[model] Update handling of ?.length in constants

This adds reporting of an error in CFE for ?.length in constant expressions and improves the message for the analyzer in the same case. The error in the analyzer was previously the invalid claim that

    The property 'length' can't be accessed on the type 'Null' in a constant expression.

Closes #60509

Change-Id: Ibbe0fa1ace3bea9d83efea2ccf3ea9716a125d74
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/421841
Reviewed-by: Konstantin Shcheglov <[email protected]>
Commit-Queue: Johnni Winther <[email protected]>
Reviewed-by: Brian Wilkerson <[email protected]>

Author: johnniwinther

pkg/_fe_analyzer_shared/lib/src/messages/codes_generated.dart

@@ -2112,6 +2112,17 @@ const MessageCode messageConstEvalNotMapInSpread = const MessageCode(
   problemMessage: r"""Only maps can be used in spreads in constant maps.""",
 );
 
+// DO NOT EDIT. THIS FILE IS GENERATED. SEE TOP OF FILE.
+const Code<Null> codeConstEvalNullAwareAccess = messageConstEvalNullAwareAccess;
+
+// DO NOT EDIT. THIS FILE IS GENERATED. SEE TOP OF FILE.
+const MessageCode messageConstEvalNullAwareAccess = const MessageCode(
+  "ConstEvalNullAwareAccess",
+  analyzerCodes: <String>["CONST_EVAL_NULL_AWARE_ACCESS"],
+  problemMessage:
+      r"""Null-aware property access can't be used in a constant expression.""",
+);
+
 // DO NOT EDIT. THIS FILE IS GENERATED. SEE TOP OF FILE.
 const Code<Null> codeConstEvalNullValue = messageConstEvalNullValue;
 

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

@@ -426,6 +426,8 @@ CompileTimeErrorCode.CONST_EVAL_FOR_ELEMENT:
   status: needsEvaluation
 CompileTimeErrorCode.CONST_EVAL_METHOD_INVOCATION:
   status: needsEvaluation
+CompileTimeErrorCode.CONST_EVAL_NULL_AWARE_ACCESS:
+  status: needsEvaluation
 CompileTimeErrorCode.CONST_EVAL_PRIMITIVE_EQUALITY:
   status: needsEvaluation
 CompileTimeErrorCode.CONST_EVAL_PROPERTY_ACCESS:

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

@@ -705,6 +705,10 @@ class ConstantVerifier extends RecursiveAstVisitor<void> {
           diagnosticCode,
           CompileTimeErrorCode.CONST_EVAL_METHOD_INVOCATION,
         ) ||
+        identical(
+          diagnosticCode,
+          CompileTimeErrorCode.CONST_EVAL_NULL_AWARE_ACCESS,
+        ) ||
         identical(
           diagnosticCode,
           CompileTimeErrorCode.CONST_EVAL_PRIMITIVE_EQUALITY,

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

@@ -1289,6 +1289,13 @@ class ConstantVisitor extends UnifyingAstVisitor<Constant> {
         return prefixResult;
       }
 
+      if (node.isNullAware) {
+        return InvalidConstant.forEntity(
+          entity: node,
+          diagnosticCode: CompileTimeErrorCode.CONST_EVAL_NULL_AWARE_ACCESS,
+        );
+      }
+
       var propertyAccessResult = _evaluatePropertyAccess(
         prefixResult,
         node.propertyName,

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

@@ -1025,6 +1025,12 @@ class CompileTimeErrorCode extends ErrorCode {
         "Methods can't be invoked in constant expressions.",
       );
 
+  static const CompileTimeErrorCode CONST_EVAL_NULL_AWARE_ACCESS =
+      CompileTimeErrorCode(
+        'CONST_EVAL_NULL_AWARE_ACCESS',
+        "Null-aware property access can't be used in a constant expression.",
+      );
+
   ///  See https://spec.dart.dev/DartLangSpecDraft.pdf#constants, "Constants",
   ///  for text about "An expression of the form e1 == e2".
   static const CompileTimeErrorCode CONST_EVAL_PRIMITIVE_EQUALITY =

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

@@ -136,6 +136,7 @@ const List<ErrorCode> errorCodeValues = [
   CompileTimeErrorCode.CONST_EVAL_EXTENSION_TYPE_METHOD,
   CompileTimeErrorCode.CONST_EVAL_FOR_ELEMENT,
   CompileTimeErrorCode.CONST_EVAL_METHOD_INVOCATION,
+  CompileTimeErrorCode.CONST_EVAL_NULL_AWARE_ACCESS,
   CompileTimeErrorCode.CONST_EVAL_PRIMITIVE_EQUALITY,
   CompileTimeErrorCode.CONST_EVAL_PROPERTY_ACCESS,
   CompileTimeErrorCode.CONST_EVAL_THROWS_EXCEPTION,

pkg/analyzer/lib/src/lint/constants.dart

@@ -49,6 +49,7 @@ class _ConstantAnalysisErrorListener extends AnalysisErrorListener {
         case CompileTimeErrorCode.CONST_EVAL_EXTENSION_METHOD:
         case CompileTimeErrorCode.CONST_EVAL_EXTENSION_TYPE_METHOD:
         case CompileTimeErrorCode.CONST_EVAL_METHOD_INVOCATION:
+        case CompileTimeErrorCode.CONST_EVAL_NULL_AWARE_ACCESS:
         case CompileTimeErrorCode.CONST_EVAL_PROPERTY_ACCESS:
         case CompileTimeErrorCode.CONST_EVAL_TYPE_BOOL:
         case CompileTimeErrorCode.CONST_EVAL_TYPE_BOOL_INT:

pkg/analyzer/messages.yaml

@@ -2824,6 +2824,9 @@ CompileTimeErrorCode:
     problemMessage: "Constant expressions don't support 'for' elements."
     correctionMessage: "Try replacing the 'for' element with a spread, or removing 'const'."
     hasPublishedDocs: false
+  CONST_EVAL_NULL_AWARE_ACCESS:
+    problemMessage: "Null-aware property access can't be used in a constant expression."
+    hasPublishedDocs: false
   CONST_EVAL_PROPERTY_ACCESS:
     problemMessage: "The property '{0}' can't be accessed on the type '{1}' in a constant expression."
     hasPublishedDocs: false

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

@@ -90,4 +90,13 @@ const x = const C().t;
       [error(CompileTimeErrorCode.CONST_EVAL_PROPERTY_ACCESS, 59, 11)],
     );
   }
+
+  test_null_aware_property_access() async {
+    await assertErrorsInCode(
+      r'''
+const String? s = null;
+const int? c = s?.length;''',
+      [error(CompileTimeErrorCode.CONST_EVAL_NULL_AWARE_ACCESS, 39, 9)],
+    );
+  }
 }

pkg/front_end/lib/src/kernel/body_builder.dart

@@ -2872,6 +2872,10 @@ class BodyBuilder extends StackListenerImpl
     ]));
     Object? send = pop();
     if (send is Selector) {
+      if (send is PropertySelector && constantContext != ConstantContext.none) {
+        addProblem(
+            cfe.messageConstEvalNullAwareAccess, token.charOffset, noLength);
+      }
       push(send.withReceiver(pop(), token.charOffset, isNullAware: true));
     } else {
       pop();