Fix pattern generation in "ordinal" mirror method (scala#17570)

The "ordinal" method generated non-sensical patterns if the cases of a
sealed trait were found in the trait itself. In that case the ordinal
method would be placed in the companion object, but still tried to
access the cases via the `this` of the companion class.

We are now more careful and fall back to type projections in
comparisons.

Fixes scala#17556

compiler/src/dotty/tools/dotc/core/SymDenotations.scala

@@ -1509,16 +1509,29 @@ object SymDenotations {
      *  See tests/pos/i10769.scala
      */
      def reachableTypeRef(using Context) =
-       TypeRef(owner.reachableThisType, symbol)
+       TypeRef(owner.reachablePrefix, symbol)
 
-    /** Like termRef, but objects in the prefix are represented by their singleton type,
+    /** The reachable typeRef with wildcard arguments for each type parameter */
+    def reachableRawTypeRef(using Context) =
+      reachableTypeRef.appliedTo(typeParams.map(_ => TypeBounds.emptyPolyKind))
+
+    /** Like termRef, if it is addressable from the current context,
+     *  but objects in the prefix are represented by their singleton type,
      *  this means we output `pre.O.member` rather than `pre.O$.this.member`.
      *
      *  This is required to avoid owner crash in ExplicitOuter.
      *  See tests/pos/i10769.scala
+     *
+     *  If the reference is to an object that is not accessible from the
+     *  current context since the object is nested in a class that is not an outer
+     *  class of the current context, fall back to a TypeRef to the module class.
+     *  Test case is tests/pos/i17556.scala.
+     *  If the reference is to some other inaccessible object, throw an AssertionError.
      */
-    def reachableTermRef(using Context) =
-      TermRef(owner.reachableThisType, symbol)
+    def reachableTermRef(using Context): Type = owner.reachablePrefix match
+      case pre: SingletonType => TermRef(pre, symbol)
+      case pre if symbol.is(ModuleVal) => TypeRef(pre, symbol.moduleClass)
+      case _ => throw AssertionError(i"cannot compute path to TermRef $this from ${ctx.owner}")
 
     /** Like thisType, but objects in the type are represented by their singleton type,
      *  this means we output `pre.O.member` rather than `pre.O$.this.member`.
@@ -1533,6 +1546,18 @@ object SymDenotations {
       else
         ThisType.raw(TypeRef(owner.reachableThisType, symbol.asType))
 
+    /** Like `reachableThisType`, except if that would refer to a class where
+     *  the `this` cannot be accessed. In that case, fall back to the
+     *  rawTypeRef of the class. E.g. instead of `A.this.X` where `A.this`
+     *  is inaccessible, use `A#X`.
+     */
+    def reachablePrefix(using Context): Type = reachableThisType match
+      case pre: ThisType
+      if !pre.cls.isStaticOwner && !ctx.owner.isContainedIn(pre.cls) =>
+        pre.cls.reachableRawTypeRef
+      case pre =>
+        pre
+
     /** The variance of this type parameter or type member as a subset of
      *  {Covariant, Contravariant}
      */

compiler/src/dotty/tools/dotc/transform/SymUtils.scala

@@ -333,10 +333,6 @@ object SymUtils:
       else owner.isLocal
     }
 
-    /** The reachable typeRef with wildcard arguments for each type parameter */
-    def reachableRawTypeRef(using Context) =
-      self.reachableTypeRef.appliedTo(self.typeParams.map(_ => TypeBounds.emptyPolyKind))
-
     /** Is symbol a type splice operation? */
     def isTypeSplice(using Context): Boolean =
       self == defn.QuotedType_splice

compiler/src/dotty/tools/dotc/transform/SyntheticMembers.scala

@@ -552,7 +552,7 @@ class SyntheticMembers(thisPhase: DenotTransformer) {
             .map((pre, child) => rawRef(child).asSeenFrom(pre, child.owner))
           case _ =>
             cls.children.map(rawRef)
-      end computeChildTypes
+
       val childTypes = computeChildTypes
       val cases =
         for (patType, idx) <- childTypes.zipWithIndex yield

tests/pos/i17556.scala

@@ -0,0 +1,8 @@
+sealed trait A {
+  // must be `object` or `case class`
+  object X extends A
+  case class Y() extends A
+}
+
+// companion object must exist
+object A