Analyzer function type parameters are not correctly scoped

[leafpetersen]

In the following, G and H are entirely equivalent, and neither of them are subtype related to F<T> for non-trivial T, but F<T> and G (but not H) are considered assignable.

typedef F<T> = int Function<S>(T x);
typedef G = int Function<T>(T x);
typedef H = int Function<Q>(Q x);

void test(bool b) {
   int Function<T>(F<T> f) f1;
   int Function<T>(G f) f2;
   int Function<T>(H f) f3;
   f1 = f2; // Should be error, IS NOT
   f1 = f3; // Should be error, is
   f2 = f1; // Should be error, IS NOT
   f2 = f3; // Should be ok, is
   f3 = f1; // Should be error, is
   f3 = f2; // Should be ok, is
}

[jmesserly]

this might be related: #30375

[jmesserly]

working through the types:

f1 is <T> (<S> T -> int ) -> int
f2 is <T> (<T'> T' -> int ) -> int
f3 is <T> (<Q> Q -> int ) -> int

The fact that f2 has the problems is scary... f2 and f3 should be identical.

We did have a capture bug here: #29778 (comment) ... any time we create a synthetic function type we hit major problems with substitutions. That could be related.

Another problem that scared me recently when reading the code, is whether our "fresh" type variables are actually treated as non-equal to existing type variables or to other "fresh" type variables that have the same name. I worried that synthetic type variables might be falling back to name-based equality. (analyzer uses name-based equality for elements, where the name is "fully-qualified". but I don't know if this can distinguish type variables declared in the same scope, like each T for f1/f2/f3, or if it takes into account synthetic type variables that don't have any parent chain.)

[jmesserly]

@leafpetersen I can help look into this if you have not started already

[leafpetersen]

All yours if you want it, though not sure it's top priority. Just getting a test written to be sure that the new front end does the right thing might be a good start.

[srawlins]

This is now fixed. In the example, there are errors reported on lines 9, 10, 11, and 13:

Analyzing 30379.dart...
  error • A value of type 'int Function<T₀>(int Function<T>(T))' can't be assigned to a variable of type 'int Function<T>(int Function<S>(T))'. • 30379.dart:9:9 • invalid_assignment
  error • A value of type 'int Function<T>(int Function<Q>(Q))' can't be assigned to a variable of type 'int Function<T>(int Function<S>(T))'. • 30379.dart:10:9 • invalid_assignment
  error • A value of type 'int Function<T>(int Function<S>(T))' can't be assigned to a variable of type 'int Function<T₀>(int Function<T>(T))'. • 30379.dart:11:9 • invalid_assignment
  error • A value of type 'int Function<T>(int Function<S>(T))' can't be assigned to a variable of type 'int Function<T>(int Function<Q>(Q))'. • 30379.dart:13:9 • invalid_assignment
4 errors found.