[analyzer] Report warnings on expressions with non-nullable types

Part of #56836

Change-Id: I3fcdceff667f371fcf4b66c12bd16e2f34e6446c
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/391621
Reviewed-by: Keerti Parthasarathy <[email protected]>
Reviewed-by: Erik Ernst <[email protected]>
Commit-Queue: Chloe Stefantsova <[email protected]>

pkg/analysis_server/lib/src/services/correction/error_fix_status.yaml

@@ -3375,6 +3375,12 @@ StaticWarningCode.INVALID_NULL_AWARE_OPERATOR:
   status: hasFix
 StaticWarningCode.INVALID_NULL_AWARE_OPERATOR_AFTER_SHORT_CIRCUIT:
   status: hasFix
+StaticWarningCode.INVALID_NULL_AWARE_ELEMENT:
+  status: needsEvaluation
+StaticWarningCode.INVALID_NULL_AWARE_MAP_ENTRY_KEY:
+  status: needsEvaluation
+StaticWarningCode.INVALID_NULL_AWARE_MAP_ENTRY_VALUE:
+  status: needsEvaluation
 StaticWarningCode.MISSING_ENUM_CONSTANT_IN_SWITCH:
   status: hasFix
 StaticWarningCode.UNNECESSARY_NON_NULL_ASSERTION:

pkg/analyzer/lib/src/error/codes.g.dart

@@ -6070,6 +6070,37 @@ class StaticWarningCode extends AnalyzerErrorCode {
     hasPublishedDocs: true,
   );
 
+  ///  No parameters.
+  static const StaticWarningCode INVALID_NULL_AWARE_ELEMENT = StaticWarningCode(
+    'INVALID_NULL_AWARE_OPERATOR',
+    "The element can't be null, so the null-aware operator '?' is unnecessary.",
+    correctionMessage: "Try removing the operator '?'.",
+    hasPublishedDocs: true,
+    uniqueName: 'INVALID_NULL_AWARE_ELEMENT',
+  );
+
+  ///  No parameters.
+  static const StaticWarningCode INVALID_NULL_AWARE_MAP_ENTRY_KEY =
+      StaticWarningCode(
+    'INVALID_NULL_AWARE_OPERATOR',
+    "The map entry key can't be null, so the null-aware operator '?' is "
+        "unnecessary.",
+    correctionMessage: "Try removing the operator '?'.",
+    hasPublishedDocs: true,
+    uniqueName: 'INVALID_NULL_AWARE_MAP_ENTRY_KEY',
+  );
+
+  ///  No parameters.
+  static const StaticWarningCode INVALID_NULL_AWARE_MAP_ENTRY_VALUE =
+      StaticWarningCode(
+    'INVALID_NULL_AWARE_OPERATOR',
+    "The map entry value can't be null, so the null-aware operator '?' is "
+        "unnecessary.",
+    correctionMessage: "Try removing the operator '?'.",
+    hasPublishedDocs: true,
+    uniqueName: 'INVALID_NULL_AWARE_MAP_ENTRY_VALUE',
+  );
+
   ///  Parameters:
   ///  0: the null-aware operator that is invalid
   ///  1: the non-null-aware operator that can replace the invalid operator

pkg/analyzer/lib/src/error/error_code_values.g.dart

@@ -958,6 +958,9 @@ const List<ErrorCode> errorCodeValues = [
   ScannerErrorCode.UNTERMINATED_MULTI_LINE_COMMENT,
   ScannerErrorCode.UNTERMINATED_STRING_LITERAL,
   StaticWarningCode.DEAD_NULL_AWARE_EXPRESSION,
+  StaticWarningCode.INVALID_NULL_AWARE_ELEMENT,
+  StaticWarningCode.INVALID_NULL_AWARE_MAP_ENTRY_KEY,
+  StaticWarningCode.INVALID_NULL_AWARE_MAP_ENTRY_VALUE,
   StaticWarningCode.INVALID_NULL_AWARE_OPERATOR,
   StaticWarningCode.INVALID_NULL_AWARE_OPERATOR_AFTER_SHORT_CIRCUIT,
   StaticWarningCode.MISSING_ENUM_CONSTANT_IN_SWITCH,

pkg/analyzer/lib/src/generated/error_verifier.dart

@@ -1157,6 +1157,21 @@ class ErrorVerifier extends RecursiveAstVisitor<void>
     super.visitListLiteral(node);
   }
 
