Regression in Type Inference engine in TypeScript 2.7.1

[benny-medflyt]

The following code works great in these TypeScript versions:

• 2.4.2
• 2.5.3
• 2.6.1
• 2.6.2

But it gives a compile error in these TypeScript versions:

• 2.7.1
• 2.7.2
• 2.8.0-dev.20180307

BugTest.ts

class Write {
    protected dummy: Write;
}

class Col<s, a> {
    protected dummy: [Col<s, a>, s, a];
}

class Table<Req, Def> {
    protected dummy: [Table<Req, Def>, Req, Def];
}

type MakeTable<T1 extends object, T2 extends object> = {
    [P in keyof T1]: Col<Write, T1[P]>;
} & {
        [P in keyof T2]: Col<Write, T2[P]>;
    };

class ConflictTarget<Cols> {
    public static tableColumns<Cols>(cols: (keyof Cols)[]): ConflictTarget<Cols> {
        return <any>{
            tableCols: cols.slice()
        };
    }

    protected dummy: [ConflictTarget<Cols>, Cols];
}



const bookTable: Table<BookReq, BookDef> = null as any

interface BookReq {
    readonly title: string;
    readonly serial: number;
}

interface BookDef {
    readonly author: string;
    readonly numPages: number | null;
}


function insertOnConflictDoNothing<Req extends object, Def extends object>(_table: Table<Req, Def>, _conflictTarget: ConflictTarget<Req & Def>): boolean {
    throw new Error();
}

function f() {
    insertOnConflictDoNothing(bookTable, ConflictTarget.tableColumns(["serial"]));  // <-- Compile error here
}

BugTest.ts(49,42): error TS2345: Argument of type 'ConflictTarget<{ serial: any; }>' is not assignable to parameter of type 'ConflictTarget<BookReq & BookDef>'.
  Type '{ serial: any; }' is not assignable to type 'BookReq & BookDef'.
    Type '{ serial: any; }' is not assignable to type 'BookReq'.
      Property 'title' is missing in type '{ serial: any; }'.

Manually adding the type argument allows the compile to succeed:

insertOnConflictDoNothing(bookTable, ConflictTarget.tableColumns<BookReq & BookDef>(["serial"]));

[RyanCavanaugh]

Looks like a change in inference priority - @ahejlsberg any thoughts?

[ahejlsberg]

Adding @sandersn. Looks like it is caused by #21271. We now infer from ["serial"] to (keyof Cols)[], producing the candidate { serial: any }. This candidate is given higher priority than the inference we make from the return type. Probably it should have lower priority, particularly since it is known to be incomplete (in that we don't have specific types for the properties).

[sandersn]

Fixed in #22525