Different inference behavior between method and function syntax as opposed to arrow functions.

[dragomirtitian]

TypeScript Version: 4.0.0-dev.20200516

Search Terms: object literal inference arrow function method

Expected behavior: M in inferred as { foo: () => void; }

Actual behavior:: When using function or method syntax M is inferred as unknown. Using arrow functions works fine.

Related Issues: #32230 But that specific case seems fixed

Code

function make<M>(o: { mutations: M,  action: (m: M) => void }) { }

make({
   mutations: {
       foo () { } // Nope
       // foo: function () { } // Nope 
       // foo: () => { } // OK
   },
   action: (a) => { a.foo() }
})

Output

"use strict";
function make(o) { }
make({
    mutations: {
        // foo () { } // Nope
        // foo: function () { } // Nope 
        foo: () => { } // OK
    },
    action: (a) => { a.foo(); }
});

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

[HerringtonDarkholme]

Personally I don't think this is a bug but rather an improvement.
A method signature will contextually type the object literal itself because this in method is the containing object literal. But an arrow function does not have this.

A user-side fix would be annotate this explicitly:

function make<M>(o: { mutations: M,  action: (m: M) => void }) { }

make({
   mutations: {
       foo(this: {}) { } // OK
   },
   action: (a) => { a.foo() }
})

However, we can skip the contextual typing if this isn't accessed in the method.

https://www.typescriptlang.org/play/?ts=4.0.0-dev.20200516#code/GYVwdgxgLglg9mABAWwIYGsCmAeAsgPgAo4AuRAbxRClVgQGczcAaRRVaeMMw5JgSkQBefIgBucGABNEAX0GVZAWABQqtFkLlVbKjTphGFHbt3A4cQlAAWMI+HRg4AdzAK5iAPSfEAeXQmcsyBHAY8qIIiFOwAdOaWgsoq8kA