3.6 regression: unions of callable types involving `this` types

TypeScript Version: Version 3.6.3, Version 3.7.0-dev.20190911

Search Terms: intersect function

Code

class A {
    public uniqueToA = "a";

    public getString(this: A): string {
        return "a";
    }
}

class B {
    public uniqueToB = "b";

    public getString(this: B): string {
        return "b";
    }
}

function f(model: A | B): string {
    return model.getString();
}

Expected behavior:

In Typescript 3.5.3, the model.getString() in function f has no error.

Actual behavior:

In Typescript 3.6.3 and 3.7.0-dev.20190911, the code above gets the following errors:

index.ts:18:12 - error TS2684: The 'this' context of type 'A | B' is not assignable to method's 'this' of type 'A & B'.
  Type 'A' is not assignable to type 'A & B'.
    Property 'uniqueToB' is missing in type 'A' but required in type 'B'.

18     return model.getString();
              ~~~~~

  index.ts:10:12
    10     public uniqueToB = "b";
                  ~~~~~~~~~
    'uniqueToB' is declared here.

Playground Link:
http://www.typescriptlang.org/play/#code/MYGwhgzhAECC0G8BQ1XQA4FcBGICWw0mAdngI6YCmAKgPbwC80ARGMwNxIppa4HQBzSgBcAysIBOeYgIAUwgBZ4IALjgBKNREnSBibmjQSRmCcRZtOhgL5JbSUJBgAhfYd75CJclTqumzNgcXO44noIi4lIy8kqq0M6a0NrResiGhsbCpuaBwTZ2XABmJMDCeLTmRbIAtrQAJpQgavAAPglJKbpuRiZm0HWNIAB0QmI6Meqc1kA

Related Issues: #32506

Reproduce the index.ts example with TypeScript 3.5.3, 3.6.3, and the referenced 3.7.0-dev version to confirm the regression. Trace the type-checking path for model.getString() and unions of callable types with this parameters; done means the example compiles without the TS2684 error while preserving the relevant this checks.