TypeScript: Incorrect inference of arrow function with intersected mapped type argument under strictFunctionTypes

TypeScript Version: 3.6.0-dev.20190801

Search Terms: inference intersection type strictFunctionTypes

Code

type MappedIntersection<T extends {}> = { [P in keyof T]: string } & {};
type MappedRaw<T extends {}> = { [P in keyof T]: string };

// Case 1: Intersection + no arrow = succeed
type ToInferNoArrowIntersection<T extends {}> = {
    fn(params: MappedIntersection<T>): void;
};

type NoArrowIntersection<T extends ToInferNoArrowIntersection<any>> = void;
declare const case1: NoArrowIntersection<{
    fn: (params: MappedIntersection<{a: 1}>) => string,
}>;


// Case 2: No intersection + arrow = succeed
type ToInferArrowNoIntersection<T extends {}> = {
    fn: (params: MappedRaw<T>) => void;
};

type ArrowNoIntersection<T extends ToInferArrowNoIntersection<any>> = void;
declare const case2: ArrowNoIntersection<{
    fn: (params: MappedRaw<{a: 1}>) => void,
}>;


// Case 3: Intersection + arrow = fail
type ToInferArrowIntersection<T extends {}> = {
    fn: (params: MappedIntersection<T>) => void;
};

type ArrowIntersection<T extends ToInferArrowIntersection<any>> = void;
declare const case3: ArrowIntersection<{ // Fails here
    fn: (params: MappedIntersection<{a: 1}>) => void,
}>;

Expected behavior: Expected case 3 to succeed.

Actual behavior: Case 3 produces:

 Type '{ fn: (params: { a: string; }) => void; }' does not satisfy the constraint 'ToInferArrowIntersection<any>'.
  Types of property 'fn' are incompatible.
    Type '(params: { a: string; }) => void' is not assignable to type '(params: { [x: string]: string; }) => void'.
      Types of parameters 'params' and 'params' are incompatible.
        Property 'a' is missing in type '{ [x: string]: string; }' but required in type '{ a: string; }'.

Seems relevant that compilation succeeds without strictFunctionTypes

Playground Link: Playground

Related Issues: https://github.com/microsoft/TypeScript/issues/29123#issuecomment-453884098 https://github.com/microsoft/TypeScript/issues/31081