Change rules for multiversal equality

 1. Don't apply the lifting rules when strictEquality is set.
 2. Be more strict about null checking
 3. Compensate for (2) by changing errorTerm in parser so that
    it has an error type instead of Null.
 4. Reorganize implementation to match new docs.

Author: odersky

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

@@ -698,16 +698,65 @@ trait Implicits { self: Typer =>
       if (ctx.inInlineMethod || enclosingInlineds.nonEmpty) ref(defn.TastyReflection_macroContext)
       else EmptyTree
 
-    /** If `formal` is of the form Eq[T, U], where no `Eq` instance exists for
-     *  either `T` or `U`, synthesize `Eq.eqAny[T, U]` as solution.
+    /** If `formal` is of the form Eq[T, U], try to synthesize an
+     *  `Eq.eqAny[T, U]` as solution.
      */
     def synthesizedEq(formal: Type)(implicit ctx: Context): Tree = {
-      //println(i"synth eq $formal / ${formal.argTypes}%, %")
+
+      /** Is there an `Eql[T, T]` instance, assuming -strictEquality? */
+      def hasEq(tp: Type)(implicit ctx: Context): Boolean = {
+        val inst = inferImplicitArg(defn.EqType.appliedTo(tp, tp), span)
+        !inst.isEmpty && !inst.tpe.isError
+      }
+
+      /** Can we assume the eqAny instance for `tp1`, `tp2`?
+       *  This is the case if assumedCanEqual(tp1, tp2), or
+       *  one of `tp1`, `tp2` has a reflexive `Eql` instance.
+       */
+      def validEqAnyArgs(tp1: Type, tp2: Type)(implicit ctx: Context) =
+        assumedCanEqual(tp1, tp2) || {
+          val nestedCtx = ctx.fresh.addMode(Mode.StrictEquality)
+          !hasEq(tp1)(nestedCtx) && !hasEq(tp2)(nestedCtx)
+        }
+
+      /** Is an `Eql[cls1, cls2]` instance assumed for predefined classes `cls1`, cls2`? */
+      def canComparePredefinedClasses(cls1: ClassSymbol, cls2: ClassSymbol): Boolean = {
+        def cmpWithBoxed(cls1: ClassSymbol, cls2: ClassSymbol) =
+          cls2 == defn.boxedType(cls1.typeRef).symbol ||
+          cls1.isNumericValueClass && cls2.derivesFrom(defn.BoxedNumberClass)
+
+        if (cls1.isPrimitiveValueClass)
+          if (cls2.isPrimitiveValueClass)
+            cls1 == cls2 || cls1.isNumericValueClass && cls2.isNumericValueClass
+          else
+            cmpWithBoxed(cls1, cls2)
+        else if (cls2.isPrimitiveValueClass)
+          cmpWithBoxed(cls2, cls1)
+        else if (cls1 == defn.NullClass)
+          cls1 == cls2 || cls2.derivesFrom(defn.ObjectClass)
+        else if (cls2 == defn.NullClass)
+          cls1.derivesFrom(defn.ObjectClass)
+        else
+          false
+      }
+
+      /** Some simulated `Eql` instances for predefined types. It's more efficient
+       *  to do this directly instead of setting up a lot of `Eql` instances to
+       *  interpret.
+       */
+      def canComparePredefined(tp1: Type, tp2: Type) =
+        tp1.classSymbols.exists(cls1 =>
+          tp2.classSymbols.exists(cls2 => canComparePredefinedClasses(cls1, cls2)))
+
       formal.argTypes match {
-        case args @ (arg1 :: arg2 :: Nil)
-        if !ctx.featureEnabled(defn.LanguageModuleClass, nme.strictEquality) &&
-           ctx.test(implicit ctx => validEqAnyArgs(arg1, arg2)) =>
-          ref(defn.Eq_eqAny).appliedToTypes(args).withSpan(span)
+        case args @ (arg1 :: arg2 :: Nil) =>
+          List(arg1, arg2).foreach(fullyDefinedType(_, "eq argument", span))
+          if (canComparePredefined(arg1, arg2)
+              ||
+              !strictEquality &&
+              ctx.test(implicit ctx => validEqAnyArgs(arg1, arg2)))
+            ref(defn.Eq_eqAny).appliedToTypes(args).withSpan(span)
+          else EmptyTree
         case _ =>
           EmptyTree
       }
@@ -736,14 +785,6 @@ trait Implicits { self: Typer =>
       }
     }
 
-    def hasEq(tp: Type): Boolean =
-      inferImplicit(defn.EqType.appliedTo(tp, tp), EmptyTree, span).isSuccess
-
-    def validEqAnyArgs(tp1: Type, tp2: Type)(implicit ctx: Context) = {
-      List(tp1, tp2).foreach(fullyDefinedType(_, "eqAny argument", span))
-      assumedCanEqual(tp1, tp2) || !hasEq(tp1) && !hasEq(tp2)
-    }
-
     /** If `formal` is of the form `scala.reflect.Generic[T]` for some class type `T`,
      *  synthesize an instance for it.
      */
@@ -884,16 +925,16 @@ trait Implicits { self: Typer =>
         em"parameter ${paramName} of $methodStr"
     }
 
-  private def assumedCanEqual(ltp: Type, rtp: Type)(implicit ctx: Context) = {
-    def eqNullable: Boolean = {
-      val other =
-        if (ltp.isRef(defn.NullClass)) rtp
-        else if (rtp.isRef(defn.NullClass)) ltp
-        else NoType
-
-      (other ne NoType) && !other.derivesFrom(defn.AnyValClass)
-    }
+  private def strictEquality(implicit ctx: Context): Boolean =
+    ctx.mode.is(Mode.StrictEquality) ||
+    ctx.featureEnabled(defn.LanguageModuleClass, nme.strictEquality)
 
+  /** An Eql[T, U] instance is assumed
+   *   - if one of T, U is an error type, or
+   *   - if one of T, U is a subtype of the lifted version of the other,
+   *     unless strict equality is set.
+   */
+  private def assumedCanEqual(ltp: Type, rtp: Type)(implicit ctx: Context) = {
     // Map all non-opaque abstract types to their upper bound.
     // This is done to check whether such types might plausibly be comparable to each other.
     val lift = new TypeMap {
@@ -909,7 +950,13 @@ trait Implicits { self: Typer =>
           if (variance > 0) mapOver(t) else t
       }
     }
-    ltp.isError || rtp.isError || ltp <:< lift(rtp) || rtp <:< lift(ltp) || eqNullable
+
+    ltp.isError ||
+    rtp.isError ||
+    !strictEquality && {
+      ltp <:< lift(rtp) ||
+      rtp <:< lift(ltp)
+    }
   }
 
   /** Check that equality tests between types `ltp` and `rtp` make sense */