Dart does not throw error if function type alias type parameter bound type is not instantiated

[iarkh]

Dart VM version: 2.9.0-17.0.dev (dev) (Thu Jun 18 10:22:39 2020 +0200) on "windows_x64"

The following source code example passes with dart and throws a compile error with analyzer:

class A<X extends A<X>> {}
typedef AAlias = Function<X extends A>();
main() {}

Seems like dart should throw a compile error here too.

Sample output is:

$> dart test.dart

$> dartanalyzer test.dart
Analyzing test.dart...
error - Type parameter bound types must be instantiated. - test.dart:2:37 - not_instantiated_bound
1 error found.

[a-siva]

/cc @crelier

[alexmarkov]

/cc @leafpetersen @johnniwinther

[johnniwinther]

Likely related to #41684

[leafpetersen]

cc @eernstg

[eernstg]

The type Function<X extends A>() is a compile-time error, because it declares a type parameter (X) with a bound (A) which is raw and doesn't have simple bounds.

So there's a missing check in the CFE for having simple bounds.

The issue arises independently of the type alias:

class A<X extends A<X>> {}
Function<X extends A>() f = throw 1;
main() {}

This program is accepted by the CFE, even though A is raw, does not have simple bounds, and is used as a type parameter bound.

[eernstg]

Cf. our discussions earlier today: Reading the error specification again, it does actually apply for formal type parameters declared by any kind of entity (in particular, it applies for formal type parameters of a function declaration and of a function type). So there is no need to change the specification after all.

[eernstg]

Re-checking: This issue should remain open at this point, dart still does not report the error (that the raw type A cannot be a bound, because it does not have simple bounds).

[chloestefantsova]

I'm working on a fix for that at https://dart-review.googlesource.com/c/sdk/+/193662.