Fix #1335: Generate null checks for extractors

Author: OlivierBlanvillain

src/dotty/tools/dotc/transform/PatternMatcher.scala

@@ -253,9 +253,13 @@ class PatternMatcher extends MiniPhaseTransform with DenotTransformer {
               )
           } else {
             assert(defn.isProductSubType(prev.tpe))
-            Block(
-              List(ValDef(b.asTerm, prev)),
-              next //Substitution(b, ref(prevSym))(next)
+            val nullCheck: Tree = prev.select(defn.Object_ne).appliedTo(Literal(Constant(null)))
+            ifThenElseZero(
+              nullCheck,
+              Block(
+                List(ValDef(b.asTerm, prev)),
+                next //Substitution(b, ref(prevSym))(next)
+              )
             )
           }
         }

tests/pos/t1335.scala

@@ -0,0 +1,11 @@
+case class MyTuple(a: Int, b: Int)
+
+object Test {
+  def main(args: Array[String]): Unit =
+    try {
+      val mt: MyTuple = null
+      val MyTuple(a, b) = mt
+    } catch {
+      case e: MatchError => ()
+    }
+}