fix #26155 - optional param passed to positional was not inferred

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

 // Copyright (c) 2014, 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.
 
 library analyzer.src.generated.resolver;
 
 import 'dart:collection';
 
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/ast/token.dart';
@@ skipping 7165 matching lines @@
       }
 
       InferenceContext.setType(node.argumentList, contextType);
     }
   }
 
   void _inferFormalParameterList(FormalParameterList node, DartType type) {
     if (typeAnalyzer.inferFormalParameterList(node, type)) {
       // TODO(leafp): This gets dropped on the floor if we're in the field
       // inference task.  We should probably keep these infos.
+      //
+      // TODO(jmesserly): this is reporting the context type, and therefore not
+      // necessarily the correct inferred type for the lambda.
+      //
+      // For example, `([x]) {}`  could be passed to `int -> void` but its type
+      // will really be `([int]) -> void`. Similar issue for named arguments.
+      // It can also happen if the return type is inferred later on to be
+      // more precise.
+      //
+      // This reporting bug defeats the deduplication of error messages and
+      // results in the same inference message being reported twice.
+      //
+      // To get this right, we'd have to delay reporting until we have the
+      // complete type including return type.
       inferenceContext.recordInference(node.parent, type);
     }
   }
 
   /**
    * If given "mayBeClosure" is [FunctionExpression] without explicit parameters types and its
    * required type is [FunctionType], then infer parameters types from [FunctionType].
    */
   void _inferFunctionExpressionParametersTypes(
       Expression mayBeClosure, DartType mayByFunctionType) {
@@ skipping 3809 matching lines @@
       return null;
     }
     if (identical(node.staticElement, variable)) {
       if (node.inSetterContext()) {
         result = true;
       }
     }
     return null;
   }
 }