Type promotion failure in for loop with type parameters

[stereotype441]

The following code:

void f<T>(Iterable<T> Function() g, void Function(T) h) {
  for (var x in g()) {
    if (x is String) {
      h(x);
    }
  }
}

Produces the analyzer error:

error • The argument type 'String' can't be assigned to the parameter type 'T'. • .../test.dart:4:9 • argument_type_not_assignable

There should be no error; the type of x should be inferred to be T, and the if test should promote it to T & String, which is a valid subtype of h's parameter type, T.

The problem bisects to 7c1fa84.
This bug is blocking an internal roll, so we're considering it blocking for D27.

[stereotype441]

Found the problem. When inferring the type of the loop variable, we were failing to account for the fact that the iterable expresion might be rewritten during analysis (in this case, it is being rewritten from a method invocation to a function expression invocation), so we were losing the inferred type; hence x's type was being inferred to be dynamic, and so it was being promoted to String, which is indeed not assignable to T.

Fix is out for review: https://dart-review.googlesource.com/c/sdk/+/123483