Add test for null spread element typing with null-safety

A spread element in a collection literal, e.g., `[...e]` or `{...?e}`,
can have an `e` of type `Null` or a subtype thereof. This CL adds a test
where this situation is created for different types (`Null`, `Never`,
`X extends Null`) with different collection literals, and with and
without elements of other types. In general, the spread does not
contribute any elements, and hence the element/key/value type is `Never`
such that it has no effect on the element/key/value type of the
collection.

Cf. dart-lang/language#1079.

Change-Id: Ib760bcba18045b1afaa50f65c0e7d0a7fe9a8ab6
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/154693
Commit-Queue: Erik Ernst <[email protected]>
Reviewed-by: Nate Bosch <[email protected]>

Author: eernstg

tests/language/spread_collections/null_spread_test.dart

@@ -0,0 +1,87 @@
+// Copyright (c) 2020, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+// Test that the inferred element type, key type, and value type of a spread
+// element of the form `...?e` where `e` has type `Null` or a potentially
+// nullable subtype thereof is `Never`; and the element, key, and value types
+// are also `Never` for `...e` where the type of `e` is a subtype of `Never`.
+
+import 'package:expect/expect.dart';
+
+Function f = uncalled; // Do not optimize away `uncalled`.
+Never myNever = throw 1;
+
+void uncalled<X extends Null>(X x) {
+  // Test empty collection, involving only type `Never`.
+  var l1 = [...?x];
+  List<Never> l1b = l1;
+  var l2 = [...myNever];
+  List<Never> l2b = l2;
+  var l3 = [...?myNever]; // Warning, but possible.
+  List<Never> l3b = l3;
+  var s1 = {...?x, if (false) throw 1};
+  Set<Never> s1b = s1;
+  var s2 = {...myNever, if (false) throw 1};
+  Set<Never> s2b = s2;
+  var s3 = {...?myNever, if (false) throw 1}; // Warning.
+  Set<Never> s3b = s3;
+  var m1 = {...?x, if (false) throw 1: throw 1};
+  Map<Never, Never> m1b = m1;
+  var m2 = {...myNever, if (false) throw 1: throw 1};
+  Map<Never, Never> m2b = m2;
+  var m3 = {...?myNever, if (false) throw 1: throw 1}; // Warning.
+  Map<Never, Never> m3b = m3;
+
+  // Test non-empty collection of `Never` and `int`.
+  var li1 = [...?x, 1];
+  List<int> li1b = li1;
+  var li2 = [...myNever, 1];
+  List<int> li2b = li2;
+  var li3 = [...?myNever, 1]; // Warning.
+  List<int> li3b = li3;
+  var si1 = {1, ...?x};
+  Set<int> si1b = si1;
+  var si2 = {1, ...myNever};
+  Set<int> si2b = si2;
+  var si3 = {1, ...?myNever}; // Warning.
+  Set<int> si3b = si3;
+  var mi1 = {1: 1, ...?x};
+  Map<int, int> mi1b = mi1;
+  var mi2 = {1: 1, ...myNever};
+  Map<int, int> mi2b = mi2;
+  var mi3 = {1: 1, ...?myNever}; // Warning.
+  Map<int, int> mi3b = mi3;
+}
+
+void main() {
+  Null myNull = null;
+
+  // Test empty collection, involving only type `Never`.
+  var l1 = [...?null];
+  List<Never> l1b = l1;
+  var l2 = [...?myNull];
+  List<Never> l2b = l2;
+  var s1 = {...?null, if (false) throw 1};
+  Set<Never> s1b = s1;
+  var s2 = {if (false) throw 1, ...?myNull};
+  Set<Never> s2b = s2;
+  var m1 = {...?null, if (false) throw 1: throw 1};
+  Map<Never, Never> m1b = m1;
+  var m2 = {if (false) throw 1: throw 1, ...?myNull};
+  Map<Never, Never> m2b = m2;
+
+  // Test non-empty collection of `Never` and `int`.
+  var li1 = [...?null, 1];
+  List<int> li1b = li1;
+  var li2 = [1, ...?myNull];
+  List<int> li2b = li2;
+  var si1 = {1, ...?null};
+  Set<int> si1b = si1;
+  var si2 = {...?myNull, 1};
+  Set<int> si2b = si2;
+  var mi1 = {1: 1, ...?null};
+  Map<int, int> mi1b = mi1;
+  var mi2 = {...?myNull, 1: 1};
+  Map<int, int> mi2b = mi2;
+}