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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Proposal: strict flag to prevent uninferrable generic types from being inferred as {}","articleBody":"I'm sure there an issue or discussion about this I missed, but I couldn't find it via search.\r\n\r\n## Search Terms\r\n\r\nGeneric, infer, default, {}, { }, empty, shape, object\r\n\r\n## Suggestion\r\n\r\nBehind a new `strict` mode flag `--strictGenericBindings`, when binding concrete types to generic type parameters, if a type can't be inferred TSC should throw an error at compile time instead of defaulting to `{}`.\r\n\r\n## Use Cases\r\n\r\nLet's say you have the following Promise:\r\n\r\n```ts\r\nconst p = new Promise(resolve =\u003e resolve(42)) // Promise\u003c{}\u003e\r\np.then(r =\u003e\r\n  r + 4 // Error: Operator '+' cannot be applied to types '{}' and '4'.\r\n)\r\n```\r\n\r\nThis gives an error when you try to use the Promise's result. This experience isn't ideal, because the error is non-local. Looking at the code that threw that error, it's not obvious why `r` is `{}`.\r\n\r\nIf I try to fix this by adding an explicit annotate when I consume `r`, it works:\r\n\r\n```ts\r\np.then((r: number) =\u003e {\r\n  r + 4 // OK\r\n})\r\n```\r\n\r\nBut if I have `strictFunctionTypes` enabled, that breaks because `{}` isn't assignable to `number`:\r\n\r\n```ts\r\np.then((r: number) =\u003e {\r\n  r + 4 // Error: Argument of type '(r: number) =\u003e void' is not assignable to parameter of type '(value: {}) =\u003e void | PromiseLike\u003cvoid\u003e'.\r\n})\r\n```\r\n\r\nThe real fix is to explicitly bind a type parameter when I consume `Promise`:\r\n\r\n```ts\r\nconst p = new Promise\u003cnumber\u003e(resolve =\u003e resolve(42)) \r\np.then(r =\u003e {\r\n  r + 4\r\n})\r\n```\r\n\r\nBut unfortunately, the error messages didn't help me get there.\r\n\r\nA possible solution to this problem is to throw a helpful error message when a generic type parameter can't be inferred. This can help prevent bugs, and help programmers catch bugs earlier:\r\n\r\n```ts\r\nconst p = new Promise(resolve =\u003e resolve(42)) // Error: Unable to infer a generic type for Promise\u003cT\u003e - please bind one explicitly. Eg. new Promise\u003cmyType\u003e(...).\r\np.then(r =\u003e {\r\n  r + 4\r\n})\r\n```\r\n\r\n## Checklist\r\n\r\nMy suggestion meets these guidelines:\r\n* [ ] This wouldn't be a breaking change in existing TypeScript / JavaScript code\r\n* [x] This wouldn't change the runtime behavior of existing JavaScript code\r\n* [x] This could be implemented without emitting different JS based on the types of the expressions\r\n* [x] This isn't a runtime feature (e.g. new expression-level syntax)\r\n\r\n## Related Issues\r\n\r\n- https://github.com/Microsoft/TypeScript/issues/26809\r\n- https://github.com/Microsoft/TypeScript/issues/10247\r\n","author":{"url":"https://github.com/bcherny","@type":"Person","name":"bcherny"},"datePublished":"2018-09-23T00:00:09.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":12},"url":"https://github.com/27288/TypeScript/issues/27288"}