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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Type error when using generic function result as argument, but not when assigned to temp variable first","articleBody":"Issue originally discovered by @eyelidlessness on Gitter.\r\n\r\n\u003c!-- 🚨 STOP 🚨 𝗦𝗧𝗢𝗣 🚨 𝑺𝑻𝑶𝑷 🚨\r\n\r\nHalf of all issues filed here are duplicates, answered in the FAQ, or not appropriate for the bug tracker. Even if you think you've found a *bug*, please read the FAQ first, especially the Common \"Bugs\" That Aren't Bugs section!\r\n\r\nPlease help us by doing the following steps before logging an issue:\r\n  * Search: https://github.com/Microsoft/TypeScript/search?type=Issues\r\n  * Read the FAQ: https://github.com/Microsoft/TypeScript/wiki/FAQ\r\n\r\nPlease fill in the *entire* template below.\r\n--\u003e\r\n\r\n\u003c!--\r\nPlease try to reproduce the issue with the latest published version. It may have already been fixed.\r\nFor npm: `typescript@next`\r\nThis is also the 'Nightly' version in the playground: http://www.typescriptlang.org/play/?ts=Nightly\r\n--\u003e\r\n**TypeScript Version:**  3.8.0-dev.20191123\r\n\r\n\u003c!-- Search terms you tried before logging this (so others can find this issue more easily) --\u003e\r\n**Search Terms:** `generic` `returntype`\r\n\r\n**Code**\r\n\r\n```ts\r\ndeclare const foo: \u003c\r\n    R1,\r\n    F1 extends () =\u003e R1,\r\n    R2,\r\n    F2 extends (input: ReturnType\u003cF1\u003e) =\u003e R2\r\n\u003e(\r\n    f1: F1,\r\n    f2: F2\r\n) =\u003e any\r\n\r\nconst f1 = () =\u003e ({ a: 'a' })\r\nconst f2 = (input: { a: string }) =\u003e ({ b: 'b' })\r\n\r\nfoo(f1, f2) // Good\r\n\r\nconst makeF1 = () =\u003e f1;\r\n\r\nfoo(makeF1(), f2) // Good\r\n\r\nconst makeF1p = \u003cT\u003e(t: T) =\u003e f1;\r\n\r\nfoo(\r\n    makeF1p(''),\r\n    f2 // Error WHY :(\r\n);\r\n\r\nconst made = makeF1p('');\r\n\r\nfoo(\r\n    made, // This is fine\r\n    f2\r\n)\r\n```\r\n\r\n**Expected behavior:**\r\n\r\nBoth `foo()` calls should compile as the only difference is whether the result of a function call is assigned to a temporary first.\r\n\r\n**Actual behavior:**\r\n\r\nThe first call to `foo()`, where `makeF1p()` is called inline, produces a type error at compile time.  The second, where the return value of `makeF1p()` is assigned to temp variable first, compiles fine.  The error is:\r\n\r\n```\r\nArgument of type '(input: { a: string; }) =\u003e { b: string; }' is not assignable to parameter of type '(input: unknown) =\u003e unknown'.\r\n  Types of parameters 'input' and 'input' are incompatible.\r\n    Type 'unknown' is not assignable to type '{ a: string; }'.\r\n```\r\n\r\nIt almost seems like the type parameter inference machinery is leaking outside the scope of the call expression or something?  It's weird, at any rate.\r\n\r\n**Playground Link:**\r\n\r\nhttps://www.typescriptlang.org/play/?ts=3.8.0-dev.20191123\u0026ssl=1\u0026ssc=1\u0026pln=33\u0026pc=1#code/CYUwxgNghgTiAEYD2A7AzgF3gMyUgXPADwBQ858ASgIwA0ZFAYtfCAB4YgrBrwAUASngBeAHxU6DcpQBM9CvEYzWHLj34BLFAAcArhkKUQGXTBQAVAJ7aQRZqKFiqMkqL5Sc1Qs3kVsM7xdHcSgUSxISZHQsbBZhfmD+AG94KEIAcih0+ABfAUjUTBxleL4tPQN4FLT4TBgtAHNcxL4UgCMMtuy8iNwkPljaYqEAehH4AHE8YAioooBbKABrEGYRBJFxWIBuXrw+RZXmQSH-UfGppBmC6PhD1eptdaJzN0rzRJ29-o975m0+Ol0gJfOR-PAxvAAKIwGBIGDwADqAAkAJrwfDuAS7G4LKCgdZ-R6A4E4vruBSLUBDSHmAAWGl4jJwWhAHn8JHyQA\r\n\r\n**Related Issues:**  I didn't find any, but @jack-williams says #30215 might be the root cause.","author":{"url":"https://github.com/fatcerberus","@type":"Person","name":"fatcerberus"},"datePublished":"2019-11-25T23:26:50.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":10},"url":"https://github.com/35339/TypeScript/issues/35339"}