From e5632198c8ac43dfd474dba0cc978aab18870968 Mon Sep 17 00:00:00 2001
From: noti0na1 <noti0na1@users.noreply.github.com>
Date: Thu, 3 Mar 2022 14:30:24 -0500
Subject: [PATCH] Fix intercept TypeApply in explicit nulls

---
 .../dotty/tools/dotc/transform/Erasure.scala  |  8 +++---
 .../explicit-nulls/pos-patmat/match-pat.scala | 27 +++++++++++++++++++
 2 files changed, 32 insertions(+), 3 deletions(-)
 create mode 100644 tests/explicit-nulls/pos-patmat/match-pat.scala

diff --git a/compiler/src/dotty/tools/dotc/transform/Erasure.scala b/compiler/src/dotty/tools/dotc/transform/Erasure.scala
index 739f7d6b06b4..d19084ac3d8c 100644
--- a/compiler/src/dotty/tools/dotc/transform/Erasure.scala
+++ b/compiler/src/dotty/tools/dotc/transform/Erasure.scala
@@ -784,9 +784,11 @@ object Erasure {
       }
 
     override def typedTypeApply(tree: untpd.TypeApply, pt: Type)(using Context): Tree = {
-      val ntree = atPhase(erasurePhase)(
-        interceptTypeApply(tree.asInstanceOf[TypeApply])
-      ).withSpan(tree.span)
+      val ntree = atPhase(erasurePhase){
+        // Use erased-type semantic to intercept TypeApply in explicit nulls
+        val interceptCtx = if ctx.explicitNulls then ctx.retractMode(Mode.SafeNulls) else ctx
+        interceptTypeApply(tree.asInstanceOf[TypeApply])(using interceptCtx)
+      }.withSpan(tree.span)
 
       ntree match {
         case TypeApply(fun, args) =>
diff --git a/tests/explicit-nulls/pos-patmat/match-pat.scala b/tests/explicit-nulls/pos-patmat/match-pat.scala
new file mode 100644
index 000000000000..c9a408bde6d4
--- /dev/null
+++ b/tests/explicit-nulls/pos-patmat/match-pat.scala
@@ -0,0 +1,27 @@
+// Ensure we don't get "the type test for argType cannot be checked at runtime" warning
+
+class Symbol {
+  type ThisName
+}
+
+type TermSymbol = Symbol { type ThisName = String }
+
+type TermSymbolOrNull = TermSymbol | Null
+
+def testSimple =
+  val x: Symbol | Null = ???
+  x match
+    case key: Symbol => 1
+    case null => 0
+
+def testWithRefinedType =
+  val x: TermSymbol | Null = ???
+  x match
+    case key: TermSymbol => 1
+    case null => 0
+
+def testWithAlias =
+  val x: TermSymbolOrNull = ???
+  x match
+    case key: TermSymbol => 1
+    case null => 0