Improve for-in statement inference.

R=brianwilkerson@google.com, paulberry@google.com

Bug: https://github.com/dart-lang/sdk/issues/31638
Change-Id: Ie15486387a6b5955e42e9fe5fc00cdba4f2ba68a
     https://github.com/dart-lang/sdk/issues/31440
Reviewed-on: https://dart-review.googlesource.com/32861
Commit-Queue: Konstantin Shcheglov <scheglov@google.com>
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>

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

@@ -5646,25 +5646,40 @@ class ResolverVisitor extends ScopedVisitor {
 
   @override
   void visitForEachStatementInScope(ForEachStatement node) {
+    Expression iterable = node.iterable;
+    DeclaredIdentifier loopVariable = node.loopVariable;
+    SimpleIdentifier identifier = node.identifier;
+
+    identifier?.accept(this);
+
+    DartType valueType;
+    if (loopVariable != null) {
+      TypeAnnotation typeAnnotation = loopVariable.type;
+      valueType = typeAnnotation?.type ?? typeProvider.dynamicType;
+    }
+    if (identifier != null) {
+      Element element = identifier.staticElement;
+      if (element is VariableElement) {
+        valueType = element.type;
+      } else if (element is PropertyAccessorElement) {
+        if (element.parameters.isNotEmpty) {
+          valueType = element.parameters[0].type;
+        }
+      }
+    }
+    if (valueType != null) {
+      InterfaceType targetType = (node.awaitKeyword == null)
+          ? typeProvider.iterableType
+          : typeProvider.streamType;
+      InferenceContext.setType(iterable, targetType.instantiate([valueType]));
+    }
+
     //
     // We visit the iterator before the loop variable because the loop variable
     // cannot be in scope while visiting the iterator.
     //
-    Expression iterable = node.iterable;
-    DeclaredIdentifier loopVariable = node.loopVariable;
-    SimpleIdentifier identifier = node.identifier;
-    if (loopVariable?.type?.type != null) {
-      InterfaceType targetType = (node.awaitKeyword == null)
-          ? typeProvider.iterableType
-          : typeProvider.streamType;
-      InferenceContext.setType(
-          iterable,
-          targetType
-              .instantiate([resolutionMap.typeForTypeName(loopVariable.type)]));
-    }
     iterable?.accept(this);
     loopVariable?.accept(this);
-    identifier?.accept(this);
     Statement body = node.body;
     if (body != null) {
       _overrideManager.enterScope();

pkg/analyzer/test/src/task/strong/dart2_inference_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/dart/ast/ast.dart';
 import 'package:analyzer/dart/element/element.dart';
 import 'package:analyzer/src/generated/engine.dart';
 import 'package:test/test.dart';
@@ -28,6 +29,86 @@ class Dart2InferenceTest extends ResolverTestCase {
   @override
   bool get enableNewAnalysisDriver => true;
 
+  test_forIn() async {
+    var code = r'''
+T f<T>() => null;
+
+void test(Iterable<num> iter) {
+  for (var w in f()) {} // 1
+  for (var x in iter) {} // 2
+  for (num y in f()) {} // 3
+}
+''';
+    var source = addSource(code);
+    var analysisResult = await computeAnalysisResult(source);
+    var unit = analysisResult.unit;
+
+    {
+      var node = EngineTestCase.findSimpleIdentifier(unit, code, 'w in');
+      VariableElement element = node.staticElement;
+      expect(node.staticType, typeProvider.dynamicType);
+      expect(element.type, typeProvider.dynamicType);
+
+      var invocation = _findMethodInvocation(unit, code, 'f()) {} // 1');
+      expect(invocation.staticType.toString(), 'Iterable<dynamic>');
+    }
+
+    {
+      var node = EngineTestCase.findSimpleIdentifier(unit, code, 'x in');
+      VariableElement element = node.staticElement;
+      expect(node.staticType, typeProvider.numType);
+      expect(element.type, typeProvider.numType);
+    }
+
+    {
+      var node = EngineTestCase.findSimpleIdentifier(unit, code, 'y in');
+      VariableElement element = node.staticElement;
+
+      expect(node.staticType, typeProvider.numType);
+      expect(element.type, typeProvider.numType);
+
+      var invocation = _findMethodInvocation(unit, code, 'f()) {} // 3');
+      expect(invocation.staticType.toString(), 'Iterable<num>');
+    }
+  }
+
+  test_forIn_identifier() async {
+    var code = r'''
+T f<T>() => null;
+
+class A {}
+
+A aTopLevel;
+void set aTopLevelSetter(A value) {}
+
+class C {
+  A aField;
+  void set aSetter(A value) {}
+  void test() {
+    A aLocal;
+    for (aLocal in f()) {} // local
+    for (aField in f()) {} // field
+    for (aSetter in f()) {} // setter
+    for (aTopLevel in f()) {} // top variable
+    for (aTopLevelSetter in f()) {} // top setter
+  }
+}''';
+    var source = addSource(code);
+    var analysisResult = await computeAnalysisResult(source);
+    var unit = analysisResult.unit;
+
+    void assertType(String prefix) {
+      var invocation = _findMethodInvocation(unit, code, prefix);
+      expect(invocation.staticType.toString(), 'Iterable<A>');
+    }
+
+    assertType('f()) {} // local');
+    assertType('f()) {} // field');
+    assertType('f()) {} // setter');
+    assertType('f()) {} // top variable');
+    assertType('f()) {} // top setter');
+  }
+
   test_inferObject_whenDownwardNull() async {
     var code = r'''
 int f(void Function(Null) f2) {}
@@ -43,4 +124,11 @@ void main() {
     expect(xNode.staticType, typeProvider.objectType);
     expect(xElement.type, typeProvider.objectType);
   }
+
+  MethodInvocation _findMethodInvocation(
+      AstNode root, String code, String prefix) {
+    return EngineTestCase.findNode(root, code, prefix, (n) {
+      return n is MethodInvocation;
+    });
+  }
 }

tests/corelib_2/corelib_2.status

@@ -345,6 +345,7 @@ integer_parsed_arith_vm_test: RuntimeError # Issue 29921
 integer_to_radix_string_test: RuntimeError # Issue 29921
 integer_to_string_test/01: RuntimeError # Issue 29921
 iterable_fold_test/02: RuntimeError # different type inference problem
+iterable_reduce_test/01: RuntimeError
 iterable_reduce_test/none: RuntimeError
 iterable_return_type_test/02: RuntimeError # Issue 29921
 iterable_to_list_test/*: RuntimeError
@@ -655,7 +656,6 @@ double_parse_test/02: Skip # Temporarily disable the following tests until we fi
 double_parse_test/03: Skip # Temporarily disable the following tests until we figure out why they started failing.
 double_parse_test/04: Skip # Temporarily disable the following tests until we figure out why they started failing.
 double_parse_test/none: Skip # Temporarily disable the following tests until we figure out why they started failing.
-iterable_reduce_test/01: CompileTimeError
 
 [ $compiler == dartkp || $compiler == precompiler ]
 apply3_test: SkipByDesign