From 901479c0f624e56d14702e4fd4493ef98ee7901c Mon Sep 17 00:00:00 2001
From: odersky <odersky@gmail.com>
Date: Mon, 25 Jul 2022 14:35:43 +0200
Subject: [PATCH 1/2] Implementation restriction: No partial functions with CFT
 results

Fixes #15741

I tried for a while to make this work, but it is just too gnarly.
---
 .../dotty/tools/dotc/transform/ExpandSAMs.scala   |  9 +++++++++
 tests/neg/i15741.scala                            | 15 +++++++++++++++
 2 files changed, 24 insertions(+)
 create mode 100644 tests/neg/i15741.scala

diff --git a/compiler/src/dotty/tools/dotc/transform/ExpandSAMs.scala b/compiler/src/dotty/tools/dotc/transform/ExpandSAMs.scala
index ac29275b2210..c3311d6495c9 100644
--- a/compiler/src/dotty/tools/dotc/transform/ExpandSAMs.scala
+++ b/compiler/src/dotty/tools/dotc/transform/ExpandSAMs.scala
@@ -66,6 +66,13 @@ class ExpandSAMs extends MiniPhase:
       tree
   }
 
+  private def checkNoContextFunction(tpt: Tree)(using Context): Unit =
+    if defn.isContextFunctionType(tpt.tpe) then
+      report.error(
+        em"""Implementation restriction: cannot convert this expression to
+            |partial function with context function result type $tpt""",
+        tpt.srcPos)
+
   /** A partial function literal:
    *
    *  ```
@@ -108,6 +115,8 @@ class ExpandSAMs extends MiniPhase:
   private def toPartialFunction(tree: Block, tpe: Type)(using Context): Tree = {
     val closureDef(anon @ DefDef(_, List(List(param)), _, _)) = tree: @unchecked
 
+    checkNoContextFunction(anon.tpt)
+
     // The right hand side from which to construct the partial function. This is always a Match.
     // If the original rhs is already a Match (possibly in braces), return that.
     // Otherwise construct a match `x match case _ => rhs` where `x` is the parameter of the closure.
diff --git a/tests/neg/i15741.scala b/tests/neg/i15741.scala
new file mode 100644
index 000000000000..45d6c3bed16d
--- /dev/null
+++ b/tests/neg/i15741.scala
@@ -0,0 +1,15 @@
+  def get(using Int): String = summon[Int].toString
+
+  def pf2: PartialFunction[String, Int ?=> String] = { // error
+    case "hoge" => get
+    case "huga" => get
+  }
+
+  type IS = Int ?=> String
+
+  def pf3: PartialFunction[String, IS] = { // error
+    case "hoge" => get
+    case "huga" => get
+  }
+
+

From 7996022a6b4b3ce6a7f4ca7f994a311dbc16405a Mon Sep 17 00:00:00 2001
From: odersky <odersky@gmail.com>
Date: Mon, 25 Jul 2022 16:07:32 +0200
Subject: [PATCH 2/2] Fix test

---
 tests/neg/i15741.scala | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/tests/neg/i15741.scala b/tests/neg/i15741.scala
index 45d6c3bed16d..b5304f83d8b3 100644
--- a/tests/neg/i15741.scala
+++ b/tests/neg/i15741.scala
@@ -1,14 +1,14 @@
   def get(using Int): String = summon[Int].toString
 
-  def pf2: PartialFunction[String, Int ?=> String] = { // error
-    case "hoge" => get
+  def pf2: PartialFunction[String, Int ?=> String] = {
+    case "hoge" => get // error
     case "huga" => get
   }
 
   type IS = Int ?=> String
 
-  def pf3: PartialFunction[String, IS] = { // error
-    case "hoge" => get
+  def pf3: PartialFunction[String, IS] = {
+    case "hoge" => get  // error
     case "huga" => get
   }