More explanations and tests

Author: odersky

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

@@ -1083,11 +1083,15 @@ object Types {
     /** Like `dealiasKeepAnnots`, but keeps only refining annotations */
     final def dealiasKeepRefiningAnnots(implicit ctx: Context): Type = dealias1(keepIfRefining)
 
-    /** If this is a synthetic opaque type, its opaque alias, otherwise the type itself */
+    /** If this is a synthetic opaque type seen from inside the opaque companion object,
+     *  its opaque alias, otherwise the type itself.
+     */
     final def followSyntheticOpaque(implicit ctx: Context): Type = this match {
       case tp: TypeProxy if tp.typeSymbol.is(SyntheticOpaque) =>
-        val AndType(alias, _) = tp.superType
-        alias
+        tp.superType match {
+          case AndType(alias, _) => alias   // in this case we are inside the companion object
+          case _ => this
+        }
       case _ => this
     }
 

tests/neg/i5481.scala

@@ -0,0 +1,20 @@
+object TypeAlias {
+
+  type Set[A] = A => Boolean
+
+  object Set {
+    def singleton[A](a: A): Set[A] = _ == a
+  }
+}
+
+object OpaqueType {
+
+  opaque type Set[A] = A => Boolean
+
+  object Set {
+    def singleton[A](a: A): Set[A] = _ == a
+    def singleton0[A](a: A): Set[A] = (_: A) == a
+  }
+
+  def singleton[A](a: A): Set[A] = _ == a       // error: missing parameter type
+}

tests/pos/i5481.scala

@@ -3,7 +3,7 @@ object TypeAlias {
   type Set[A] = A => Boolean
 
   object Set {
-    def singleton[A](a: A): Set[A] = _ == a       // Works
+    def singleton[A](a: A): Set[A] = _ == a
   }
 }
 
@@ -12,7 +12,8 @@ object OpaqueType {
   opaque type Set[A] = A => Boolean
 
   object Set {
-    def singleton[A](a: A): Set[A] = _ == a     // Does not compile
-    def singleton0[A](a: A): Set[A] = (_: A) == a // Works
+    def singleton[A](a: A): Set[A] = _ == a
+    def singleton0[A](a: A): Set[A] = (_: A) == a
   }
+
 }