Possible regression of intersection type inference in 5.7.2 compared to 5.6.3

🔎 Search Terms

inference generic types

🕗 Version & Regression Information

• This changed between versions 5.6.3 and 5.7.2
• This changed in commit or PR https://github.com/microsoft/TypeScript/pull/52095

⏯ Playground Link

https://www.typescriptlang.org/play/?ts=5.7.2#code/MYGwhgzhAEBqYCcCWYBGICmAeAqgGmgGVoMAPAFwwDsATGCc5KgcwD5oBvAKGmmAHsqDBAFdg5fggAUABxHokwaBABcRAJSdoAXx7QRAQjU4A3F11cAZiKrikg6AFsA1gHFqGZMFwFiZSrT0jEgsrFKqGpx6CBjkIghU0FQYAO5wiCjo2PhEYRDq5lxc5ACeMhjQAKKkYOIAggjMAHJgjhgQWO7JXgAqZRgEAMKCwmAh5OwAvNBdnop95dAAZFG8ANoA0tAh0M4YJfyWMx69-QC6alv+1HS7+4fQw0KMY1Tk0AD8x93z-Ztn0DUyQAbp5CsV+tA6iAQPwUvBkGhMB0elN0oislgegQwFQSuwAD5aTbbRJ7A5HHoXdGZTBY-44vGE6A9f46IqlRY9drkBrMGDTbi8CD8Np8oEiRyoMEWTkVbkMPkQADqSHIAAsEQKoTC4Qjae0sTylawitZbOR7IlyGA9hBZl4sLASBQbjBobD4RkkYaFbzGhBWGEwGpqrV-c1WobYAQ-Sb1GpgfwkDQohZMO9gdBpja7Q7FFIOCKxY01C588ApAAiUhV9TadRmIA

💻 Code

class Variable<U, S extends string> {
  constructor(public s: S) { }
  u!: U;
}

function mkGeneric<U, S extends string>(s: S) {
  return new Variable<U, S>(s)
}

type ExactArgNames<GenericType, Constraint> = GenericType & {
  [K in keyof GenericType]: K extends keyof Constraint ? GenericType[K] : never
}

type AllowVariables<T> = Variable<T, any> | { [K in keyof T]: Variable<T[K], any> | T[K] }

type TestArgs = {
  someArg: number
}

type TestArgsWithVars = AllowVariables<TestArgs>

function takesGeneric<V extends AllowVariables<TestArgs>>(a: ExactArgNames<V, TestArgs>): void {
}

let v = takesGeneric({someArg: mkGeneric("x")});

🙁 Actual behavior

VVersion 5.7.2 of the compiler infers the variable type as Variable<uknown, "x"> and subsequently generates an error

🙂 Expected behavior

VVersion 5.6.3 of the compiler infers the variable type correctly as Variable<number, "x">

Additional information about the issue

This is breaking most code that uses typed-graphql-builder https://github.com/typed-graphql-builder/typed-graphql-builder/issues/85

Start by running the linked Playground reproduction with TypeScript 5.6.3 and 5.7.2 to confirm the inference difference. Read the change in PR #52095 and trace the type-inference path involved in the example. Done means the code infers Variable<number, "x"> without the reported error while preserving the expected behavior in both versions' regression tests.