Bug in subtyping with records and FutureOr

[lrhn]

Running on tip-of-tree VM (5b3f387).

Example:

import "dart:async";
void main() {
  var o = (null, null, null, 42);
  (int?, String?, bool?, int?) nullable = o;
  FutureOr<(int?, String?, bool?, int?)> futornullable1 = nullable;
  FutureOr<(int?, String?, bool?, int?)> futornullable2 = o;  
}

yields

recchk.dart:6:59: Error: A value of type '(Null, Null, Null, int)' can't be assigned to a variable of type 'FutureOr<(int?, String?, bool?, int?)>'.
  FutureOr<(int?, String?, bool?, int?)> futornullable2 = o;
                                                          ^

There is no error on the previous lines, not even the = nullable one where the same value is assigned, with a different type.

Changing the 42 to Null makes no difference.

Changing the other values to non-nullable makes no difference:

recchk.dart:6:59: Error: A value of type '(int, String, bool, int)' can't be assigned to a variable of type 'FutureOr<(int?, String?, bool?, int?)>' because 'int?' is nullable and 'int' isn't.
  FutureOr<(int?, String?, bool?, int?)> futornullable2 = o;
                                                          ^

The argument here seems backwards, since we're assigning int to int?, so maybe a type-check is reversed somewhere, or maybe just a nullability check.

If I try to add:

  var fo = Future.value(o);
  FutureOr<(int?, String?, bool?, int?)> futornullable3 = fo;

I get an error in the first line, saying:

recchk.dart:8:25: Error: The argument type '(int, String, bool, int)' can't be assigned to the parameter type 'FutureOr<(int, String, bool, int)>?' because '(int, String, bool, int)?' is nullable and '(int, String, bool, int)' isn't.
  var fo = Future.value(o);
                        ^

Again, the record type cannot be assigned to FutureOr of the same record type, because ... something is nullable?

(And yes, it blocks my CL 😉)