René's URL Explorer Experiment

{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"extends clause leaks temporary type","articleBody":"**TypeScript Version:** 3.5.0-dev.20190420\r\n\r\n**Code**\r\n\r\n```ts\r\n// type Weird = U\r\ntype Weird = any extends infer U ? U : never;\r\n\r\n// Type 'null' is not assignable to type 'U'.\r\nconst a: Weird = null;\r\n\r\n// Type 'U' is not assignable to type 'string'.\r\nconst b: string = a;\r\n\r\n// type AlsoWeird = U\r\ntype AlsoWeird = unknown extends infer U ? U : never;\r\n```\r\n\r\n**Expected behavior:** Infer some type\r\n\r\n**Actual behavior:** The extends clause leaks a temporary type U. Happens for `any` and `unknown`\r\n\r\n**Playground Link:** [Playground](https://www.typescriptlang.org/play/#src=%2F%2F%20type%20Weird%20%3D%20U%0D%0Atype%20Weird%20%3D%20any%20extends%20infer%20U%20%3F%20U%20%3A%20never%3B%0D%0A%0D%0A%2F%2F%20Type%20'null'%20is%20not%20assignable%20to%20type%20'U'.%0D%0Aconst%20a%3A%20Weird%20%3D%20null%3B%0D%0A%0D%0A%2F%2F%20Type%20'U'%20is%20not%20assignable%20to%20type%20'string'.%0D%0Aconst%20b%3A%20string%20%3D%20a%3B%0D%0A%0D%0A%2F%2F%20type%20AlsoWeird%20%3D%20U%0D%0Atype%20AlsoWeird%20%3D%20unknown%20extends%20infer%20U%20%3F%20U%20%3A%20never%3B)\r\n","author":{"url":"https://github.com/SimonMeskens","@type":"Person","name":"SimonMeskens"},"datePublished":"2019-04-24T22:42:38.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":0},"url":"https://github.com/31099/TypeScript/issues/31099"}