Survive bottom values in pattern matches

See test case. This caused two spurious overloding errors,
one when looking for equality evidence and then again in the
emit method of the pattern matcher.

A spurious error was also eliminated in neg/ensureReported.scala.

Author: odersky

compiler/src/dotty/tools/dotc/ast/tpd.scala

@@ -824,7 +824,10 @@ object tpd extends Trees.Instance[Type] with TypedTreeInfo {
 
     /** `tree == that` */
     def equal(that: Tree)(implicit ctx: Context) =
-      applyOverloaded(tree, nme.EQ, that :: Nil, Nil, defn.BooleanType)
+      if (that.tpe.widen.isRef(defn.NothingClass))
+        Literal(Constant(false))
+      else
+        applyOverloaded(tree, nme.EQ, that :: Nil, Nil, defn.BooleanType)
 
     /** `tree.isInstanceOf[tp]`, with special treatment of singleton types */
     def isInstance(tp: Type)(implicit ctx: Context): Tree = tp.dealias match {

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

@@ -2653,7 +2653,8 @@ class Typer extends Namer
     tree match {
       case _: RefTree | _: Literal
         if !isVarPattern(tree) &&
-          !(tree.tpe <:< pt) (ctx.addMode(Mode.GADTflexible)) =>
+           !(pt <:< tree.tpe) &&
+           !(tree.tpe <:< pt)(ctx.addMode(Mode.GADTflexible)) =>
         val cmp =
           untpd.Apply(
             untpd.Select(untpd.TypedSplice(tree), nme.EQ),

tests/neg/NoneMatch.scala

@@ -0,0 +1,7 @@
+object Test {
+
+  None match {
+    case Some(0) => ??? // error: unreachable
+  }
+
+}

tests/neg/ensureReported.scala

@@ -1,6 +1,6 @@
 object AnonymousF {
   val f = {
-    case l @ List(1) => // error: missing parameter type // error: Ambiguous overload
+    case l @ List(1) => // error: missing parameter type
       Some(l)
   }
 }