Fix scala#1665: Check that != has an operand on the left.

Author: nicolasstucki

src/dotty/tools/dotc/transform/InterceptedMethods.scala

@@ -96,14 +96,31 @@ class InterceptedMethods extends MiniPhaseTransform {
         s"that means the intercepted methods set doesn't match the code")
       tree
     }
-    lazy val Select(qual, _) = tree.fun
+    lazy val qual = tree.fun match {
+      case Select(qual, _) => qual
+      case ident @ Ident(_) =>
+        ident.tpe match {
+          case TermRef(prefix: TermRef, name) =>
+            tpd.ref(prefix)
+          case TermRef(prefix: ThisType, name) =>
+            tpd.This(prefix.cls)
+        }
+
+    }
     val Any_## = this.Any_##
     val Any_!= = defn.Any_!=
     val rewrite: Tree = tree.fun.symbol match {
       case Any_## =>
           poundPoundValue(qual)
       case Any_!= =>
-          qual.select(defn.Any_==).appliedToArgs(tree.args).select(defn.Boolean_!)
+        assert(tree.args.size == 1)
+        val lhs = qual.tpe
+        val rhs = tree.args.head.tpe
+        if (!(lhs <:< rhs || rhs <:< lhs) && rhs != defn.NullType && lhs != defn.NullType) {
+          ctx.warning(ex"comparing values of types ${lhs.widenDealias} and ${rhs.widenDealias} using `!=' will always yield true",
+              tree.pos)
+        }
+        qual.select(defn.Any_==).appliedToArgs(tree.args).select(defn.Boolean_!)
         /*
         /* else if (isPrimitiveValueClass(qual.tpe.typeSymbol)) {
             // todo: this is needed to support value classes

tests/pos/i1665-nonfuzzy.scala

@@ -0,0 +1,7 @@
+
+object Test {
+  !=(1) // warning: comparing values of types Test and Int using `!=' will always yield true
+  !=("abc") // warning: comparing values of types Test and String using `!=' will always yield true
+  1 != this // warning: comparing values of types Int and Test using `!=' will always yield true
+  !=(this)
+}

tests/pos/i1665.scala

@@ -0,0 +1,12 @@
+
+import scala.reflect.runtime.universe._
+import scala.reflect.runtime.{currentMirror => cm}
+
+object Test extends App {
+  val mutant = new { val x = 2 }
+  val c = cm.classSymbol(mutant.getClass)
+  !=(c) // warning
+  println(c.fullName)
+  c.info.toString.lines
+    .filter(_ != "  private var bitmap$init$0: Boolean") foreach println
+}