Allow ambiguous references under Scala-2 mode

And offer an automatic rewrite rule.

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

@@ -125,9 +125,6 @@ class Typer extends Namer
     val refctx = ctx
     val noImports = ctx.mode.is(Mode.InPackageClauseName)
 
-    def ambiguous(newPrec: BindingPrec, prevPrec: BindingPrec, prevCtx: Context)(using Context) =
-      refctx.error(AmbiguousReference(name, newPrec, prevPrec, prevCtx), posd.sourcePos)
-
     /** A symbol qualifies if it really exists and is not a package class.
      *  In addition, if we are in a constructor of a pattern, we ignore all definitions
      *  which are methods and not accessors (note: if we don't do that
@@ -173,7 +170,7 @@ class Typer extends Namer
           found
         else
           if !scala2pkg && !previous.isError && !found.isError then
-            ambiguous(newPrec, prevPrec, prevCtx)
+            refctx.error(AmbiguousReference(name, newPrec, prevPrec, prevCtx), posd.sourcePos)
           previous
 
       /** Recurse in outer context. If final result is same as `previous`, check that it
@@ -317,7 +314,13 @@ class Typer extends Namer
               if scope.lookup(name).exists then
                 val symsMatch = scope.lookupAll(name).exists(denot.containsSym)
                 if !symsMatch then
-                  ambiguous(Definition, Inheritance, prevCtx)(using outer)
+                  refctx.errorOrMigrationWarning(
+                    AmbiguousReference(name, Definition, Inheritance, prevCtx)(using outer),
+                    posd.sourcePos)
+                  if ctx.scala2CompatMode then
+                    patch(Span(posd.span.start),
+                      if prevCtx.owner == refctx.owner.enclosingClass then "this."
+                      else s"${prevCtx.owner.name}.this.")
               else
                 checkNoOuterDefs(denot, outer, prevCtx)
 

tests/neg/ambiref.check

@@ -22,3 +22,11 @@ longer explanation available when compiling with `-explain`
    |              and inherited subsequently in anonymous class D {...}
 
 longer explanation available when compiling with `-explain`
+-- [E049] Reference Error: tests/neg/ambiref.scala:25:16 ---------------------------------------------------------------
+25 |        println(y)  // error
+   |                ^
+   |                Reference to y is ambiguous,
+   |                it is both defined in method c
+   |                and inherited subsequently in class E
+
+longer explanation available when compiling with `-explain`

tests/neg/ambiref.scala

@@ -17,6 +17,14 @@ object test2:
       println(y)  // error
 
 object test3:
+  def c(y: Float) =
+    class D:
+      val y = 2
+    class E extends D:
+      class F:
+        println(y)  // error
+
+object test4:
 
   class C:
     val x = 0
@@ -26,7 +34,7 @@ object test3:
       def x(y: Int) = 3
       val y: Int = this.x // OK
       val z: Int = x      // OK
-end test3
+end test4
 
 val global = 0
 class C:

tests/pos-scala2/rewrites.scala

@@ -34,3 +34,26 @@ class Stream[+A] {
 
 }
 
+object test2 {
+  def c(y: Float) = {
+    class D {
+      val y = 2
+    }
+    new D {
+      println(y)
+    }
+  }
+}
+
+object test3 {
+  def c(y: Float) = {
+    class D {
+      val y = 2
+    }
+    class E extends D {
+      class F {
+        println(y)
+      }
+    }
+  }
+}