It is disabled when augmentations feature is enabled.
This is not a breaking change, because CFE did report this error.
The relevant reason is indirect, not a single sentence saying “on A
implements A is forbidden”.
Spec chain:
1. In dartLangSpec.tex:6019, a mixin declaration is modeled as:
`mixin N<T...> on T1, ..., Tn implements I1, ..., Ik { ... }`
2. In dartLangSpec.tex:6065, the mixin interface M_I is defined as if
by:
`abstract class N<T...> implements T1, ..., Tn, I1, ..., Ik { ... }`
and dartLangSpec.tex:6078 says it is a compile-time error for the mixin
if that synthetic class declaration would be a compile-time error.
3. In dartLangSpec.tex:5189, class superinterfaces say:
It is a compile-time error if two elements in the type list of the
IMPLEMENTS clause of a class C specifies the same type T.
So:
`mixin M on A implements A {}`
is checked like:
`abstract class M implements A, A {}`
which violates the duplicate implements type rule.
Change-Id: I29e5fa17c47d111346ef6a6e321ab5dbab985ad1
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/503501
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Commit-Queue: Konstantin Shcheglov <scheglov@google.com>
Reviewed-by: Paul Berry <paulberry@google.com>
Konstantin Shcheglov
5ed538a532