[analyzer] Support ?.length on null

This adds support for constant evaluation of null aware access of length
on null. Null aware access of length on String was already supported in
both analyzer and CFE, and the spec has been updated to support it.

Closes #60509

Change-Id: Ie7e26d3e969bc82f5c65147f243350fbf4c68e67
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/429921
Commit-Queue: Johnni Winther <[email protected]>
Reviewed-by: Konstantin Shcheglov <[email protected]>

Author: johnniwinther

pkg/analyzer/lib/src/dart/constant/evaluation.dart

@@ -1214,6 +1214,7 @@ class ConstantVisitor extends UnifyingAstVisitor<Constant> {
           prefixResult,
           node.identifier,
           node,
+          isNullAware: false,
         );
         if (propertyAccessResult != null) {
           return propertyAccessResult;
@@ -1293,6 +1294,7 @@ class ConstantVisitor extends UnifyingAstVisitor<Constant> {
         prefixResult,
         node.propertyName,
         node,
+        isNullAware: node.isNullAware,
       );
       if (propertyAccessResult != null) {
         return propertyAccessResult;
@@ -1854,8 +1856,9 @@ class ConstantVisitor extends UnifyingAstVisitor<Constant> {
   Constant? _evaluatePropertyAccess(
     DartObjectImpl targetResult,
     SimpleIdentifier identifier,
-    AstNode errorNode,
-  ) {
+    AstNode errorNode, {
+    required bool isNullAware,
+  }) {
     var propertyElement = identifier.element;
     if (propertyElement is GetterElement && propertyElement.isStatic) {
       return null;
@@ -1878,10 +1881,12 @@ class ConstantVisitor extends UnifyingAstVisitor<Constant> {
     var targetType = targetResult.type;
 
     // Evaluate a constant that reads the length of a `String`.
-    if (identifier.name == 'length' &&
-        targetType is InterfaceType &&
-        targetType.isDartCoreString) {
-      return _dartObjectComputer.stringLength(errorNode, targetResult);
+    if (identifier.name == 'length') {
+      if (targetType is InterfaceType && targetType.isDartCoreString) {
+        return _dartObjectComputer.stringLength(errorNode, targetResult);
+      } else if (targetType.isDartCoreNull && isNullAware) {
+        return ConstantEvaluationEngine._nullObject(_library);
+      }
     }
 
     var element = identifier.element;

pkg/analyzer/test/src/dart/constant/evaluation_test.dart

@@ -1099,6 +1099,66 @@ void f(Object? x) {
 ''');
   }
 
+  test_propertyAccess_nullAware_dynamic_length_notNull() async {
+    await assertNoErrorsInCode(r'''
+const dynamic d = 'foo';
+const int? c = d?.length;
+''');
+    var result = _topLevelVar('c');
+    assertDartObjectText(result, '''
+int 3
+  variable: <testLibrary>::@topLevelVariable::c
+''');
+  }
+
+  test_propertyAccess_nullAware_dynamic_length_null() async {
+    await assertNoErrorsInCode(r'''
+const dynamic d = null;
+const int? c = d?.length;
+''');
+    var result = _topLevelVar('c');
+    assertDartObjectText(result, '''
+Null null
+  variable: <testLibrary>::@topLevelVariable::c
+''');
+  }
+
+  test_propertyAccess_nullAware_list_length_null() async {
+    await assertNoErrorsInCode(r'''
+const List? l = null;
+const int? c = l?.length;
+''');
+    var result = _topLevelVar('c');
+    assertDartObjectText(result, '''
+Null null
+  variable: <testLibrary>::@topLevelVariable::c
+''');
+  }
+
+  test_propertyAccess_nullAware_string_length_notNull() async {
+    await assertNoErrorsInCode(r'''
+const String? s = 'foo';
+const int? c = s?.length;
+''');
+    var result = _topLevelVar('c');
+    assertDartObjectText(result, '''
+int 3
+  variable: <testLibrary>::@topLevelVariable::c
+''');
+  }
+
+  test_propertyAccess_nullAware_string_length_null() async {
+    await assertNoErrorsInCode(r'''
+const String? s = null;
+const int? c = s?.length;
+''');
+    var result = _topLevelVar('c');
+    assertDartObjectText(result, '''
+Null null
+  variable: <testLibrary>::@topLevelVariable::c
+''');
+  }
+
   test_recordTypeAnnotation() async {
     await assertNoErrorsInCode(r'''
 const a = ('',) is (int,);

pkg/analyzer/test/src/diagnostics/const_eval_property_access_test.dart

@@ -47,6 +47,21 @@ const a = const A();
     );
   }
 
+  test_length_dynamic_notNull() async {
+    await assertNoErrorsInCode(r'''
+const dynamic d = 'foo';
+const int? c = d.length;''');
+  }
+
+  test_length_dynamic_null() async {
+    await assertErrorsInCode(
+      r'''
+const dynamic d = null;
+const int? c = d.length;''',
+      [error(CompileTimeErrorCode.CONST_EVAL_PROPERTY_ACCESS, 39, 8)],
+    );
+  }
+
   test_length_invalidTarget() async {
     await assertErrorsInCode(
       '''
@@ -90,4 +105,34 @@ const x = const C().t;
       [error(CompileTimeErrorCode.CONST_EVAL_PROPERTY_ACCESS, 59, 11)],
     );
   }
+
+  test_nullAware_isEven_null() async {
+    await assertErrorsInCode(
+      r'''
+const int? s = null;
+const bool? c = s?.isEven;''',
+      [error(CompileTimeErrorCode.CONST_EVAL_PROPERTY_ACCESS, 37, 9)],
+    );
+  }
+
+  test_nullAware_length_dynamic_null() async {
+    await assertNoErrorsInCode(r'''
+const dynamic d = 'foo';
+const int? c = d?.length;''');
+  }
+
+  test_nullAware_length_list_notNull() async {
+    await assertErrorsInCode(
+      r'''
+const List? l = [];
+const int? c = l?.length;''',
+      [error(CompileTimeErrorCode.CONST_EVAL_PROPERTY_ACCESS, 35, 9)],
+    );
+  }
+
+  test_nullAware_length_string_notNull() async {
+    await assertNoErrorsInCode(r'''
+const String? s = '';
+const int? c = s?.length;''');
+  }
 }