Type promotion is confusing on type parameters

[vsmenon]

The following is showing a strong mode error in foo but not foo2.

void bar(List l) {
  print(l.length);
}

/*=T*/ foo/*<T>*/(/*=T*/ arg) {
  if (arg is List) {
    bar(arg);
  }
}

foo2(arg) {
  if (arg is List) {
    bar(arg);
  }
}

void main() {
  foo([1, 2, 3]);
}

The error is on the call to bar in foo:

[error] The argument type 'T' cannot be assigned to the parameter type 'List'.

@sigmundch was running into this

[jmesserly]

chatted with folks ( @leafpetersen @sigmundch ) but this is by design in the language right now.

[vsmenon]

Changed 'broken' to 'confusing'.

This is actually not a strong mode specific issue. Type promotion - by design - doesn't kick in because T and List are unrelated.

[vsmenon]

This is one of the issues already covered in #25565 - closing in favor of that.