well-formedness of compound types: spec is stricter than implementation

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from the spec:

A compound type T1 with...with Tn {R} represents objects with members as
given in the component types T1 ,..., Tn and the refinement {R}. A refinement {R}
contains declarations and type definitions. If a declaration or definition overrides a
declaration or definition in one of the component types T1 ,..., Tn , the usual rules
for overriding (�5.1.4) apply;

5.1.4 Overriding
A member M of class C that matches (�5.1.3) a non-private member M' of a base
class of C is said to override that member. In this case the binding of the overriding
member M must subsume (�3.5.2) the binding of the overridden member M' .

5.1.3 Definition 5.1.4 A member definition M matches a member definition M' , if M and
M' bind the same name, and one of following holds.

1. Neither M nor M' is a method definition.

3.5.2 A declaration or definition in some compound type of class type C subsumes an-
other declaration of the same name in some compound type or class type C' , if one
of the following holds.

• A type alias type t [T1 ,..., Tn ] = T subsumes a type alias
  type t [T1 ,..., Tn ] = T' if T ≡ T' .
• A type declaration type t [T1 ,..., Tn ] >: L <: U subsumes a type declaration
  type t [T1 ,..., Tn ] >: L′ <: U' if L′ <: L and U <: U' .

code examples that say otherwise:

object T {
  trait A {
    type X <: Int
  }
  
  type S = A{type X = String}
}

this should give a type error according the spec;
since it is not specified when compound types need to adhere to the rules from the first paragraph above,
it would be reasonable to assume they should always adhere
I guess it's a spec issue, not an implementation one
see also #3419 and neg/bug1275

object Bug1275 {
  trait Seq[+t] {
    type MyType[+t] <: Seq[t]

    def f: MyType[t]
  }

  // illegal abstract type member refinement: changes the arity of MyType
  // the error is pretty strange, since the compiler forms the illegal type xs.MyType[a] anyway
  def span[a, s <: Seq[a] { type MyType/*look ma, no type parameters!*/ <: s } ](xs: s): s
     = xs f
//        ^
// found   : xs.MyType[a]
// required: s  
}

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Imported From: https://issues.scala-lang.org/browse/SI-3512?orig=1
Reporter: @adriaanm

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@adriaanm said:
(In r22754) closes #3419: test files

omit check that wouldn't work with separate compilation,
not needed anymore because compiler has become more robust

the actual fix was committed as part of r22512, see #3374

also see #3512

no review

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@gkossakowski said:
Unassigning and rescheduling to M7 as previous deadline was missed.