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.

Author: odersky

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.

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
+  }
+
+