Dealias val aliases to modules so they're =:=

[dwijnand]

This changes the definition of =:= in the smallest possible way:
if you have a singleton type that widens to a module,
then widen to that module.

Fixes scala/bug#12186.

[lrytz]

Here's an example. In 2.13.3:

scala> :power
scala> import reflect.internal.util.{shortClassOfInstance => c}

scala> object O; val x: O.type = O
object O
val x: O.type = O$@1545b9da

scala> val oSingle = typeOf[O.type]; println(c(oSingle))
UniqueSingleType
val oSingle: $r.intp.global.Type = O.type

scala> val oModule = oSingle.underlying; println(c(oModule))
ModuleTypeRef
val oModule: $r.intp.global.Type = O.type

scala> val xSingle = typeOf[x.type]; println(c(xSingle))
UniqueSingleType
val xSingle: $r.intp.global.Type = x.type

scala> xSingle =:= oSingle
val res3: Boolean = true

scala> xSingle =:= oModule
val res4: Boolean = false

This PR fixes the last line.

[retronym]

We should unit test this by adding some test cases to

scala/test/junit/scala/reflect/internal/TypesTest.scala

Line 39 in 33478bd

def testTransitivityWithModuleTypeRef(): Unit = {

See the previous tweaks to normalizePlus that I test drive with that test: 6f8c057

[retronym]

e.g.

diff --git a/test/junit/scala/reflect/internal/TypesTest.scala b/test/junit/scala/reflect/internal/TypesTest.scala
index ab0cdd954e..fc30775b4a 100644
--- a/test/junit/scala/reflect/internal/TypesTest.scala
+++ b/test/junit/scala/reflect/internal/TypesTest.scala
@@ -46,9 +46,15 @@ class TypesTest {
     val tp1 = TypeRef(ThisType(EmptyPackageClass), moduleClass, Nil)
     val tp2 = SingleType(ThisType(EmptyPackageClass), module)
     val tp3 = ThisType(moduleClass)
-    val tps = List(tp1, tp2, tp3)
+
+    val (otherModule, otherModuleClass) = EmptyPackageClass.newModuleAndClassSymbol(TermName("Other"), NoPosition, 0L)
+    val aliasSym = otherModuleClass.newTermSymbol(TermName("alias")).setInfo(tp2)
+    val tp4 = singleType(TypeRef(ThisType(EmptyPackageClass), otherModuleClass, Nil), aliasSym)
+
+
+    val tps = List(tp1, tp2, tp3, tp4)
     val results = mutable.Buffer[String]()
-    tps.permutations.foreach {
+    tps.combinations(3).foreach {
       case ts @ List(a, b, c) =>
         def tsShownRaw = ts.map(t => showRaw(t)).mkString(", ")
         if (a <:< b && b <:< c && !(a <:< c)) results += s"<:< intransitive: $tsShownRaw"

[dwijnand]

Thanks, Jason. I was getting some detail there wrong so I was struggling. Yours works very well, failing without the patch.

[dwijnand]

Briefly touched on this one with Adriaan and he said he agrees that scala.Nil.type shouldn't be different to scala.collection.immutable.Nil.type. Any chance we can merge this?