+  @override
+  void visitMapLiteralEntry(MapLiteralEntry node) {
+    if (node.keyQuestion != null) {
+      _checkForUnnecessaryNullAware(node.key, node.keyQuestion!,
+          nullAwareElementOrMapEntryKind:
+              _NullAwareElementOrMapEntryKind.mapEntryKey);
+    }
+    if (node.valueQuestion != null) {
+      _checkForUnnecessaryNullAware(node.value, node.valueQuestion!,
+          nullAwareElementOrMapEntryKind:
+              _NullAwareElementOrMapEntryKind.mapEntryValue);
+    }
+    super.visitMapLiteralEntry(node);
+  }
+
   @override
   void visitMethodDeclaration(covariant MethodDeclarationImpl node) {
     var element = node.declaredElement!;
@@ -1296,6 +1311,14 @@ class ErrorVerifier extends RecursiveAstVisitor<void>
     super.visitNativeFunctionBody(node);
   }
 
+  @override
+  void visitNullAwareElement(NullAwareElement node) {
+    _checkForUnnecessaryNullAware(node.value, node.question,
+        nullAwareElementOrMapEntryKind:
+            _NullAwareElementOrMapEntryKind.element);
+    super.visitNullAwareElement(node);
+  }
+
   @override
   void visitPatternVariableDeclarationStatement(
     covariant PatternVariableDeclarationStatementImpl node,
@@ -5551,7 +5574,8 @@ class ErrorVerifier extends RecursiveAstVisitor<void>
     }
   }
 
-  void _checkForUnnecessaryNullAware(Expression target, Token operator) {
+  void _checkForUnnecessaryNullAware(Expression target, Token operator,
+      {_NullAwareElementOrMapEntryKind? nullAwareElementOrMapEntryKind}) {
     if (target is SuperExpression) {
       return;
     }
@@ -5560,9 +5584,20 @@ class ErrorVerifier extends RecursiveAstVisitor<void>
     Token endToken = operator;
     List<Object> arguments = const [];
     if (operator.type == TokenType.QUESTION) {
-      errorCode = StaticWarningCode.INVALID_NULL_AWARE_OPERATOR;
-      endToken = operator.next!;
-      arguments = ['?[', '['];
+      if (nullAwareElementOrMapEntryKind == null) {
+        errorCode = StaticWarningCode.INVALID_NULL_AWARE_OPERATOR;
+        endToken = operator.next!;
+        arguments = ['?[', '['];
+      } else {
+        switch (nullAwareElementOrMapEntryKind) {
+          case _NullAwareElementOrMapEntryKind.element:
+            errorCode = StaticWarningCode.INVALID_NULL_AWARE_ELEMENT;
+          case _NullAwareElementOrMapEntryKind.mapEntryKey:
+            errorCode = StaticWarningCode.INVALID_NULL_AWARE_MAP_ENTRY_KEY;
+          case _NullAwareElementOrMapEntryKind.mapEntryValue:
+            errorCode = StaticWarningCode.INVALID_NULL_AWARE_MAP_ENTRY_VALUE;
+        }
+      }
     } else if (operator.type == TokenType.QUESTION_PERIOD) {
       errorCode = StaticWarningCode.INVALID_NULL_AWARE_OPERATOR;
       arguments = [operator.lexeme, '.'];
@@ -7164,6 +7199,10 @@ class _MacroTypeAnnotationLocationConverter {
   }
 }
 
+/// Signals the kind of the null-aware element or entry observed in list, set,
+/// or map literals.
+enum _NullAwareElementOrMapEntryKind { element, mapEntryKey, mapEntryValue }
+
 /// Recursively visits a type annotation, looking uninstantiated bounds.
 class _UninstantiatedBoundChecker extends RecursiveAstVisitor<void> {
   final ErrorReporter _errorReporter;

pkg/analyzer/messages.yaml

@@ -22673,6 +22673,14 @@ StaticWarningCode:
       has the value `null`, the cast will fail and the invocation of `length`
       will not happen.
 
