Refactor decomposition logic

This commit renames isClosedSum to isDecomposable, and moves that
function and the decompose function to where they are used locally.
The logic in question only makes sense at the precise point where it's
used (see the added comment), so having those two functions defined
locally helps to avoid confusion.

Author: OlivierBlanvillain

compiler/src/dotty/tools/dotc/core/TypeComparer.scala

@@ -2526,17 +2526,6 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
         case _ => false
       })
 
-    /** Can we enumerate all instantiations of this type? */
-    def isClosedSum(tp: Symbol): Boolean =
-      tp.is(Sealed) && !tp.hasAnonymousChild
-
-    /** Splits a closed type into a disjunction of smaller types.
-     *  It should hold that `tp` and `decompose(tp).reduce(_ or _)`
-     *  denote the same set of values.
-     */
-    def decompose(sym: Symbol, tp: Type): List[Type] =
-      sym.children.map(x => refineUsingParent(tp, x)).filter(_.exists)
-
     def fullyInstantiated(tp: Type): Boolean = new TypeAccumulator[Boolean] {
       override def apply(x: Boolean, t: Type) =
         x && {
@@ -2558,6 +2547,10 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
       case (tp1: TypeRef, tp2: TypeRef) if tp1.symbol.isClass && tp2.symbol.isClass =>
         val cls1 = tp1.classSymbol
         val cls2 = tp2.classSymbol
+        def isDecomposable(tp: Symbol): Boolean =
+           tp.is(Sealed) && !tp.hasAnonymousChild
+        def decompose(sym: Symbol, tp: Type): List[Type] =
+          sym.children.map(x => refineUsingParent(tp, x)).filter(_.exists)
         if (cls1.derivesFrom(cls2) || cls2.derivesFrom(cls1))
           false
         else
@@ -2570,9 +2563,12 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
             // subtype, so they must be unrelated by single inheritance
             // of classes.
             true
-          else if (isClosedSum(cls1))
+          else if (isDecomposable(cls1))
+            // At this point, !cls1.derivesFrom(cls2): we know that `new cls2`
+            // is disjoint from `tp2`. Therefore, we can safely decompose
+            // `cls1` using `.children`, even if `cls1` is non abstract.
             decompose(cls1, tp1).forall(x => provablyDisjoint(x, tp2))
-          else if (isClosedSum(cls2))
+          else if (isDecomposable(cls2))
             decompose(cls2, tp2).forall(x => provablyDisjoint(x, tp1))
           else
             false