Generic type erased after type guard with strictNullChecks

[jgoz]

TypeScript Version: 2.0.2

Code

interface Pet {
    name: string;
}

function isPet(pet: any): pet is Pet {
    return typeof pet.name === "string";
}

export function speak<TPet extends Pet>(pet: TPet, voice: (pet: TPet) => string): string {
    if (!isPet(pet)) {
        throw new Error("Expected \"pet\" to be a Pet");
    }

    return voice(pet); // line 14
}

Expected behavior:
Module compiles without error when --strictNullChecks is enabled OR disabled.

Actual behavior:

• Module compiles without error when --strictNullChecks is disabled.

• The following error is produced when --strictNullChecks is enabled

  bug.ts(14,18): error TS2345: Argument of type 'Pet' is not assignable to parameter of type 'TPet'.
  

[ahejlsberg]

Issue here is that in the true branch of isPet(pet), we should keep type TPet extends Pet for pet instead of narrowing to type Pet because the former is a more specific type. It's an easy fix, I will put up a PR soon. BTW, this doesn't appear to have anything to do with --strictNullChecks, the error is reported in either mode.