[wildcards] report dead late wildcard variable initializers

Fixes: #55905

Change-Id: I78f40c754ae2daf0894b7a2b114a739c60214f72
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/375200
Reviewed-by: Brian Wilkerson <[email protected]>
Commit-Queue: Phil Quitslund <[email protected]>

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

@@ -45,7 +45,7 @@
 #
 # Stats:
 # -  42 "needsEvaluation"
-# - 306 "needsFix"
+# - 307 "needsFix"
 # - 441 "hasFix"
 # - 517 "noFix"
 
@@ -3389,6 +3389,10 @@ WarningCode.DEAD_CODE:
   status: hasFix
 WarningCode.DEAD_CODE_CATCH_FOLLOWING_CATCH:
   status: hasFix
+WarningCode.DEAD_CODE_LATE_WILDCARD_VARIABLE_INITIALIZER:
+  status: needsFix
+  notes: |-
+    Remove the initializer or remove the late keyword.  
 WarningCode.DEAD_CODE_ON_CATCH_SUBTYPE:
   status: hasFix
 WarningCode.DEPRECATED_EXPORT_USE:

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

@@ -6288,6 +6288,19 @@ class WarningCode extends AnalyzerErrorCode {
     hasPublishedDocs: true,
   );
 
+  ///  No parameters.
+  static const WarningCode DEAD_CODE_LATE_WILDCARD_VARIABLE_INITIALIZER =
+      WarningCode(
+    'DEAD_CODE',
+    "Dead code: The assigned-to wildcard variable is marked late and can never "
+        "be referenced so this initializer will never be evaluated.",
+    correctionMessage:
+        "Try removing the code, removing the late modifier or changing the "
+        "variable to a non-wildcard.",
+    hasPublishedDocs: true,
+    uniqueName: 'DEAD_CODE_LATE_WILDCARD_VARIABLE_INITIALIZER',
+  );
+
   ///  Dead code is code that is never reached. This case covers cases where the
   ///  user has an on-catch clause such as `on A catch (e)`, where a supertype of
   ///  `A` was already caught.

pkg/analyzer/lib/src/error/dead_code_verifier.dart

@@ -111,6 +111,23 @@ class DeadCodeVerifier extends RecursiveAstVisitor<void> {
     });
   }
 
+  @override
+  void visitVariableDeclaration(VariableDeclaration node) {
+    var initializer = node.initializer;
+    if (initializer != null && node.isLate) {
+      var element = node.declaredElement;
+      // TODO(pq): ask the LocalVariableElement once implemented
+      if (_wildCardVariablesEnabled &&
+          element is LocalVariableElement &&
+          element.name == '_') {
+        _errorReporter.atNode(initializer,
+            WarningCode.DEAD_CODE_LATE_WILDCARD_VARIABLE_INITIALIZER);
+      }
+    }
+
+    super.visitVariableDeclaration(node);
+  }
+
   /// Resolve the names in the given [combinator] in the scope of the given
   /// [library].
   void _checkCombinator(LibraryElement library, Combinator combinator) {

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

@@ -983,6 +983,7 @@ const List<ErrorCode> errorCodeValues = [
   WarningCode.CONSTANT_PATTERN_NEVER_MATCHES_VALUE_TYPE,
   WarningCode.DEAD_CODE,
   WarningCode.DEAD_CODE_CATCH_FOLLOWING_CATCH,
+  WarningCode.DEAD_CODE_LATE_WILDCARD_VARIABLE_INITIALIZER,
   WarningCode.DEAD_CODE_ON_CATCH_SUBTYPE,
   WarningCode.DEPRECATED_EXPORT_USE,
   WarningCode.DEPRECATED_EXTENDS_FUNCTION,

pkg/analyzer/messages.yaml

@@ -23360,6 +23360,12 @@ WarningCode:
         }
       }
       ```
+  DEAD_CODE_LATE_WILDCARD_VARIABLE_INITIALIZER:
+    problemMessage: "Dead code: The assigned-to wildcard variable is marked late and can never be referenced so this initializer will never be evaluated."
+    correctionMessage: Try removing the code, removing the late modifier or changing the variable to a non-wildcard.
+    sharedName: DEAD_CODE
+    hasPublishedDocs: true
+    comment: No parameters.
   DEAD_CODE_ON_CATCH_SUBTYPE:
     problemMessage: "Dead code: This on-catch block won't be executed because '{0}' is a subtype of '{1}' and hence will have been caught already."
     correctionMessage: Try reordering the catch clauses so that this block can be reached, or removing the unreachable catch clause.

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

@@ -178,7 +178,17 @@ void f(Object x) {
 
 @reflectiveTest
 class DeadCodeTest_Language219 extends PubPackageResolutionTest
-    with WithLanguage219Mixin, DeadCodeTestCases_Language212 {}
+    with WithLanguage219Mixin, DeadCodeTestCases_Language212 {
+  @override
+  test_lateWildCardVariable_initializer() async {
+    await assertNoErrorsInCode(r'''
+f() {
+  // Not a wildcard variable.
+  late var _ = 0;
+}
+''');
+  }
+}
 
 mixin DeadCodeTestCases_Language212 on PubPackageResolutionTest {
   @override
@@ -1192,6 +1202,24 @@ void g(Never f) {
     ]);
   }
 
+  test_lateWildCardVariable_initializer() async {
+    await assertErrorsInCode(r'''
+f() {
+  late var _ = 0;
+}
+''', [
+      error(WarningCode.DEAD_CODE_LATE_WILDCARD_VARIABLE_INITIALIZER, 21, 1),
+    ]);
+  }
+
+  test_lateWildCardVariable_noInitializer() async {
+    await assertNoErrorsInCode(r'''
+f() {
+  late var _;
+}
+''');
+  }
+
   test_notUnassigned_propertyAccess() async {
     await assertNoErrorsInCode(r'''
 void f(int? i) {

pkg/analyzer/tool/diagnostics/diagnostics.md

@@ -3651,6 +3651,9 @@ void g() {
 
 _Dead code._
 
+_Dead code: The assigned-to wildcard variable is marked late and can never be
+referenced so this initializer will never be evaluated._
+
 #### Description
 
 The analyzer produces this diagnostic when code is found that won't be