[extension types] Specify that a method shadows an "inherited" setter, and vice versa (#3470)

This PR changes the extension types feature specification such that a method declaration in an extension type will shadow an "inherited" setter, and vice versa. This eliminates a setter/method conflict which couldn't otherwise be resolved.

Author: eernstg

accepted/future-releases/extension-types/feature-specification.md

@@ -15,6 +15,11 @@ information about the process, including in their change logs.
 [1]: https://github.com/dart-lang/language/blob/master/working/1426-extension-types/feature-specification-views.md
 [2]: https://github.com/dart-lang/language/blob/master/working/extension_structs/overview.md
 
+2023.11.14
+  - Specify that a method declaration will shadow an otherwise "inherited"
+    setter with the same basename, and vice versa. This eliminates a
+    method/setter conflict that would otherwise be impossible to avoid.
+
 2023.10.31
   - Simplify the rules about the relationship between extension types and the
     types `Object` and `Object?`.
@@ -516,14 +521,43 @@ type declaration, but this section is concerned with invocations of
 extension type members.*
 
 We need to introduce a concept that is similar to existing concepts
-for regular classes.
+for regular classes, namely that an extension type _has_ a certain member.
+
+First, we say that an extension type member declaration _DM_ _precludes_ an
+extension type member declaration _DM2_ if they have the same name, or the
+basename of _DM_ is the same as the basename of _DM2_, and one of _DM_ and
+_DM2_ is a setter declaration, and the other is a method declaration.
+
+Moreover, we say that an extension type member declaration _DM_ _precludes_
+a non-extension type member signature `m` if they have the same name, or
+the basename of _DM_ is the same as the basename of `m`, and _DM_ is a
+setter declaration and `m` is a method signature, or _DM_ is a method
+declaration and `m` is a setter signature.
+
+*We use this concept with superinterfaces of DM. DM may have multiple
+non-extension type superinterfaces, and they may differ with respect to the
+precise member signature for a given member name, but they will not differ
+with respect to the kind: `m` may be a method or a setter or a getter, but
+it cannot be, e.g., a method in one superinterface and a getter in another
+one. Hence, it is well-defined to talk about `m` being a method or a setter
+signature even though there may be multiple member signatures with the
+given name.*
 
 We say that an extension type declaration _DV_ _has_ an extension type
-member named `n` in the case where _DV_ declares a member named `n`, and in
-the case where _DV_ has no such declaration, but _DV_ has a direct
-extension type superinterface `V` that has an extension type member named
-`n`. In both cases, when this is unique, _the extension type member
-declaration named `n` that DV has_ is said declaration.
+member named `n` in the cases where:
+
+- _DV_ declares a member named `n`.
+- _DV_ has no such declaration, but _DV_ has a direct extension type
+  superinterface `V` that has an extension type member named `n` due to a
+  member declaration _DM2_, and _DV_ does not declare a member that
+  precludes _DM2_.
+
+*Note that it is well-defined which member declaration causes an extension
+type to have a given extension type member because a compile-time error
+occurs whenever this is ambiguous: Either _DV_ contains a declaration
+named `n`, or at most one superinterface has an extension type member named
+`n`, which is then (by induction) due to a uniquely determined
+extension type member declaration.*
 
 The type (function type for a method, return type for a getter) of this
 declaration relative to this invocation is determined by repeatedly
@@ -534,10 +568,14 @@ type parameters of the extension type into the type of that declaration.
 
 Similarly, we say that an extension type declaration _DV_ _has_ a
 non-extension type member named `n` in the case where _DV_ does not declare
-a member named `n`, but _DV_ has a direct extension type superinterface `V`
-that has a non-extension type member named `n`, or _DV_ has a direct
-non-extension type superinterface `T` whose interface contains a member
-signature named `n`.
+a member named `n`, and one of the following criteria is satisfied:
+
+- _DV_ has a direct extension type superinterface `V` that has a
+  non-extension type member with signature `m` and name `n`, and _DV_ does
+  not declare a member that precludes `m`.
+- _DV_ has a direct non-extension type superinterface whose interface
+  contains a member signature `m` named `n`, and _DV_ does not declare a
+  member that precludes `m`.
 
 The member signature of such a member is the combined member signature of
 all non-extension type members named `n` that _DV_ has, again using a