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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Sub-optimal type parameter inference with strictFunctionTypes enabled","articleBody":"These are cases I've extracted from #49929; they are cases where if you don't specify the type parameters yourself, the inference isn't good.\r\n\r\nThis is potentially a full duplicate of #49924, however, the fix suggested in that issue does not work properly in our codebase. To test these, it may be easiest to just clone https://github.com/microsoft/TypeScript/pull/49929 with the explicit type parameters removed and see what works and what doesn't.\r\n\r\n\r\n```ts\r\nenum SyntaxKind {\r\n    Block,\r\n    Identifier,\r\n    CaseClause,\r\n    FunctionExpression,\r\n    FunctionDeclaration,\r\n}\r\n\r\ninterface Node { kind: SyntaxKind; }\r\ninterface Expression extends Node { _expressionBrand: any; }\r\ninterface Declaration extends Node { _declarationBrand: any; }\r\ninterface Block extends Node { kind: SyntaxKind.Block; }\r\ninterface Identifier extends Expression, Declaration { kind: SyntaxKind.Identifier; }\r\ninterface CaseClause extends Node { kind: SyntaxKind.CaseClause; }\r\ninterface FunctionDeclaration extends Declaration { kind: SyntaxKind.FunctionDeclaration; }\r\n\r\ntype HasLocals = Block | FunctionDeclaration;\r\ndeclare function canHaveLocals(node: Node): node is HasLocals;\r\n\r\ndeclare function assertNode\u003cT extends Node, U extends T\u003e(node: T | undefined, test: (node: T) =\u003e node is U): asserts node is U;\r\ndeclare function assertNode(node: Node | undefined, test: ((node: Node) =\u003e boolean) | undefined): void;\r\n\r\nfunction foo(node: FunctionDeclaration | CaseClause) {\r\n    assertNode(node, canHaveLocals)\r\n\r\n    node;\r\n    // ^?\r\n\r\n    assertNode\u003cNode, HasLocals\u003e(node, canHaveLocals)\r\n\r\n    node;\r\n    // ^?\r\n}\r\n\r\n\r\ndeclare function isExpression(node: Node): node is Expression;\r\n\r\ndeclare function cast\u003cTOut extends TIn, TIn = any\u003e(value: TIn | undefined, test: (value: TIn) =\u003e value is TOut): TOut;\r\n\r\nfunction bar(node: Identifier | FunctionDeclaration) {\r\n    const a = cast(node, isExpression);\r\n    //    ^?\r\n\r\n    const b = cast\u003cExpression\u003e(node, isExpression);\r\n    //    ^?\r\n}\r\n```\r\n\r\n\r\n\r\n\u003cdetails\u003e\u003csummary\u003e\u003cb\u003eOutput\u003c/b\u003e\u003c/summary\u003e\r\n\r\n```ts\r\n\"use strict\";\r\nvar SyntaxKind;\r\n(function (SyntaxKind) {\r\n    SyntaxKind[SyntaxKind[\"Block\"] = 0] = \"Block\";\r\n    SyntaxKind[SyntaxKind[\"Identifier\"] = 1] = \"Identifier\";\r\n    SyntaxKind[SyntaxKind[\"CaseClause\"] = 2] = \"CaseClause\";\r\n    SyntaxKind[SyntaxKind[\"FunctionExpression\"] = 3] = \"FunctionExpression\";\r\n    SyntaxKind[SyntaxKind[\"FunctionDeclaration\"] = 4] = \"FunctionDeclaration\";\r\n})(SyntaxKind || (SyntaxKind = {}));\r\nfunction foo(node) {\r\n    assertNode(node, canHaveLocals);\r\n    node;\r\n    // ^?