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Analyzer fails to recognize subtype relation in F-bounded type parameter. #34728

lrhn opened this Issue Oct 9, 2018 · 0 comments


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lrhn commented Oct 9, 2018


// Arbitrary generic class.                                                     
class A<X> {}

class B<X extends A<A<X>>> {}
class C<X extends A<X>> extends B<X> {}  // Analyzer error: 

// Example class.
class E extends A<E> {}

main() {
 A<A<E>> e1 = E();

The analyzer reports the following error:

Analyzing test.dart...                                                          
  error • 'X' doesn't extend 'A<A<X>>' at test.dart:5:35 • type_argument_not_matching_bounds

However, since X <: A<X>, by covariance of generics we also know that A<X> <: A<A<X>>, and by transitivity we know that X <: A<A<X>>, so the error is incorrect.

The code runs without issues on the CFE.

(Might be related to #34726).

@bwilkerson bwilkerson added this to the Dart2.2 milestone Oct 10, 2018

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