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Different inference behavior between method and function syntax as opposed to arrow functions. #38623
Description
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
Activity
HerringtonDarkholme commentedon Aug 3, 2020
Personally I don't think this is a bug but rather an improvement.
A method signature will contextually type the object literal itself because
thisin method is the containing object literal. But an arrow function does not havethis.A user-side fix would be annotate
thisexplicitly:However, we can skip the contextual typing if
thisisn't accessed in the method.https://www.typescriptlang.org/play/?ts=4.0.0-dev.20200516#code/GYVwdgxgLglg9mABAWwIYGsCmAeAsgPgAo4AuRAbxRClVgQGczcAaRRVaeMMw5JgSkQBefIgBucGABNEAX0GVZAWABQqtFkLlVbKjTphGFHbt3A4cQlAAWMI+HRg4AdzAK5iAPSfEAeXQmcsyBHAY8qIIiFOwAdOaWgsoq8kA
test(types): add prop validator test
test(types): add prop validator test (#1769)