Record<string, string> is assignable to Record<T, string>

[bela53]

TypeScript Version: 3.8.3-3.9.0-dev.20200324

Search Terms:
mapped type, record, assignability, assignability compatibility, generic type, type parameter, generic mapped type, string index signature

Code

function foo<T extends string>(
    a1: Record<T, string>, a2: Record<string, string>, a3: { [P in T]: string }
): Record<T, string> {
    a2 = a1 // no error, expected
    a3 = a2 // error, expected

    a1 = a2 // no error, not expected (!)
    return a2 // no error, not expected (!)
};

Expected behavior:
a2: Record<string, string> is not assignable to a1: Record<T, string>. Also returned a2 is invalid.

Actual behavior:
a1 = a2 compiles and returned value is assignable to declared return type.

Playground Link:
here

Related Issues:
#28798

[jack-williams]

Duplicate of #29698, I think.