignable to Injector<{foo: string}>, since it cannot provide a value for token 'foo'. This was the case in TS 3.7. However, since TS 3.8, Injector<{}> is assignable to Injector<{foo: string}> 😪.

declare const rootInjector: Injector<{}>;
const fooInjector: Injector<{ foo: string}> = rootInjector;

Expected behavior: Type 'Injector<{}>' is not assignable to type 'Injector<{ foo: string; }>'.

Actual behavior: No error

Related Issues: Couldn't find any 🤷‍♂️

Code

I think I've trimmed it down to the essentials.

interface Injector<TContext> {
 injectFunction<Tokens extends (keyof TContext)[]>(todo:
 (...args: { [K in keyof Tokens]: Tokens[K] extends keyof TContext ? TContext[Tokens[K]] : never; }) => void): void;
} 

declare const rootInjector: Injector<{}>;
const fooInjector: Injector<{ foo: string}> = rootInjector;

Output

(none)

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

Simpler contravariant examples like this still work as expected.

type Action<T> = (arg: T) => void;
declare let b: Action<{}>;
declare let a: Action<{foo: number}>;
b = a
// => Error! Type 'Action<{ foo: number; }>' is not assignable to type 'Action<{}>'.

[RyanCavanaugh]

Tried replacing method-syntax declarations with arrow function equivalents; no effect.

Nico Jansen (@nicojs) the simpler the repro the sooner we can look at this; any progress would be appreciated

[nicojs]

Thanks for the encouragement Ryan Cavanaugh (@RyanCavanaugh) 💖

I've tried to trim it down further, with moderate success:

Playground link

Maybe some more clues. Performance of compiling a project with typed-inject degraded significantly with 3.8 compared to 3.7 (about 3~4 times slower). See the integration test output snippets below.

TypeScript 3.7 (link)

 typed-inject
 ✓ dependency-graph.ts (5308ms)
 ✓ forgot-tokens-class.ts (4536ms)
 ✓ forgot-tokens-function.ts (3395ms)
 ✓ injector-too-variant.ts (3386ms)
 ✓ override-token-should-change-type.ts (3326ms)
 ✓ tokens-of-type-string.ts (3445ms)
 ✓ unknown-token.ts (3562ms)
 ✓ wrong-order-tokens.ts (3209ms)

TypeScript 3.8 (link)

 typed-inject
 ✓ dependency-graph.ts (15980ms)
 ✓ forgot-tokens-class.ts (16109ms)
 ✓ forgot-tokens-function.ts (15598ms)
 1) injector-too-variant.ts
 ✓ override-token-should-change-type.ts (15747ms)
 ✓ tokens-of-type-string.ts (15607ms)
 ✓ unknown-token.ts (15563ms)
 ✓ wrong-order-tokens.ts (15932ms)

[nicojs]

I think I've trimmed it down to the essentials.

playground link

interface Injector<TContext> {
 injectFunction<Tokens extends (keyof TContext)[]>(todo:
 (...args: { [K in keyof Tokens]: Tokens[K] extends keyof TContext ? TContext[Tokens[K]] : never; }) => void): void;
} 

declare const rootInjector: Injector<{}>;
const fooInjector: Injector<{ foo: string}> = rootInjector; // Missing error here in TS 3.8

The error seems to occur when combining a conditional type with spreading parameter lists with tuples. I'll also update the title and the initial message to better reflect this.

[nicojs]

Wesley Wigham (@weswigham) or Ryan Cavanaugh (@RyanCavanaugh) any updates on this? Would this make it into TypeScript 4.0? 😇