instances are infinite sets (see #3031)

matcher.ml

@@ -828,7 +828,7 @@ let rec all_ctors mctx t =
 	| TAnon a ->
 		h,true
 	| TInst(_,_) ->
-		h,false
+		h,true
 	| _ ->
 		h,true
 

tests/unit/issues/Issue3031.hx

@@ -0,0 +1,34 @@
+package unit.issues;
+
+private class X {
+	public var s:String;
+	public function new(s:String) {
+		this.s = s;
+	}
+}
+
+class Issue3031 extends Test {
+	function testArty() {
+		var a = "";
+		switch (new StringBuf()) {
+			case null: a = "null";
+			case notNull: a = "not null";
+		}
+		eq("not null", a);
+	}
+
+	function testHeinz() {
+		function doSwitch(x1:Null<X>, x2:Null<X>) {
+			return switch [x1,x2] {
+				case [null,null]: "";
+				case [a,null]: a.s;
+				case [null,b]: b.s;
+				case [a,b]: a.s + b.s;
+			}
+		}
+		eq("", doSwitch(null, null));
+		eq("foo", doSwitch(new X("foo"), null));
+		eq("bar", doSwitch(null, new X("bar")));
+		eq("foobar", doSwitch(new X("foo"), new X("bar")));
+	}
+}