Improved warning for insensible comparisons.

Utilize knowledge of case class synthetic equals to rule out
some comparisons statically.  Closes SI-5426.

Author: paulp

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

@@ -1034,18 +1034,21 @@ abstract class RefChecks extends InfoTransform with reflect.internal.transform.R
         /** Symbols which limit the warnings we can issue since they may be value types */
         val isMaybeValue = Set(AnyClass, AnyRefClass, AnyValClass, ObjectClass, ComparableClass, JavaSerializableClass)
 
-        // Whether def equals(other: Any) is overridden
-        def isUsingDefaultEquals      = {
+        // Whether def equals(other: Any) is overridden or synthetic
+        def isUsingWarnableEquals = {
           val m = receiver.info.member(nme.equals_)
-          (m == Object_equals) || (m == Any_equals)
+          (m == Object_equals) || (m == Any_equals) || (m.isSynthetic && m.owner.isCase)
         }
         // Whether this == or != is one of those defined in Any/AnyRef or an overload from elsewhere.
         def isUsingDefaultScalaOp = {
           val s = fn.symbol
           (s == Object_==) || (s == Object_!=) || (s == Any_==) || (s == Any_!=)
         }
         // Whether the operands+operator represent a warnable combo (assuming anyrefs)
-        def isWarnable           = isReferenceOp || (isUsingDefaultEquals && isUsingDefaultScalaOp)
+        // Looking for comparisons performed with ==/!= in combination with either an
+        // equals method inherited from Object or a case class synthetic equals (for
+        // which we know the logic.)
+        def isWarnable           = isReferenceOp || (isUsingDefaultScalaOp && isUsingWarnableEquals)
         def isEitherNullable     = (NullClass.tpe <:< receiver.info) || (NullClass.tpe <:< actual.info)
         def isBoolean(s: Symbol) = unboxedValueClass(s) == BooleanClass
         def isUnit(s: Symbol)    = unboxedValueClass(s) == UnitClass

test/files/neg/checksensible.check

@@ -28,6 +28,9 @@ 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:33: error: comparing values of types Some[Int] and Int using `==' will always yield false
+  Some(1) == 1      // as above
+          ^
 checksensible.scala:38: error: comparing a fresh object using `==' will always yield false
   new AnyRef == 1
              ^
@@ -94,4 +97,4 @@ 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)
                          ^
-32 errors found
+33 errors found

test/files/neg/t5426.check

@@ -0,0 +1,13 @@
+t5426.scala:2: error: comparing values of types Some[Int] and Int using `==' will always yield false
+  def f1 = Some(5) == 5
+                   ^
+t5426.scala:3: error: comparing values of types Int and Some[Int] using `==' will always yield false
+  def f2 = 5 == Some(5)
+             ^
+t5426.scala:8: error: comparing values of types Int and Some[Int] using `==' will always yield false
+  (x1 == x2)
+      ^
+t5426.scala:9: error: comparing values of types Some[Int] and Int using `==' will always yield false
+  (x2 == x1)
+      ^
+four errors found

test/files/neg/t5426.flags

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

test/files/neg/t5426.scala

@@ -0,0 +1,10 @@
+class A {
+  def f1 = Some(5) == 5
+  def f2 = 5 == Some(5)
+  
+  val x1 = 5
+  val x2 = Some(5)
+  
+  (x1 == x2)
+  (x2 == x1)
+}