Use MirrorSource.reduce result for companionPath

Fixes #20187

Author: EugeneFlesselle

compiler/src/dotty/tools/dotc/ast/tpd.scala

@@ -471,26 +471,6 @@ object tpd extends Trees.Instance[Type] with TypedTreeInfo {
     case ConstantType(value) => Literal(value)
   }
 
-  /** A path that corresponds to the given type `tp`. Error if `tp` is not a refinement
-   *  of an addressable singleton type.
-   */
-  def pathFor(tp: Type)(using Context): Tree = {
-    def recur(tp: Type): Tree = tp match {
-      case tp: NamedType =>
-        tp.info match {
-          case TypeAlias(alias) => recur(alias)
-          case _: TypeBounds => EmptyTree
-          case _ => singleton(tp)
-        }
-      case tp: TypeProxy => recur(tp.superType)
-      case _ => EmptyTree
-    }
-    recur(tp).orElse {
-      report.error(em"$tp is not an addressable singleton type")
-      TypeTree(tp)
-    }
-  }
-
   /** A tree representing a `newXYZArray` operation of the right
    *  kind for the given element type in `elemTpe`. No type arguments or
    *  `length` arguments are given.

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

@@ -116,21 +116,6 @@ class TypeUtils {
 
     def refinedWith(name: Name, info: Type)(using Context) = RefinedType(self, name, info)
 
-    /** The TermRef referring to the companion of the underlying class reference
-     *  of this type, while keeping the same prefix.
-     */
-    def mirrorCompanionRef(using Context): TermRef = self match {
-      case AndType(tp1, tp2) =>
-        val c1 = tp1.classSymbol
-        val c2 = tp2.classSymbol
-        if c1.isSubClass(c2) then tp1.mirrorCompanionRef
-        else tp2.mirrorCompanionRef // precondition: the parts of the AndType have already been checked to be non-overlapping
-      case self @ TypeRef(prefix, _) if self.symbol.isClass =>
-        prefix.select(self.symbol.companionModule).asInstanceOf[TermRef]
-      case self: TypeProxy =>
-        self.superType.mirrorCompanionRef
-    }
-
     /** Is this type a methodic type that takes at least one parameter? */
     def takesParams(using Context): Boolean = self.stripPoly match
       case mt: MethodType => mt.paramNames.nonEmpty || mt.resType.takesParams

compiler/src/dotty/tools/dotc/typer/Synthesizer.scala

@@ -15,6 +15,7 @@ import transform.SyntheticMembers.*
 import util.Property
 import ast.Trees.genericEmptyTree
 import annotation.{tailrec, constructorOnly}
+import ast.tpd
 import ast.tpd.*
 import Synthesizer.*
 import sbt.ExtractDependencies.*
@@ -265,10 +266,10 @@ class Synthesizer(typer: Typer)(using @constructorOnly c: Context):
       .refinedWith(tpnme.MirroredType, TypeAlias(mirroredType))
       .refinedWith(tpnme.MirroredLabel, TypeAlias(ConstantType(Constant(label.toString))))
 
-  /** A path referencing the companion of class type `clsType` */
-  private def companionPath(clsType: Type, span: Span)(using Context) =
-    val ref = pathFor(clsType.mirrorCompanionRef)
-    assert(ref.symbol.is(Module) && (clsType.classSymbol.is(ModuleClass) || (ref.symbol.companionClass == clsType.classSymbol)))
+  /** A path referencing the companion of `cls` with prefix `pre` */
+  private def companionPath(pre: Type, cls: Symbol, span: Span)(using Context) =
+    val ref = tpd.ref(TermRef(pre, cls.companionModule))
+    assert(ref.symbol.is(Module) && (cls.is(ModuleClass) || ref.symbol.companionClass == cls))
     ref.withSpan(span)
 
   private def checkFormal(formal: Type)(using Context): Boolean =
@@ -428,7 +429,7 @@ class Synthesizer(typer: Typer)(using @constructorOnly c: Context):
           .refinedWith(tpnme.MirroredElemLabels, TypeAlias(elemsLabels))
       }
       val mirrorRef =
-        if cls.useCompanionAsProductMirror then companionPath(mirroredType, span)
+        if cls.useCompanionAsProductMirror then companionPath(pre, cls, span)
         else if defn.isTupleClass(cls) then newTupleMirror(typeElems.size) // TODO: cls == defn.PairClass when > 22
         else anonymousMirror(monoType, MirrorImpl.OfProduct(pre), span)
       withNoErrors(mirrorRef.cast(mirrorType).withSpan(span))
@@ -438,7 +439,7 @@ class Synthesizer(typer: Typer)(using @constructorOnly c: Context):
       case Right(msrc) => msrc match
         case MirrorSource.Singleton(_, tref) =>
           val singleton = tref.termSymbol // prefer alias name over the orignal name
-          val singletonPath = pathFor(tref).withSpan(span)
+          val singletonPath = tpd.singleton(tref).withSpan(span)
           if tref.classSymbol.is(Scala2x) then // could be Scala 3 alias of Scala 2 case object.
             val mirrorType = formal.constrained_& {
               mirrorCore(defn.Mirror_SingletonProxyClass, mirroredType, mirroredType, singleton.name)
@@ -571,7 +572,7 @@ class Synthesizer(typer: Typer)(using @constructorOnly c: Context):
             .refinedWith(tpnme.MirroredElemLabels, TypeAlias(labels))
         }
       val mirrorRef =
-        if cls.useCompanionAsSumMirror then companionPath(mirroredType, span)
+        if cls.useCompanionAsSumMirror then companionPath(pre, cls, span)
         else anonymousMirror(monoType, MirrorImpl.OfSum(childPres), span)
       withNoErrors(mirrorRef.cast(mirrorType))
     else if acceptableMsg.nonEmpty then

tests/pos/i20187/A_1.scala

@@ -0,0 +1,19 @@
+import scala.deriving.Mirror
+
+enum E:
+  case Foo1()
+  case Foo2()
+
+class Outer:
+  case class Inner()
+val o = new Outer
+
+type F = E.Foo1
+type G = Tuple.Head[E.Foo1 *: E.Foo2 *: EmptyTuple]
+type H = Tuple.Head[o.Inner *: EmptyTuple]
+type I = Tuple.Last[E *: EmptyTuple]
+
+def local =
+  case class Bar()
+  type B = Tuple.Head[Bar *: EmptyTuple]
+  summon[Mirror.Of[B]]

tests/pos/i20187/B_2.scala

@@ -0,0 +1,7 @@
+import scala.deriving.Mirror
+
+def Test =
+  summon[Mirror.Of[F]] // ok
+  summon[Mirror.Of[G]] // was crash
+  summon[Mirror.Of[H]] // was crash
+  summon[Mirror.Of[I]] // was crash