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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Conditional type triggers \"No error for last overload signature\" exception","articleBody":"# Bug Report\r\n\r\n\u003c!--\r\n  Please fill in each section completely. Thank you!\r\n--\u003e\r\n\r\n### 🔎 Search Terms\r\n\r\n**\"No error for last overload signature\"** yields [stale issue 35186](https://github.com/microsoft/TypeScript/issues/35186), and a couple of more recent issues that got resolved: [48636](https://github.com/microsoft/TypeScript/issues/48636), [37974](https://github.com/microsoft/TypeScript/issues/37974). (The stale 35186 appears to be a different issue as it also occurs in versions before v5.0.4.)\r\n\u003c!--\r\n  What search terms did you use when trying to find an existing bug report?\r\n  List them here so people in the future can find this one more easily.\r\n--\u003e\r\n\r\n### 🕗 Version \u0026 Regression Information\r\n\r\n\u003c!-- When did you start seeing this bug occur?\r\n\r\n\"Bugs\" that have existed in TS for a long time are very likely to be FAQs; refer to\r\n  https://github.com/Microsoft/TypeScript/wiki/FAQ#common-bugs-that-arent-bugs\r\n\r\nIf possible, please try testing the nightly version of TS to see if it's 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\r\nNote: The TypeScript Playground can be used to try older versions of TypeScript.\r\n\r\nPlease keep and fill in the line that best applies:\r\n--\u003e\r\n- This is a crash\r\n- This changed between versions 4.9.5 and 5.0.4, and still occurs up to typescript@next.\r\n\r\n### ⏯ Playground Link\r\n\r\n\u003c!--\r\n  A link to a TypeScript Playground \"Share\" link which shows this behavior.\r\n  This should have the same code as the code snippet below, and use whichever settings are relevant to your report.\r\n\r\n  As a last resort, you can link to a repo, but these will be slower for us to investigate.\r\n--\u003e\r\n[Bug-workbench link with relevant code](https://www.typescriptlang.org/dev/bug-workbench/?ts=5.2.0-beta#code/PTAEAEDMEsBsFMB2BDAtvAXKALgTwA7wDOAdNkQFDwAe+A9gE7Y4HygAKDdAJgK4DG2AHJo2AXlAByZJNAAfKQCNJFKrUbM8hUAGVei-Fz6CR6AGKNOPAcNEAedqBrYk3IhyM3T8AHygJjs6u7ryIANaIdADuiBSgoAD8oGHwuHSQoACiiNjQucQAQrhWxrboANrsALpxoFiI8ABu8AyqWmzZuflERSVeov6gAN61yFhDoMgAjOOgjciwvJhSyAC007IAvqCbtYqzijPDcwtLWJKKq4dbOxS7avRMLNoAItBE-AzQqNAoLtwAVUQ0DoiDstUC1BciDcHmsJgGAU8CPQABpapknFDgrp9IZ4WV4BYGH0UfAHH4JHoDMjCcTSYSKejKRwsdDYQzvLV4klMUEYe5qfjSt56bTvBTuaAQIksmycaEItFENKwAB1PIAC1AisiMVROE1bBo-Hg+FyoNA3DoxEQkmYmuQ+EIiBIoClPKyOTy0EKxXFokqVXKmRq8XDdVADWarQj9SaLVUMqgcCQoiwTvwZEo30ezAmQoD5ksRfgBreHy+Pz+8EBwNBBs5A22kC4qCkJGA7SIKgeGlA-FBRGYil4AHNBg55QK4SLRD4ABQASn8fhG8UHiGHoCIeNpRCwFc+31+yH+QJBYPYBsLBNFJbv9nYPh85SqgzftV3NIJpFQToXBcAH1D3eY9qzPWsL1BFcxD8RBeFgWAlzuCggA)\r\n\r\n### 💻 Code\r\n\r\n```ts\r\n// @filename: types.ts\r\nexport type ProductName = 'a' | 'b'\r\n\r\nexport type SubproductNameForProductName\u003cP extends ProductName\u003e = P extends unknown\r\n  ? keyof EntitiesByProductName[P]\r\n  : never\r\n\r\ntype EntitiesByProductName = {\r\n  a: { a1: { value: 'a-a1' } }\r\n  b: { b1: { value: 'b-b1' } }\r\n}\r\n\r\nexport type DiscriminatedUnion\u003c\r\n  P extends ProductName = ProductName,\r\n  E extends SubproductNameForProductName\u003cP\u003e = SubproductNameForProductName\u003cP\u003e,\r\n\u003e = P extends ProductName\r\n    ? E extends SubproductNameForProductName\u003cP\u003e\r\n    // ? E extends unknown // With unknown, the exception doesn't happen. \r\n      ? EntitiesByProductName[P][E]\r\n      : never\r\n    : never\r\n\r\n// @filename: app.ts\r\nimport { SubproductNameForProductName, DiscriminatedUnion, ProductName } from './types'\r\n\r\nexport const bug = \u003cP extends ProductName\u003e() =\u003e {\r\n  const subproducts: DiscriminatedUnion\u003cP, SubproductNameForProductName\u003cP\u003e\u003e[] = []\r\n  subproducts.map((_: DiscriminatedUnion) =\u003e null)\r\n}\r\n```\r\n\r\n### 🙁 Actual behavior\r\n\r\nThe compiler throws an exception:\r\n```\r\n/Users/martijn/git/Insify/insify-core/node_modules/typescript/lib/tsc.js:114747\r\n      throw e;\r\n      ^\r\n\r\nError: Debug Failure. No error for last overload signature\r\n    at resolveCall (/Users/martijn/git/Insify/insify-core/node_modules/typescript/lib/tsc.js:70223:19)\r\n    at resolveCallExpression (/Users/martijn/git/Insify/insify-core/node_modules/typescript/lib/tsc.js:70605:12)\r\n    at resolveSignature (/Users/martijn/git/Insify/insify-core/node_modules/typescript/lib/tsc.js:70988:16)\r\n    at getResolvedSignature (/Users/martijn/git/Insify/insify-core/node_modules/typescript/lib/tsc.js:71008:18)\r\n    at checkCallExpression (/Users/martijn/git/Insify/insify-core/node_modules/typescript/lib/tsc.js:71120:23)\r\n    at checkExpressionWorker (/Users/martijn/git/Insify/insify-core/node_modules/typescript/lib/tsc.js:74278:16)\r\n    at checkExpression (/Users/martijn/git/Insify/insify-core/node_modules/typescript/lib/tsc.js:74189:32)\r\n    at maybeCheckExpression (/Users/martijn/git/Insify/insify-core/node_modules/typescript/lib/tsc.js:73280:28)\r\n    at BinaryExpressionStateMachine.onLeft (/Users/martijn/git/Insify/insify-core/node_modules/typescript/lib/tsc.js:73216:16)\r\n    at Array.left (/Users/martijn/git/Insify/insify-core/node_modules/typescript/lib/tsc.js:25140:30)\r\n```\r\n\r\n### 🙂 Expected behavior\r\n\r\nNo compiler exception.\r\n","author":{"url":"https://github.com/Oblosys","@type":"Person","name":"Oblosys"},"datePublished":"2023-07-31T16:50:25.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":4},"url":"https://github.com/55217/TypeScript/issues/55217"}