Revert SAM condition to what it was before (#21684)

Fixes #21676

Author: odersky

compiler/src/dotty/tools/dotc/typer/Typer.scala

@@ -4568,7 +4568,7 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
 
       // convert function literal to SAM closure
       tree match {
-        case closure(Nil, id @ Ident(nme.ANON_FUN), _)
+        case blockEndingInClosure(Nil, id @ Ident(nme.ANON_FUN), _)
         if defn.isFunctionNType(wtp) && !defn.isFunctionNType(pt) =>
           pt match {
             case SAMType(samMeth, samParent)
@@ -4577,6 +4577,8 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
               // but this prevents case blocks from implementing polymorphic partial functions,
               // since we do not know the result parameter a priori. Have to wait until the
               // body is typechecked.
+	          // Note: Need to come back to this when we clean up SAMs/PartialFunctions
+    	      // These conditions would most likely be affected by a precise spec.
               return toSAM(tree, samParent)
             case _ =>
           }

tests/pos/i21676.scala

@@ -0,0 +1,18 @@
+def Test =
+  val members = collection.immutable.SortedSet.empty[String]
+  members.collect {
+    var upNumber = 0
+    {
+      case m =>
+        // upNumber += 1
+        m
+    }
+  }
+
+  members.collect {
+    var upNumber = 0
+    {
+      m => m
+    }
+  }
+