+      The following code produces this diagnostic because `s` can't be `null`:
+
+      ```dart
+      List<String> makeSingletonList(String s) {
+        return <String>[[!?!]s];
+      }
+      ```
+
       #### Common fixes
 
       Replace the null-aware operator with a non-null-aware equivalent; for
@@ -22695,6 +22703,24 @@ StaticWarningCode:
       Parameters:
       0: the null-aware operator that is invalid
       1: the non-null-aware operator that can replace the invalid operator
+  INVALID_NULL_AWARE_ELEMENT:
+    sharedName: INVALID_NULL_AWARE_OPERATOR
+    problemMessage: "The element can't be null, so the null-aware operator '?' is unnecessary."
+    correctionMessage: "Try removing the operator '?'."
+    hasPublishedDocs: true
+    comment: No parameters.
+  INVALID_NULL_AWARE_MAP_ENTRY_KEY:
+    sharedName: INVALID_NULL_AWARE_OPERATOR
+    problemMessage: "The map entry key can't be null, so the null-aware operator '?' is unnecessary."
+    correctionMessage: "Try removing the operator '?'."
+    hasPublishedDocs: true
+    comment: No parameters.
+  INVALID_NULL_AWARE_MAP_ENTRY_VALUE:
+    sharedName: INVALID_NULL_AWARE_OPERATOR
+    problemMessage: "The map entry value can't be null, so the null-aware operator '?' is unnecessary."
+    correctionMessage: "Try removing the operator '?'."
+    hasPublishedDocs: true
+    comment: No parameters.
   MISSING_ENUM_CONSTANT_IN_SWITCH:
     problemMessage: "Missing case clause for '{0}'."
     correctionMessage: Try adding a case clause for the missing constant, or adding a default clause.

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

@@ -0,0 +1,53 @@
+// Copyright (c) 2024, 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.
+
+import 'package:analyzer/src/error/codes.dart';
+import 'package:test_reflective_loader/test_reflective_loader.dart';
+
+import '../dart/resolution/context_collection_resolution.dart';
+
+main() {
+  defineReflectiveSuite(() {
+    defineReflectiveTests(InvalidNullAwareElementsErrorTest);
+  });
+}
+
+@reflectiveTest
+class InvalidNullAwareElementsErrorTest extends PubPackageResolutionTest {
+  test_invalid_null_aware_element_in_list() async {
+    await assertErrorsInCode('''
+const stringConst = "";
+const list = [0, ?stringConst];
+''', [
+      error(StaticWarningCode.INVALID_NULL_AWARE_ELEMENT, 41, 1),
+    ]);
+  }
+
+  test_invalid_null_aware_element_in_set() async {
+    await assertErrorsInCode('''
+const stringConst = "";
+const set = {0, ?stringConst};
+''', [
+      error(StaticWarningCode.INVALID_NULL_AWARE_ELEMENT, 40, 1),
+    ]);
+  }
+
+  test_invalid_null_aware_key_in_map() async {
+    await assertErrorsInCode('''
+const intConst = 0;
+const map = {?0: intConst};
+''', [
+      error(StaticWarningCode.INVALID_NULL_AWARE_MAP_ENTRY_KEY, 33, 1),
+    ]);
+  }
+
+  test_invalid_null_aware_value_in_map() async {
+    await assertErrorsInCode('''
+const intConst = 0;
+const map = {0: ?intConst};
+''', [
+      error(StaticWarningCode.INVALID_NULL_AWARE_MAP_ENTRY_VALUE, 36, 1),
+    ]);
+  }
+}