Fix #2473: Check self for inner class redefinitions

Author: nicolasstucki

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

@@ -1316,12 +1316,27 @@ class Typer extends Namer with TypeAssigner with Applications with Implicits wit
         result
       }
 
+    /** Checks if one of the decls is a type with the same name as class type member in selfType */
+    def classExistsOnSelf(decls: Scope, selfType: Type): Boolean = {
+      if (!selfType.exists || (selfType.classSymbol eq cls)) false
+      else {
+        def memberInSelfButNotThis(decl: Symbol) =
+          selfType.member(decl.name).symbol.filter(other => other.isClass && other.owner != cls)
+        decls.iterator.filter(_.isType).foldLeft(false) { (foundRedef, decl) =>
+          val other = memberInSelfButNotThis(decl)
+          if (other.exists)
+            ctx.error(CannotHaveSameNameAs(decl, other), decl.pos)
+          foundRedef || other.exists
+        }
+      }
+    }
+
     completeAnnotations(cdef, cls)
     val constr1 = typed(constr).asInstanceOf[DefDef]
     val parentsWithClass = ensureFirstIsClass(parents mapconserve typedParent, cdef.namePos)
     val parents1 = ensureConstrCall(cls, parentsWithClass)(superCtx)
     val self1 = typed(self)(ctx.outer).asInstanceOf[ValDef] // outer context where class members are not visible
-    if (self1.tpt.tpe.isError) {
+    if (self1.tpt.tpe.isError || classExistsOnSelf(cls.unforcedDecls, self1.tpt.tpe)) {
       // fail fast to avoid typing the body with an error type
       cdef.withType(UnspecifiedErrorType)
     } else {

tests/neg/i2473.scala

@@ -0,0 +1,7 @@
+trait Foo { trait Inner }
+trait Bar { foo: Foo =>
+  type Inner <: foo.Inner // error: type Inner cannot have the same name as trait Inner in trait Foo -- class definitions cannot be overridden
+}
+trait Baz { baz: Foo =>
+  class Inner // error: class Inner cannot have the same name as trait Inner in trait Foo -- class definitions cannot be overridden
+}