\r\n    assertNode(node, canHaveLocals);\r\n    node;\r\n    // ^?\r\n}\r\nfunction bar(node) {\r\n    const a = cast(node, isExpression);\r\n    //    ^?\r\n    const b = cast(node, isExpression);\r\n    //    ^?\r\n}\r\n\r\n```\r\n\r\n\r\n\u003c/details\u003e\r\n\r\n\r\n\u003cdetails\u003e\u003csummary\u003e\u003cb\u003eCompiler Options\u003c/b\u003e\u003c/summary\u003e\r\n\r\n```json\r\n{\r\n  \"compilerOptions\": {\r\n    \"strict\": true,\r\n    \"noImplicitAny\": true,\r\n    \"strictNullChecks\": true,\r\n    \"strictFunctionTypes\": true,\r\n    \"strictPropertyInitialization\": true,\r\n    \"strictBindCallApply\": true,\r\n    \"noImplicitThis\": true,\r\n    \"noImplicitReturns\": true,\r\n    \"alwaysStrict\": true,\r\n    \"esModuleInterop\": true,\r\n    \"declaration\": true,\r\n    \"experimentalDecorators\": true,\r\n    \"emitDecoratorMetadata\": true,\r\n    \"target\": \"ES2017\",\r\n    \"jsx\": \"react\",\r\n    \"module\": \"ESNext\",\r\n    \"moduleResolution\": \"node\"\r\n  }\r\n}\r\n```\r\n\r\n\r\n\u003c/details\u003e\r\n\r\n**Playground Link:** [Provided](https://www.typescriptlang.org/play?#code/KYOwrgtgBAygniALgQwB4GkCWIAmUDeAUFCVAEIA2A9gMYDWANMaQJI6iKYBmmwATk1JQAwsgDOwYRWRgJg0gDEwIGpyogAoqgAOfYGLGZ18kkpVqQAEWA1pfZBaYBfQoWyJ+XZDWBQAclTsBFB02DgAXLAIKBhhANxQLu6e3r5auvqG6lDAqB64Yv6BvvhQAPq5GQZGIGT2uJHIIHAJSUgpPlDWtsj2Fjl5oDiFAUGlZew9fTV1TRFQTS2Jbu18Xp2UtHQD+cNFYyFhkfBIaFi4AHSb9K0rHmupUGwc3Lx8O0OF6XrVxl02dgcNWCoQaUVOsUuzyQr34t2SD06ogkUhkEg+BX2JUOYJOMXOOAuyMk0lkwHhq3WvjMqhq3UB-Vyu0K9N6QOypVB8zxZzCFxpFlZ03Ut0IiDg2l8AAlxAAZWjICiFAC85Go9CgAB8oAK6QC2RY4oRJnZfFxlLTsjQmjKAG7AeXWpUAChAxUio2AAEpIm6gphCjKxI7FWIjcb9XooObzMDxBI+IhPQAeAAqGL2noYUAAqhnCqmAHyu91QdPa5TsHggYA4bMeMSISIl9iRVNeqDKwtQP2+AO5n0LAz8RCFXtQfs5o0m3pmi39eMjz0t4Ae4paqCV4DV2v1-RNqDOldr9gdrtQABGVCoFGATQ7Fdw2+wtcHtqomBw4ZjlpA0evx46vOepTOyf7asSqJkh2RBCIuibLr22bWiAdoOgqSpeq4Qi9kaQgAPT4VAAB6AD82GkPBSbFMmWZQEGIZKsWSFQChaGMWIWHMCQuHcVAhEkeRLiuDOUY-v0AbfJkNSAZ6g7jv2Um-CA4aiXOsZWuIiBpgA8mAiD5mWLAgNmqbGZ2CzNMWtqKmAq5GeBm5PjudZQA2B7OjZFB2W2xlnt2Xl2ROBZ6Ygg6pqF37AdkF69IB0KcDw-Abrq6hCmBMF8TQ6iNgsFnWo2K7ZpJOg-FkIBenhpACaQZEUSQ2UgLlF75VpyZKeVzHFMVYgdTUlV8TVJB1U4QA)\r\n      ","author":{"url":"https://github.com/jakebailey","@type":"Person","name":"jakebailey"},"datePublished":"2023-01-05T00:10:41.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":3},"url":"https://github.com/52111/TypeScript/issues/52111"}