Improve comments around constrainPatternType

Author: abgruszecki

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

@@ -14,7 +14,9 @@ trait PatternTypeConstrainer { self: TypeComparer =>
   /** Derive type and GADT constraints that necessarily follow from a pattern with the given type matching
    *  a scrutinee of the given type.
    *
-   *  We have the following situation in case of a (dynamic) pattern match:
+   *  This function breaks down scrutinee and pattern types into subcomponents between which there must be
+   *  a subtyping relationship, and derives constraints from those relationships. We have the following situation
+   *  in case of a (dynamic) pattern match:
    *
    *       StaticScrutineeType           PatternType
    *                         \            /

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

@@ -1094,9 +1094,9 @@ trait Applications extends Compatibility { self: Typer with Dynamic =>
             fullyDefinedType(unapplyArgType, "pattern selector", tree.span)
             selType.dropAnnot(defn.UncheckedAnnot) // need to drop @unchecked. Just because the selector is @unchecked, the pattern isn't.
           } else {
-            // note that we simply ignore whether constraining actually succeeded or not
-            // in theory, constraining should only fail if the pattern cannot possibly match
-            // however, during exhaustivity checks, we perform a strictly better check
+            // We ignore whether constraining the pattern succeeded.
+            // Constraining only fails if the pattern cannot possibly match,
+            // but useless pattern checks detect more such cases, so we simply rely on them instead.
             ctx.addMode(Mode.GADTflexible).typeComparer.constrainPatternType(unapplyArgType, selType)
             val patternBound = maximizeType(unapplyArgType, tree.span, fromScala2x)
             if (patternBound.nonEmpty) unapplyFn = addBinders(unapplyFn, patternBound)