Fix #6849: support irrefutable sequence match

Author: liufengyun

compiler/src/dotty/tools/dotc/transform/PatternMatcher.scala

@@ -8,7 +8,7 @@ import collection.mutable
 import Symbols._, Contexts._, Types._, StdNames._, NameOps._
 import ast.Trees._
 import util.Spans._
-import typer.Applications.{isProductMatch, isGetMatch, isProductSeqMatch, productSelectors, productArity}
+import typer.Applications.{isProductMatch, isGetMatch, isProductSeqMatch, productSelectors, productArity, unapplySeqTypeElemTp}
 import SymUtils._
 import Flags._, Constants._
 import Decorators._
@@ -323,10 +323,13 @@ object PatternMatcher {
                 .map(ref(unappResult).select(_))
               matchArgsPlan(selectors, args, onSuccess)
             }
-            else if (isProductSeqMatch(unapp.tpe.widen, args.length, unapp.sourcePos) && isUnapplySeq) {
+            else if (isUnapplySeq && isProductSeqMatch(unapp.tpe.widen, args.length, unapp.sourcePos)) {
               val arity = productArity(unapp.tpe.widen, unapp.sourcePos)
               unapplyProductSeqPlan(unappResult, args, arity)
             }
+            else if (isUnapplySeq && unapplySeqTypeElemTp(unapp.tpe.widen.finalResultType).exists) {
+              unapplySeqPlan(unappResult, args)
+            }
             else {
               assert(isGetMatch(unapp.tpe))
               val argsPlan = {

compiler/src/dotty/tools/dotc/transform/patmat/Space.scala

@@ -284,6 +284,21 @@ object SpaceEngine {
       isEmptyTp <:< ConstantType(Constant(true))
     }
   }
+
+  /** Is the unapplySeq irrefutable?
+   *  @param  unapp   The unapplySeq function reference
+   */
+  def isIrrefutableUnapplySeq(unapp: tpd.Tree, patSize: Int)(implicit ctx: Context): Boolean = {
+    val unappResult = unapp.tpe.widen.finalResultType
+    unappResult.isRef(defn.SomeClass) ||
+    (unapp.symbol.is(Synthetic) && unapp.symbol.owner.linkedClass.is(Case)) ||  // scala2 compatibility
+    unapplySeqTypeElemTp(unappResult).exists ||
+    isProductSeqMatch(unappResult, patSize) ||
+    {
+      val isEmptyTp = extractorMemberType(unappResult, nme.isEmpty, unapp.sourcePos)
+      isEmptyTp <:< ConstantType(Constant(true))
+    }
+  }
 }
 
 /** Scala implementation of space logic */
@@ -336,9 +351,9 @@ class SpaceEngine(implicit ctx: Context) extends SpaceLogic {
         else {
           val (arity, elemTp, resultTp) = unapplySeqInfo(fun.tpe.widen.finalResultType, fun.sourcePos)
           if (elemTp.exists)
-            Prod(erase(pat.tpe.stripAnnots), fun.tpe, fun.symbol, projectSeq(pats) :: Nil, isIrrefutableUnapply(fun, -1))
+            Prod(erase(pat.tpe.stripAnnots), fun.tpe, fun.symbol, projectSeq(pats) :: Nil, isIrrefutableUnapplySeq(fun, pats.size))
           else
-            Prod(erase(pat.tpe.stripAnnots), fun.tpe, fun.symbol, pats.take(arity - 1).map(project) :+ projectSeq(pats.drop(arity - 1)), full = true)
+            Prod(erase(pat.tpe.stripAnnots), fun.tpe, fun.symbol, pats.take(arity - 1).map(project) :+ projectSeq(pats.drop(arity - 1)), isIrrefutableUnapplySeq(fun, pats.size))
         }
       else
         Prod(erase(pat.tpe.stripAnnots), erase(fun.tpe), fun.symbol, pats.map(project), isIrrefutableUnapply(fun, pats.length))