fixes #24507, allow known functions to be treated as strict arrows

pkg/analyzer/lib/src/task/strong/checker.dart

 // Copyright (c) 2015, 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.
 
 // TODO(jmesserly): this was ported from package:dev_compiler, and needs to be
 // refactored to fit into analyzer.
 library analyzer.src.task.strong.checker;
 
 import 'package:analyzer/analyzer.dart';
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/ast/visitor.dart';
 import 'package:analyzer/dart/element/element.dart';
 import 'package:analyzer/dart/element/type.dart';
+import 'package:analyzer/src/dart/element/element.dart';
 import 'package:analyzer/src/dart/element/type.dart';
 import 'package:analyzer/src/generated/resolver.dart' show TypeProvider;
 import 'package:analyzer/src/generated/scanner.dart' show Token, TokenType;
 import 'package:analyzer/src/generated/type_system.dart';
 
 import 'info.dart';
 
 DartType _elementType(Element e) {
   if (e == null) {
     // Malformed code - just return dynamic.
@@ skipping 534 matching lines @@
     node.visitChildren(this);
   }
 
   @override
   void visitYieldStatement(YieldStatement node) {
     _checkReturnOrYield(node.expression, node, yieldStar: node.star != null);
     node.visitChildren(this);
   }
 
   StaticInfo _checkAssignment(Expression expr, DartType toT) {
-    final fromT = expr.staticType ?? DynamicTypeImpl.instance;
+    final fromT = _getStaticType(expr);
     final Coercion c = _coerceTo(fromT, toT);
     if (c is Identity) return null;
     if (c is CoercionError) return new StaticTypeError(rules, expr, toT);
     var reason = null;
 
     var errors = <String>[];
 
     var ok = _inferExpression(expr, toT, errors);
     if (ok) return InferredType.create(rules, expr, toT);
     reason = (errors.isNotEmpty) ? errors.first : null;
@@ skipping 192 matching lines @@
       return type;
     } else if (type is InterfaceType && type.element == expectedType.element) {
       return type.typeArguments[0];
     } else {
       // Malformed type - fallback on analyzer error.
       return null;
     }
   }
 
   DartType _getStaticType(Expression expr) {
-    return expr.staticType ?? DynamicTypeImpl.instance;
+    DartType t = expr.staticType ?? DynamicTypeImpl.instance;
+
+    // Remove fuzzy arrow if possible.
+    if (t is FunctionType && StaticInfo.isKnownFunction(expr)) {
+      t = _removeFuzz(t);
+    }
+
+    return t;
+  }
+
+  /// Remove "fuzzy arrow" in this function type.
+  ///
+  /// Normally we treat dynamically typed parameters as bottom for function
+  /// types. This allows type tests such as `if (f is SingleArgFunction)`.
+  /// It also requires a dynamic check on the parameter type to call these
+  /// functions.
+  ///
+  /// When we convert to a strict arrow, dynamically typed parameters become
+  /// top. This is safe to do for known functions, like top-level or local
+  /// functions and static methods. Those functions must already be essentially
+  /// treating dynamic as top.
+  ///
+  /// Only the outer-most arrow can be strict. Any others must be fuzzy, because
+  /// we don't know what function value will be passed there.
+  FunctionType _removeFuzz(FunctionType t) {
+    bool foundFuzz = false;
+    List<ParameterElement> parameters = <ParameterElement>[];
+    for (ParameterElement p in t.parameters) {
+      ParameterElement newP = _removeParameterFuzz(p);
+      parameters.add(newP);
+      if (p != newP) foundFuzz = true;
+    }
+    if (!foundFuzz) {
+      return t;
+    }
+
+    FunctionElementImpl function = new FunctionElementImpl("", -1);
+    function.synthetic = true;
+    function.returnType = t.returnType;
+    function.shareTypeParameters(t.typeFormals);
+    function.shareParameters(parameters);
+    return function.type = new FunctionTypeImpl(function);
+  }
+
+  /// Removes fuzzy arrow, see [_removeFuzz].
+  ParameterElement _removeParameterFuzz(ParameterElement p) {
+    if (p.type.isDynamic) {
+      return new ParameterElementImpl.synthetic(
+          p.name, typeProvider.objectType, p.parameterKind);
+    }
+    return p;
   }
 
   /// Given an expression, return its type assuming it is
   /// in the caller position of a call (that is, accounting
   /// for the possibility of a call method).  Returns null
   /// if expression is not statically callable.
   FunctionType _getTypeAsCaller(Expression node) {
     DartType t = node.staticType;
     if (node is SimpleIdentifier) {
       Expression parent = node.parent;
@@ skipping 446 matching lines @@
     } while (!current.isObject && !visited.contains(current));
   }
 
   void _recordMessage(StaticInfo info) {
     if (info == null) return;
     var error = info.toAnalysisError();
     if (error.errorCode.errorSeverity == ErrorSeverity.ERROR) _failure = true;
     _reporter.onError(error);
   }
 }