Merge pull request #15751 from dotty-staging/backport-15658

Backport #15658: Harden erasure of TermRefs

Author: Kordyjan

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

@@ -602,7 +602,9 @@ class TypeErasure(sourceLanguage: SourceLanguage, semiEraseVCs: Boolean, isConst
       else if (tp.isRepeatedParam) apply(tp.translateFromRepeated(toArray = sourceLanguage.isJava))
       else if (semiEraseVCs && isDerivedValueClass(tycon.classSymbol)) eraseDerivedValueClass(tp)
       else apply(tp.translucentSuperType)
-    case _: TermRef | _: ThisType =>
+    case tp: TermRef =>
+      this(underlyingOfTermRef(tp))
+    case _: ThisType =>
       this(tp.widen)
     case SuperType(thistpe, supertpe) =>
       SuperType(this(thistpe), this(supertpe))
@@ -687,6 +689,19 @@ class TypeErasure(sourceLanguage: SourceLanguage, semiEraseVCs: Boolean, isConst
       tp
   }
 
+  /** Widen term ref, skipping any `()` parameter of an eventual getter. Used to erase a TermRef.
+   *  Since getters are introduced after erasure, one would think that erasing a TermRef
+   *  could just use `widen`. However, it's possible that the TermRef got read from a class
+   *  file after Getters (i.e. in the backend). In that case, the reference will not get
+   *  an earlier denotation even when time travelling forward to erasure. Hence, we
+   *  need to take the extra precaution of going from nullary method types to their resuls.
+   *  A test case where this is needed is pos/i15649.scala, which fails non-deterministically
+   *  if `underlyingOfTermRef` is replaced by `widen`.
+   */
+  private def underlyingOfTermRef(tp: TermRef)(using Context) = tp.widen match
+    case tpw @ MethodType(Nil) if tp.symbol.isGetter => tpw.resultType
+    case tpw => tpw
+
   private def eraseArray(tp: Type)(using Context) = {
     val defn.ArrayOf(elemtp) = tp: @unchecked
     if (isGenericArrayElement(elemtp, isScala2 = sourceLanguage.isScala2)) defn.ObjectType
@@ -832,7 +847,7 @@ class TypeErasure(sourceLanguage: SourceLanguage, semiEraseVCs: Boolean, isConst
       case JavaArrayType(elem) =>
         sigName(elem) ++ "[]"
       case tp: TermRef =>
-        sigName(tp.widen)
+        sigName(underlyingOfTermRef(tp))
       case ExprType(rt) =>
         sigName(defn.FunctionOf(Nil, rt))
       case tp: TypeVar =>

tests/pos/i15649.scala

@@ -0,0 +1,11 @@
+trait ConfigSourceModule:
+  object ConfigSource:
+    class R
+
+object M extends ConfigSourceModule
+
+object Foo:
+  implicit class FromConfigSource(c: M.ConfigSource.type)
+
+object FooBar:                                      // problem disappears if we rename as `Bar`
+  def foo: M.ConfigSource.R = new M.ConfigSource.R  // problem disappears if we use `???` as rhs