From 01d8eadb92473754dac89b344dc2078c9803bc0e Mon Sep 17 00:00:00 2001
From: Jan Chyb <jan.chyb.99@gmail.com>
Date: Mon, 7 Jul 2025 11:02:34 +0200
Subject: [PATCH] Extend caching in the space engine beyond local context

Previously isSubspace calls would be cached only as part of individual
Space object, so when recursing through their ADT contents, we were not
able to reuse the previously computed information. Now the cache is
shared across the whole space engine run.
---
 .../tools/dotc/transform/patmat/Space.scala   | 14 ++++++----
 tests/pos/i23317.scala                        | 26 +++++++++++++++++++
 2 files changed, 35 insertions(+), 5 deletions(-)
 create mode 100644 tests/pos/i23317.scala

diff --git a/compiler/src/dotty/tools/dotc/transform/patmat/Space.scala b/compiler/src/dotty/tools/dotc/transform/patmat/Space.scala
index ab5885e6278c..8514c897d01e 100644
--- a/compiler/src/dotty/tools/dotc/transform/patmat/Space.scala
+++ b/compiler/src/dotty/tools/dotc/transform/patmat/Space.scala
@@ -48,11 +48,12 @@ import SpaceEngine.*
  *
  */
 
+/** A key to be used in a context property that caches the unpickled trees */
+private val IsSubspaceCacheKey = new Property.Key[mutable.HashMap[(Space, Space), Boolean]]
+
 /** space definition */
 sealed trait Space extends Showable:
 
-  @sharable private val isSubspaceCache = mutable.HashMap.empty[Space, Boolean]
-
   def isSubspace(b: Space)(using Context): Boolean =
     val a = this
     val a2 = a.simplify
@@ -60,7 +61,8 @@ sealed trait Space extends Showable:
     if (a ne a2) || (b ne b2) then a2.isSubspace(b2)
     else if a == Empty then true
     else if b == Empty then false
-    else isSubspaceCache.getOrElseUpdate(b, computeIsSubspace(a, b))
+    else
+      ctx.property(IsSubspaceCacheKey).get.getOrElseUpdate((a, b), computeIsSubspace(a, b))
 
   @sharable private var mySimplified: Space | Null = null
 
@@ -968,6 +970,8 @@ object SpaceEngine {
   end checkReachability
 
   def checkMatch(m: Match)(using Context): Unit =
-    if exhaustivityCheckable(m.selector) then checkExhaustivity(m)
-    if reachabilityCheckable(m.selector) then checkReachability(m)
+    inContext(ctx.withProperty(IsSubspaceCacheKey, Some(mutable.HashMap.empty))) {
+      if exhaustivityCheckable(m.selector) then checkExhaustivity(m)
+      if reachabilityCheckable(m.selector) then checkReachability(m)
+    }
 }
diff --git a/tests/pos/i23317.scala b/tests/pos/i23317.scala
new file mode 100644
index 000000000000..1381aad452d2
--- /dev/null
+++ b/tests/pos/i23317.scala
@@ -0,0 +1,26 @@
+import scala.quoted.*
+
+def goImpl(using Quotes): Expr[Int] =
+  List.empty[Type[?]] match
+    case Nil =>
+      Expr(0)
+    case '[t1] :: Nil =>
+      Expr(1)
+    case '[t1] :: '[t2] :: Nil =>
+      Expr(2)
+    case '[t1] :: '[t2] :: '[t3] :: Nil =>
+      Expr(3)
+    case '[t1] :: '[t2] :: '[t3] :: '[t4] :: Nil =>
+      Expr(4)
+    case '[t1] :: '[t2] :: '[t3] :: '[t4] :: '[t5] :: Nil =>
+      Expr(5)
+    case '[t1] :: '[t2] :: '[t3] :: '[t4] :: '[t5] :: '[t6] :: Nil =>
+      Expr(6)
+    case '[t1] :: '[t2] :: '[t3] :: '[t4] :: '[t5] :: '[t6] :: '[t7] :: Nil =>
+      Expr(7)
+    case '[t1] :: '[t2] :: '[t3] :: '[t4] :: '[t5] :: '[t6] :: '[t7] :: '[t8] :: Nil =>
+      Expr(8)
+    case '[t1] :: '[t2] :: '[t3] :: '[t4] :: '[t5] :: '[t6] :: '[t7] :: '[t8] :: '[t9] :: Nil =>
+      Expr(9)
+    case _ =>
+      Expr(999)