Fine. Formal parameters are not in scope of the signature of a function expression.

Specification: 9.2 Formal Parameters

The formal parameter list of a function declaration introduces a new
scope known as the function’s formal parameter scope. The formal
parameter scope △ of a non-generic function f is enclosed in the scope
where f is declared. The formal parameter scope of a generic function f
is enclosed in the type parameter scope of f. Every formal parameter
introduces a local variable into the formal parameter scope. The current
scope for the function’s signature is the scope that encloses the formal
parameter scope.

This means that in a generic function declaration, the return type and
parameter type annotations can use the formal type parameters, but the
formal parameters are not in scope in the signature.

The body of a function declaration introduces a new scope known as the
function’s body scope. The body scope of a function f is enclosed in the
scope △ introduced by the formal parameter scope of f.

Change-Id: Icc795c11f489fadd6ed25b1a5904c5dfd1601d09
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/457885
Reviewed-by: Johnni Winther <[email protected]>
Commit-Queue: Konstantin Shcheglov <[email protected]>

Author: scheglov

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

@@ -5178,11 +5178,12 @@ class ScopeResolverVisitor extends UnifyingAstVisitor<void> {
         return;
       }
 
-      nameScope = FormalParameterScope(
-        TypeParameterScope(nameScope, element.typeParameters),
-        element.formalParameters,
-      );
-      super.visitFunctionExpression(node);
+      nameScope = TypeParameterScope(nameScope, element.typeParameters);
+      node.typeParameters?.accept(this);
+      node.parameters?.accept(this);
+
+      nameScope = FormalParameterScope(nameScope, element.formalParameters);
+      node.body.accept(this);
     } finally {
       nameScope = outerScope;
       _enclosingClosure = outerClosure;

pkg/analyzer/test/src/dart/analysis/driver_test.dart

@@ -75167,6 +75167,47 @@ const a = -'' + 2;
     );
   }
 
+  test_manifest_constInitializer_functionExpression_formalParameter_defaultValue_formalParameter() async {
+    await _runLibraryManifestScenario(
+      initialCode: r'''
+final f = ({int x = x}) {};
+''',
+      expectedInitialEvents: r'''
+[operation] linkLibraryCycle SDK
+[operation] linkLibraryCycle
+  package:test/test.dart
+    hashForRequirements: #H0
+    declaredGetters
+      f: #M0
+    declaredVariables
+      f: #M1
+    exportMapId: #M2
+    exportMap
+      f: #M0
+''',
+      updatedCode: r'''
+final f = ({int x = x}) {};
+class A {}
+''',
+      expectedUpdatedEvents: r'''
+[operation] linkLibraryCycle
+  package:test/test.dart
+    hashForRequirements: #H1
+    declaredClasses
+      A: #M3
+        interface: #M4
+    declaredGetters
+      f: #M0
+    declaredVariables
+      f: #M1
+    exportMapId: #M5
+    exportMap
+      A: #M3
+      f: #M0
+''',
+    );
+  }
+
   test_manifest_constInitializer_identifier_addIdentifier() async {
     configuration.withElementManifests = true;
     await _runLibraryManifestScenario(

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

@@ -2,6 +2,7 @@
 // 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 'context_collection_resolution.dart';
@@ -93,6 +94,42 @@ FunctionExpression
     element: null@null
       type: Null Function<T extends num>()
   staticType: Null Function<T extends num>()
+''');
+  }
+
+  test_signatureScope_noFormalParameters() async {
+    await assertErrorsInCode(
+      '''
+var f = ({int x = x}) {};
+''',
+      [error(CompileTimeErrorCode.undefinedIdentifier, 18, 1)],
+    );
+
+    var node = findNode.singleFormalParameterList;
+    assertResolvedNodeText(node, r'''
+FormalParameterList
+  leftParenthesis: (
+  leftDelimiter: {
+  parameter: DefaultFormalParameter
+    parameter: SimpleFormalParameter
+      type: NamedType
+        name: int
+        element: dart:core::@class::int
+        type: int
+      name: x
+      declaredElement: <testLibraryFragment> x@14
+        element: isPublic
+          type: int
+    separator: =
+    defaultValue: SimpleIdentifier
+      token: x
+      element: <null>
+      staticType: InvalidType
+    declaredElement: <testLibraryFragment> x@14
+      element: isPublic
+        type: int
+  rightDelimiter: }
+  rightParenthesis: )
 ''');
   }
 }