From 90a9067ba4f650beb0014d23bf97b88b0fb99d16 Mon Sep 17 00:00:00 2001
From: Emil Ejbyfeldt <emil.ejbyfeldt@choreograph.com>
Date: Fri, 13 Jun 2025 21:03:59 +0200
Subject: [PATCH] Support Mirror for generic tuples arity > 22

---
 .../dotty/tools/dotc/typer/Synthesizer.scala  | 22 +++++++++++--------
 tests/{neg => pos}/i14127.scala               |  0
 tests/pos/i15398.scala                        |  9 ++++++++
 3 files changed, 22 insertions(+), 9 deletions(-)
 rename tests/{neg => pos}/i14127.scala (100%)
 create mode 100644 tests/pos/i15398.scala

diff --git a/compiler/src/dotty/tools/dotc/typer/Synthesizer.scala b/compiler/src/dotty/tools/dotc/typer/Synthesizer.scala
index 4f596776d497..58f21121f629 100644
--- a/compiler/src/dotty/tools/dotc/typer/Synthesizer.scala
+++ b/compiler/src/dotty/tools/dotc/typer/Synthesizer.scala
@@ -432,13 +432,24 @@ class Synthesizer(typer: Typer)(using @constructorOnly c: Context):
       makeProductMirror(typeElems, elemLabels, tpnme.NamedTuple, mirrorRef)
     end makeNamedTupleProductMirror
 
+    def makeTupleProductMirror(tps: List[Type]): TreeWithErrors =
+      val arity = tps.size
+      if arity < Definitions.MaxTupleArity then
+        val tupleCls = defn.TupleType(arity).nn.classSymbol
+        makeClassProductMirror(tupleCls.owner.reachableThisType, tupleCls, Some(tps))
+      else
+        val elemLabels = (for i <- 1 to arity yield ConstantType(Constant(s"_$i"))).toList
+        val mirrorRef: Type => Tree = _ => newTupleMirror(arity)
+        makeProductMirror(tps, elemLabels, s"Tuple$arity".toTermName, mirrorRef)
+    end makeTupleProductMirror
+
     def makeClassProductMirror(pre: Type, cls: Symbol, tps: Option[List[Type]]) =
       val accessors = cls.caseAccessors
       val elemLabels = accessors.map(acc => ConstantType(Constant(acc.name.toString)))
       val typeElems = tps.getOrElse(accessors.map(mirroredType.resultType.memberInfo(_).widenExpr))
       val mirrorRef = (monoType: Type) =>
         if cls.useCompanionAsProductMirror then companionPath(pre, cls, span)
-        else if defn.isTupleClass(cls) then newTupleMirror(typeElems.size) // TODO: cls == defn.PairClass when > 22
+        else if defn.isTupleClass(cls) then newTupleMirror(typeElems.size)
         else anonymousMirror(monoType, MirrorImpl.OfProduct(pre), span)
       makeProductMirror(typeElems, elemLabels, cls.name, mirrorRef)
     end makeClassProductMirror
@@ -478,14 +489,7 @@ class Synthesizer(typer: Typer)(using @constructorOnly c: Context):
             }
             withNoErrors(singletonPath.cast(mirrorType).withSpan(span))
         case MirrorSource.GenericTuple(tps) =>
-          val maxArity = Definitions.MaxTupleArity
-          val arity = tps.size
-          if tps.size <= maxArity then
-            val tupleCls = defn.TupleType(arity).nn.classSymbol
-            makeClassProductMirror(tupleCls.owner.reachableThisType, tupleCls, Some(tps))
-          else
-            val reason = s"it reduces to a tuple with arity $arity, expected arity <= $maxArity"
-            withErrors(i"${defn.PairClass} is not a generic product because $reason")
+          makeTupleProductMirror(tps)
         case MirrorSource.NamedTuple(nameTypePairs) =>
           makeNamedTupleProductMirror(nameTypePairs)
         case MirrorSource.ClassSymbol(pre, cls) =>
diff --git a/tests/neg/i14127.scala b/tests/pos/i14127.scala
similarity index 100%
rename from tests/neg/i14127.scala
rename to tests/pos/i14127.scala
diff --git a/tests/pos/i15398.scala b/tests/pos/i15398.scala
new file mode 100644
index 000000000000..15ab57ff6661
--- /dev/null
+++ b/tests/pos/i15398.scala
@@ -0,0 +1,9 @@
+object i15398 {
+  type Tuple23 = (Int, Int, Int, Int, Int, Int, Int, Int, Int, Int, Int, Int, Int, Int, Int, Int, Int, Int, Int, Int, Int, Int, Int)
+
+  summon[Tuple.Size[Tuple23] =:= 23]
+  val m = summon[scala.deriving.Mirror.Of[Tuple23]]
+  summon[m.MirroredLabel =:= "Tuple23"]
+  summon[m.MirroredElemTypes =:= Tuple23]
+  summon[m.MirroredElemLabels =:= ("_1", "_2", "_3", "_4", "_5", "_6", "_7", "_8", "_9", "_10", "_11", "_12", "_13", "_14", "_15", "_16", "_17", "_18", "_19", "_20", "_21", "_22", "_23")]
+}