Tone down insensible-equality warning.

Closes SI-5175.

src/compiler/scala/tools/nsc/typechecker/RefChecks.scala

@@ -1092,7 +1092,7 @@ abstract class RefChecks extends InfoTransform with reflect.internal.transform.R
         }
 
         // possibleNumericCount is insufficient or this will warn on e.g. Boolean == j.l.Boolean
-        if (nullCount == 0 && !(isSpecial(receiver) && isSpecial(actual))) {
+        if (isWarnable && nullCount == 0 && !(isSpecial(receiver) && isSpecial(actual))) {
           if (actual isSubClass receiver) ()
           else if (receiver isSubClass actual) ()
           // warn only if they have no common supertype below Object

test/files/neg/checksensible.check

@@ -28,12 +28,6 @@ checksensible.scala:27: error: comparing values of types Int and Unit using `=='
 checksensible.scala:29: error: comparing values of types Int and String using `==' will always yield false
   1 == "abc"
     ^
-checksensible.scala:32: error: String and Int are unrelated: they will most likely never compare equal
-  "abc" == 1        // warns because the lub of String and Int is Any
-        ^
-checksensible.scala:33: error: Some[Int] and Int are unrelated: they will most likely never compare equal
-  Some(1) == 1      // as above
-          ^
 checksensible.scala:38: error: comparing a fresh object using `==' will always yield false
   new AnyRef == 1
              ^
@@ -100,4 +94,4 @@ checksensible.scala:84: error: comparing values of types EqEqRefTest.this.C3 and
 checksensible.scala:95: error: comparing values of types Unit and Int using `!=' will always yield true
     while ((c = in.read) != -1)
                          ^
-34 errors found
+32 errors found

test/files/pos/t5175.flags

@@ -0,0 +1 @@
+-Xfatal-warnings

test/files/pos/t5175.scala

@@ -0,0 +1,9 @@
+object Test {
+  def ==(p: Phase): Int = 0
+
+  def foo {
+    ==(new Phase())
+  }
+}
+
+class Phase