Check types for overriding conditions.

Closes #241 -- that took a while!

Author: odersky

Diff for: src/dotty/tools/dotc/typer/RefChecks.scala

@@ -172,8 +172,8 @@ object RefChecks {
       val sym1 = sym.underlyingSymbol
       def info = self.memberInfo(sym1)
       i"${if (showLocation) sym1.showLocated else sym1}${
-        if (sym1.isAliasType) i", which equals $info"
-        else if (sym1.isAbstractType) i" with bounds $info"
+        if (sym1.isAliasType) i", which equals ${info.bounds.hi}"
+        else if (sym1.isAbstractType) i" with bounds$info"
         else if (sym1.is(Module)) ""
         else if (sym1.isTerm) i" of type $info"
         else ""
@@ -229,6 +229,18 @@ object RefChecks {
           (if (otherAccess == "") "public" else "at least " + otherAccess))
       }
 
+      def compatibleTypes =
+        if (member.isType) { // intersection of bounds to refined types must be nonempty
+          member.is(BaseTypeArg) ||
+          (memberTp <:< otherTp) || {
+            val jointBounds = (memberTp.bounds & otherTp.bounds).bounds
+            jointBounds.lo <:< jointBounds.hi
+          }
+        }
+        else
+          isDefaultGetter(member.name) || // default getters are not checked for compatibility
+          memberTp.overrides(otherTp)
+
       //Console.println(infoString(member) + " overrides " + infoString(other) + " in " + clazz);//DEBUG
 
       // return if we already checked this combination elsewhere
@@ -321,9 +333,7 @@ object RefChecks {
         overrideError("cannot be used here - term macros cannot override abstract methods")
       } else if (other.is(Macro) && !member.is(Macro)) { // (1.10)
         overrideError("cannot be used here - only term macros can override term macros")
-      } else if (member.isTerm && !isDefaultGetter(member.name) && !(memberTp overrides otherTp)) {
-        // types don't need to have their bounds in an overriding relationship
-        // since we automatically form their intersection when selecting.
+      } else if (!compatibleTypes) {
         overrideError("has incompatible type" + err.whyNoMatchStr(memberTp, otherTp))
       } else {
         checkOverrideDeprecated()

Diff for: test/dotc/tests.scala

@@ -121,7 +121,7 @@ class tests extends CompilerTest {
   @Test def neg_autoTupling2 = compileFile(negDir, "autoTuplingTest", xerrors = 3)
   @Test def neg_companions = compileFile(negDir, "companions", xerrors = 1)
   @Test def neg_over = compileFile(negDir, "over", xerrors = 3)
-  @Test def neg_overrides = compileFile(negDir, "overrides", xerrors = 10)
+  @Test def neg_overrides = compileFile(negDir, "overrides", xerrors = 12)
   @Test def neg_overrideClass = compileFile(negDir, "overrideClass", List("-language:Scala2"), xerrors = 1)
   @Test def neg_i39 = compileFile(negDir, "i39", xerrors = 2)
   @Test def neg_i50_volatile = compileFile(negDir, "i50-volatile", xerrors = 6)

Diff for: tests/neg/overrides.scala

@@ -1,3 +1,20 @@
+class Foo {
+  type A = Int
+  type B >: Int <: Int
+  def get: A = 42
+}
+class Bar extends Foo {
+  override type A = Any   // error
+  type B >: String <: Any  // error
+  override def get: A = "bar"
+}
+object Test {
+  def main(args: Array[String]): Unit = {
+    val foo: Foo = new Bar
+    val i: Int = foo.get
+  }
+}
+
 package p1 {
   abstract class T1 {
     protected def bug(p: Int = 1): Int // without 'protected' compiles fine
@@ -102,3 +119,4 @@ class C extends A {
   override def m: Int = 42 // error: has incompatible type
 }
 }
+

Diff for: tests/pos/overrides.scala

@@ -11,12 +11,3 @@ class B extends A[Int] {
   f(2)()
 
 }
-
-class A1
-class A2
-class X1 {
-  type T = A1
-}
-class Y1 extends X1 {
-  override type T = A2
-}