Make null check to participate in null shorting.

Bug: dart-lang/language#1163
Change-Id: I6e648f76413c5036a867b2125359f28eb03027c2
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/163727
Reviewed-by: Brian Wilkerson <[email protected]>
Commit-Queue: Konstantin Shcheglov <[email protected]>

Author: scheglov

pkg/analyzer/lib/src/dart/ast/ast.dart

@@ -8092,8 +8092,7 @@ class PostfixExpressionImpl extends ExpressionImpl
   }
 
   @override
-  bool _extendsNullShorting(Expression child) =>
-      operator.type != TokenType.BANG && identical(child, operand);
+  bool _extendsNullShorting(Expression child) => identical(child, operand);
 }
 
 /// An identifier that is prefixed or an access to an object property where the

pkg/analyzer/lib/src/dart/resolver/postfix_expression_resolver.dart

@@ -232,6 +232,7 @@ class PostfixExpressionResolver {
     var type = _typeSystem.promoteToNonNull(operandType);
     _inferenceHelper.recordStaticType(node, type);
 
+    _resolver.nullShortingTermination(node);
     _flowAnalysis?.flow?.nonNullAssert_end(operand);
   }
 }

pkg/analyzer/test/src/dart/resolution/postfix_expression_test.dart

@@ -641,6 +641,67 @@ int g() => f(null)!;
     );
   }
 
+  /// See https://github.com/dart-lang/language/issues/1163
+  test_nullCheck_participatesNullShorting() async {
+    await assertErrorsInCode('''
+class A {
+  int zero;
+  int? zeroOrNull;
+
+  A(this.zero, [this.zeroOrNull]);
+}
+
+void test1(A? a) => a?.zero!;
+void test2(A? a) => a?.zeroOrNull!;
+void test3(A? a) => a?.zero!.isEven;
+void test4(A? a) => a?.zeroOrNull!.isEven;
+
+class Foo {
+  Bar? bar;
+
+  Foo(this.bar);
+
+  Bar? operator [](int? index) => null;
+}
+
+class Bar {
+  int baz;
+
+  Bar(this.baz);
+
+  int operator [](int index) => index;
+}
+
+void test5(Foo? foo) => foo?.bar!;
+void test6(Foo? foo) => foo?.bar!.baz;
+void test7(Foo? foo, int a) => foo?.bar![a];
+void test8(Foo? foo, int? a) => foo?[a]!;
+void test9(Foo? foo, int? a) => foo?[a]!.baz;
+void test10(Foo? foo, int? a, int b) => foo?[a]![b];
+''', [
+      error(StaticWarningCode.UNNECESSARY_NON_NULL_ASSERTION, 107, 1),
+      error(StaticWarningCode.UNNECESSARY_NON_NULL_ASSERTION, 173, 1),
+    ]);
+
+    void assertTestType(int index, String expected) {
+      var function = findNode.functionDeclaration('test$index(');
+      var body = function.functionExpression.body as ExpressionFunctionBody;
+      assertType(body.expression, expected);
+    }
+
+    assertTestType(1, 'int?');
+    assertTestType(2, 'int?');
+    assertTestType(3, 'bool?');
+    assertTestType(4, 'bool?');
+
+    assertTestType(5, 'Bar?');
+    assertTestType(6, 'int?');
+    assertTestType(7, 'int?');
+    assertTestType(8, 'Bar?');
+    assertTestType(9, 'int?');
+    assertTestType(10, 'int?');
+  }
+
   test_nullCheck_superExpression() async {
     await assertErrorsInCode(r'''
 class A {