[3.9.2] Regression - combination of mapped type, intersection and generic produces error under infer

[nth-commit]

TypeScript Version: 3.9.2, 3.9.1-rc (works in 3.8.3)

Search Terms: infer, intersection, generic, mapped types

Code

type UnknownExtensions = Record<string, unknown>;

type IdentityMappedType<T> = {
  [P in keyof T]: T[P];
};

type Structure<Extensions extends UnknownExtensions> =
  IdentityMappedType<{ someBaseProperty: string } & Extensions>;

type InferStructureExtensions<TStructure extends Structure<UnknownExtensions>> =
  TStructure extends Structure<infer U> ? U : never; // Error: Type 'U' does not satisfy the constraint 'Record<string, unknown>'.

// Without mapped

type Structure_WithoutMappedType<Extensions extends UnknownExtensions> = 
  { someBaseProperty: string } & Extensions;

type InferStructureExtensions_WithoutMappedType<TStructure extends Structure<UnknownExtensions>> =
  TStructure extends Structure_WithoutMappedType<infer U> ? U : never; // Ok

// Without generic intersection

type Structure_WithoutGenericIntersection<Extensions extends UnknownExtensions> = 
  IdentityMappedType<{ someBaseProperty: string }>;

type InferStructureExtensions_WithoutGenericIntersection<TStructure extends Structure<UnknownExtensions>> =
  TStructure extends Structure_WithoutMappedType<infer U> ? U : never; // Ok

// Without concrete intersection

type Structure_WithoutConcreteIntersection<Extensions extends UnknownExtensions> = 
  IdentityMappedType<Extensions>;

type InferStructureExtensions_WithoutConcreteIntersection<TStructure extends Structure<UnknownExtensions>> =
  TStructure extends Structure_WithoutMappedType<infer U> ? U : never; // Ok

// With less strict typing of the generic

type AnyExtensions = Record<string, any>;

type Structure_WithAnyExtensions<Extensions extends AnyExtensions> = 
  IdentityMappedType<Extensions>;

type InferStructureExtensions_WithAnyExtensions<TStructure extends Structure<AnyExtensions>> =
  TStructure extends Structure_WithoutMappedType<infer U> ? U : never; // Ok

Expected behavior:

Should be able to infer U, given the code above.

Actual behavior:

An error is produced: "Error: Type 'U' does not satisfy the constraint 'Record<string, unknown>'."

Note, the example given outlines similar situations where no error is produced.

Playground Link: https://www.typescriptlang.org/play?ts=3.9.0-beta#code/C4TwDgpgBAqgdgazgewO5wKIA9gTgZwEtkCoBeKAJQgGNkAnAEwB59h7C4BzAGigFdEKdAD4A3ACgJoSFACSjPMEKgAsgEMwkRgBVwEZjpHkoAbwlQoAbQAKUTlAQQQyAGZQdAXQBcH258kAX0lpfSgAZXZ+GmB+egNsXAJiUggcPEZ8WCE0THTkknxjMgt5RThlNU1tPUhmUyh8ZABbCAAhdXwIG3pkSHpQXzYObihAqAAyKES8IkLxKRloOThXCHpI+mjY+JmCgkNN7bjoNKTMiKiYk+Z4JFy9uYIRYtKdI+v4qDOMrI+dgycNb0WDGAD8sCgvjgEAAbusxFAAPRI6b0Xr0Xy1aAAchgOKgjGQECyKGAjXUynwrhAUGAAAtoHQCOx1JxyTjqHQmKx2JxeAIcqIcQA6KQoqAAdRU9OQ-HJzWqEEYizC-xOAH1pQy5cANFpldjmI8UlkfnALndhHkkk8iiZSg0mq0Ol0en11oNGnzRuMpibCiElvJVut1bt8natTLdfqavpDlcAd98hdwwYrQ9I6aXuQ3umU+c-knNdrZfK44aE0D1qCoBCYFCoDD4fRERKAPIIcWosu6qBcPDrQg0ewVdZdGIpVWydPRnXygDiQ44NBWuHok+UJGN2cKhd+2Xu6ADzwdlgUShUIErugTTpa7U63V6-S9w35YwWoVkK2B6dPfB53LYBlxhVd1wnWhtzgRMtk+U5U2LeCAVuIUbVmHNXksd4Sy+c00zwiBgNjJU7zqGsQRgcFIWhOEEWRVEux7KUY3lKBmRoeJcDHDct2nH9oDnPt5QAYRILiIFwSDN2glJd1tU0DwtLJMxPPczwoUpLwqa9byNQDv2DP8wyIwCSLEiTuIgGT+J3XCUJOZTCMc+I0OPDD9iKbCPALAjkOOeILL1MijUousGybFsGM7bsJAlMsoAAGxJLIPxiOlwE-Nw6UZAcVxHGdoAAQTgEBAJMLkGBYD9uD4dQyqMtUiOA0ryo0-AFMw-d-KgNrDPPMoryqA1yISDqmt-UMNjMjrWrKwC4MCxCi0uVyDH6iafIc5bnIChDgv06tpoi2jm3ottGKgLsgA

Related Issues:

[fox1t]

I am facing this issue on 3.9.2 using https://github.com/sinclairzx81/typebox

Type 'unknown' is not assignable to type 'TSchema'.
    Type 'unknown' is not assignable to type 'TNull'.
      Type 'unknown' is not assignable to type '{ type: "null"; }'.
        Type 'U' is not assignable to type 'TNull'.
          Type 'unknown' is not assignable to type 'TNull'.
            Type 'unknown' is not assignable to type '{ type: "null"; }'.
              Type 'U' is not assignable to type '{ type: "null"; }'.
                Type 'unknown' is not assignable to type '{ type: "null"; }'.

Works fine in 3.8.3

[Andarist]

This getReducedType call is responsible for the circularity.

1. we are in the getResolvedBaseConstraint for U and that creates a recursion stack
2. that leads to getInferredTypeParameterConstraint for U
3. and eventually circles back to getResolvedBaseConstraint through that mentioned getReducedType call

Not a lot of cases break without that getReducedType call (mainly types like Partial<never> are not reduced to just never) so perhaps it could be replaced with a different strategy. 🤔 Or maybe under certain conditions we could detect the cycle and skip re-requesting that constraint?