From e2b52290d8847a63a995154c3f9cb1d3d8c5cb76 Mon Sep 17 00:00:00 2001
From: Eugene Flesselle <eugene@flesselle.net>
Date: Sun, 4 Feb 2024 15:37:56 +0100
Subject: [PATCH] Fix untupling of functions in for comprehensions

---
 compiler/src/dotty/tools/dotc/typer/Typer.scala | 7 ++++---
 tests/pos/i19576.scala                          | 6 ++++++
 2 files changed, 10 insertions(+), 3 deletions(-)
 create mode 100644 tests/pos/i19576.scala

diff --git a/compiler/src/dotty/tools/dotc/typer/Typer.scala b/compiler/src/dotty/tools/dotc/typer/Typer.scala
index 7727c125d1e4..5648f651d8e9 100644
--- a/compiler/src/dotty/tools/dotc/typer/Typer.scala
+++ b/compiler/src/dotty/tools/dotc/typer/Typer.scala
@@ -1621,15 +1621,16 @@ 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 = if params1.hasSameLengthAs(elems)
+            then cpy.Function(tree)(params1, rhs)
+            else desugar.makeCaseLambda(cases, desugar.MatchCheck.IrrefutablePatDef, protoFormals.length)
         case _ =>
 
     if desugared.isEmpty then
diff --git a/tests/pos/i19576.scala b/tests/pos/i19576.scala
new file mode 100644
index 000000000000..030107b3c63c
--- /dev/null
+++ b/tests/pos/i19576.scala
@@ -0,0 +1,6 @@
+
+object Test:
+  val z = Seq(0 -> 1, 2 -> 3).lazyZip(Seq("A", "B"))
+  for case ((beg, end), c) <- z yield c // Ok: a withFilter is inserted before map
+  for (range, c) <- z yield c           // Ok: exact shape
+  for ((beg, end), c) <- z yield c      // Error before changes: Wrong number of parameters, expected 2