Fix untupling of functions in for comprehensions

compiler/src/dotty/tools/dotc/ast/Desugar.scala

@@ -1547,10 +1547,22 @@ object desugar {
    *
    *  If `nparams` != 1, expand instead to
    *
-   *       (x$1, ..., x$n) => (x$0, ..., x${n-1} @unchecked?) match { cases }
+   *       (x$1, ..., x$n) => (x$1, ..., x$n @unchecked?) match { cases }
+   *
+   *  Unless there is a single irrefutable case, then can reuse the rhs
+   *
+   *       { case (a1, ..., an) => rhs }
+   *  ==>
+   *       (a1, ..., an) => rhs
    */
   def makeCaseLambda(cases: List[CaseDef], checkMode: MatchCheck, nparams: Int = 1)(using Context): Function = {
-    val params = (1 to nparams).toList.map(makeSyntheticParameter(_))
+    val params = cases match
+      case List(CaseDef(untpd.Tuple(elems), untpd.EmptyTree, rhs)) if elems.sizeIs == nparams =>
+        patternsToParams(elems) match
+          case params if params.sizeIs == nparams => params
+          case _ => (1 to nparams).toList.map(makeSyntheticParameter(_))
+      case _ => (1 to nparams).toList.map(makeSyntheticParameter(_))
+
     val selector = makeTuple(params.map(p => Ident(p.name)))
     Function(params, Match(makeSelector(selector, checkMode), cases))
   }

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

@@ -1621,15 +1621,14 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
         case untpd.Annotated(scrut1, _) => isParamRef(scrut1)
         case untpd.Ident(id) => id == params.head.name
       fnBody match
-        case untpd.Match(scrut, untpd.CaseDef(untpd.Tuple(elems), untpd.EmptyTree, rhs) :: Nil)
+        case untpd.Match(scrut, cases @ untpd.CaseDef(untpd.Tuple(elems), untpd.EmptyTree, rhs) :: Nil)
         if scrut.span.isSynthetic && isParamRef(scrut) && elems.hasSameLengthAs(protoFormals) =>
           // If `pt` is N-ary function type, convert synthetic lambda
           //   x$1 => x$1 match case (a1, ..., aN) => e
           // to
           //   (a1, ..., aN) => e
-          val params1 = desugar.patternsToParams(elems)
-          if params1.hasSameLengthAs(elems) then
-            desugared = cpy.Function(tree)(params1, rhs)
+          desugared = desugar.makeCaseLambda(
+            cases, desugar.MatchCheck.IrrefutablePatDef, protoFormals.length)
         case _ =>
 
     if desugared.isEmpty then

tests/pos/i19576.scala

@@ -0,0 +1,5 @@
+
+object Test:
+  val a = Seq(0 -> 1, 2 -> 3)
+  val c = Seq("A", "B")
+  val z = for ((beg, end), c) <- a.lazyZip(c) yield c // Error before changes