Unify "object Foo" and "Foo.type".

The source of many bugs over the years is that the first is
represented as a TypeRef and the second a SingleType.  Over a
great period of time I figured out how to shield us from the
more obvious bug manifestations, but a recent comment by adriaan
jarred me into realizing that we can fix it at the source.

This commit changes <:< and =:= to recognize when those two
representations are being compared and to treat them as equivalent
regardless of which is on the left.  The reason I don't quash one
representation entirely is that a fair bit of code depends on
singleton types having an underlying type which is not the same,
and regardless of that it would entail more changes and more risk.

The change allows removing the type inference conditions which
worried about this, and also fixes SI-4910.

scala> val t1 = typeRef(ScalaPackageClass.thisType, NoneModule.moduleClass, Nil)
t1: $r.intp.global.Type = None.type

scala> val t2 = t1.narrow
t2: $r.intp.global.Type = None.type

scala> (t1.getClass, t2.getClass)
res20: (Class[?0], Class[?0]) forSome { type ?0 <: $r.intp.global.Type; type ?0 <: $r.intp.global.Type } =
  (class scala.reflect.internal.Types$ModuleTypeRef,class scala.reflect.internal.Types$UniqueSingleType)

scala> ((t1 =:= t2, t2 =:= t1, t1 <:< t2, t2 <:< t1))
res21: (Boolean, Boolean, Boolean, Boolean) = (true,true,true,true)

3 files changed, 9 insertions, 9 deletions

diff --git a/src/compiler/scala/reflect/internal/Types.scala b/src/compiler/scala/reflect/internal/Types.scala
index 5afa5343ed..786b680ff8 100644
--- a/src/compiler/scala/reflect/internal/Types.scala
+++ b/src/compiler/scala/reflect/internal/Types.scala
@@ -5059,6 +5059,8 @@ trait Types extends api.Types { self: SymbolTable =>
                  case (_, tv @ TypeVar(_,_)) => tv.registerTypeSelection(tr2.sym, tr1)
                  case _ => false
                })
+          case _: SingleType =>
+            return isSameType2(tp2, tp1)  // put singleton type on the left, caught below
           case _ =>
         }
       case tt1: ThisType =>
@@ -5071,6 +5073,8 @@ trait Types extends api.Types { self: SymbolTable =>
         tp2 match {
           case st2: SingleType =>
             if (equalSymsAndPrefixes(st1.sym, st1.pre, st2.sym, st2.pre)) return true
+          case TypeRef(pre2, sym2, Nil) =>
+            if (sym2.isModuleClass && equalSymsAndPrefixes(st1.sym, st1.pre, sym2.sourceModule, pre2)) return true
           case _ =>
         }
       case ct1: ConstantType =>
@@ -5481,7 +5485,7 @@ trait Types extends api.Types { self: SymbolTable =>
      *   - handle typerefs, refined types, notnull and singleton types.
      */
     def fourthTry = tp1 match {
-      case tr1 @ TypeRef(_, sym1, _) =>
+      case tr1 @ TypeRef(pre1, sym1, _) =>
         sym1 match {
           case NothingClass => true
           case NullClass =>
@@ -5495,8 +5499,8 @@ trait Types extends api.Types { self: SymbolTable =>
             if (isRaw(sym1, tr1.args))
               isSubType(rawToExistential(tp1), tp2, depth)
             else if (sym1.isModuleClass) tp2 match {
-              case SingleType(_, sym2)  => sym1 == sym2
-              case _                    => false
+              case SingleType(pre2, sym2) => equalSymsAndPrefixes(sym1.sourceModule, pre1, sym2, pre2)
+              case _                      => false
             }
             else if (sym1.isRefinementClass)
               isSubType(sym1.info, tp2, depth)
diff --git a/src/compiler/scala/tools/nsc/typechecker/Infer.scala b/src/compiler/scala/tools/nsc/typechecker/Infer.scala
index a1ca4904f4..dba31f7bca 100644
--- a/src/compiler/scala/tools/nsc/typechecker/Infer.scala
+++ b/src/compiler/scala/tools/nsc/typechecker/Infer.scala
@@ -484,8 +484,7 @@ trait Infer {
           else Some(
             if (targ.typeSymbol == RepeatedParamClass)     targ.baseType(SeqClass)
             else if (targ.typeSymbol == JavaRepeatedParamClass) targ.baseType(ArrayClass)
-            // this infers Foo.type instead of "object Foo" (see also widenIfNecessary)
-            else if (targ.typeSymbol.isModuleClass || ((opt.experimental || opt.virtPatmat) && tvar.constr.avoidWiden)) targ
+            else if ((opt.experimental || opt.virtPatmat) && tvar.constr.avoidWiden) targ
             else targ.widen
           )
         ))
diff --git a/src/compiler/scala/tools/nsc/typechecker/Namers.scala b/src/compiler/scala/tools/nsc/typechecker/Namers.scala
index 8604366bf2..1b505d1e5d 100644
--- a/src/compiler/scala/tools/nsc/typechecker/Namers.scala
+++ b/src/compiler/scala/tools/nsc/typechecker/Namers.scala
@@ -793,10 +793,7 @@ trait Namers extends MethodSynthesis {
       val tpe1 = dropRepeatedParamType(tpe.deconst)
       val tpe2 = tpe1.widen
 
-      // This infers Foo.type instead of "object Foo"
-      // See Infer#adjustTypeArgs for the polymorphic case.
-      if (tpe.typeSymbolDirect.isModuleClass) tpe1
-      else if (sym.isVariable || sym.isMethod && !sym.hasAccessorFlag)
+      if (sym.isVariable || sym.isMethod && !sym.hasAccessorFlag)
         if (tpe2 <:< pt) tpe2 else tpe1
       else if (isHidden(tpe)) tpe2
       // In an attempt to make pattern matches involving method local vals