Remove deep future flattening from analyzer and DDC

Change-Id: Ic48f29adec3a46744b259ba5b2b8ad7d97caa6c2
Reviewed-on: https://dart-review.googlesource.com/37424
Reviewed-by: Jenny Messerly <jmesserly@google.com>

CHANGELOG.md

@@ -11,6 +11,19 @@
 
 #### Strong Mode
 
+* Future flattening is now done only as specified in the Dart 2.0 spec, rather
+than more broadly.  This means that the following code will now have an error on
+the assignment to `y`.
+
+  ```dart
+    test() {
+      Future<int> f;
+      var x = f.then<Future<List<int>>>((x) => []);
+      Future<List<int>> y = x;
+    }
+    ```
+
+
 ### Core library changes
 
 * `dart:async`

pkg/analyzer/lib/src/dart/element/type.dart

@@ -2127,33 +2127,6 @@ class InterfaceTypeImpl extends TypeImpl implements InterfaceType {
       return this;
     }
 
-    if (isDartAsyncFuture && newTypeArguments.isNotEmpty) {
-      //
-      // In strong mode interpret Future< T > as Future< flatten(T) >
-      //
-      // For example, Future<Future<T>> will flatten to Future<T>.
-      //
-      // In the Dart 3rd edition spec, this flatten operation is used for
-      // `async` and `await`. In strong mode, we extend it to all Future<T>
-      // instantiations. This allows typing of Future-related operations
-      // in dart:async in a way that matches their runtime behavior and provides
-      // precise return types for users of these APIs.
-      //
-      // For example:
-      //
-      //     abstract class Future<T> {
-      //       Future<S> then<S>(S onValue(T value), ...);
-      //     }
-      //
-      // Given a call where S <: Future<R> for some R, we will need to flatten
-      // the return type so it is Future< flatten(S) >, yielding Future<R>.
-      //
-      if (element.library.context.analysisOptions.strongMode) {
-        TypeImpl t = newTypeArguments[0];
-        newTypeArguments[0] = t.flattenFutures(new StrongTypeSystemImpl(null));
-      }
-    }
-
     InterfaceTypeImpl newType = new InterfaceTypeImpl(element, prune);
     newType.typeArguments = newTypeArguments;
     return newType;

pkg/analyzer/test/src/task/strong/inferred_type_test.dart

@@ -1642,7 +1642,7 @@ m1() {
   Future<int> f;
   var x = f.then<Future<List<int>>>(/*info:INFERRED_TYPE_CLOSURE*/
                                     (x) => /*error:RETURN_OF_INVALID_TYPE_FROM_CLOSURE*/[]);
-  Future<List<int>> y = x;
+  Future<List<int>> y = /*error:INVALID_ASSIGNMENT*/x;
 }
 m2() {
   Future<int> f;

pkg/dev_compiler/lib/src/analyzer/code_generator.dart

@@ -1232,10 +1232,6 @@ class CodeGenerator extends Object
 
     var genericCall = _callHelper('generic(#)', [genericArgs]);
 
-    if (element.library.isDartAsync &&
-        (element.name == "Future" || element.name == "_Future")) {
-      genericCall = _callHelper('flattenFutures(#)', [genericCall]);
-    }
     var genericName = _emitTopLevelNameNoInterop(element, suffix: '\$');
     return js.statement('{ # = #; # = #(); }',
         [genericName, genericCall, _emitTopLevelName(element), genericName]);

pkg/dev_compiler/lib/src/kernel/compiler.dart

@@ -579,10 +579,6 @@ class ProgramCompiler
 
     var genericCall = _callHelper('generic(#)', [genericArgs]);
 
-    if (getLibrary(c) == coreTypes.asyncLibrary &&
-        (name == "Future" || name == "_Future")) {
-      genericCall = _callHelper('flattenFutures(#)', [genericCall]);
-    }
     var genericName = _emitTopLevelNameNoInterop(c, suffix: '\$');
     return js.statement('{ # = #; # = #(); }',
         [genericName, genericCall, _emitTopLevelName(c), genericName]);

pkg/dev_compiler/tool/input_sdk/private/ddc_runtime/classes.dart

@@ -58,24 +58,6 @@ final _originalDeclaration = JS('', 'Symbol("originalDeclaration")');
 
 final mixinNew = JS('', 'Symbol("dart.mixinNew")');
 
-/// Wrap a generic class builder function with future flattening.
-flattenFutures(builder) => JS('', '''(() => {
-  function flatten(T) {
-    if (!T) return $builder($dynamic);
-    let futureClass = $getGenericClass($Future);
-    //TODO(leafp): This only handles the direct flattening case.
-    // It would probably be good to at least search up the class
-    // hierarchy.  If we keep doing flattening long term, we may
-    // want to implement the full future flattening per spec.
-    if ($getGenericClass(T) == futureClass) {
-      let args = $getGenericArgs(T);
-      if (args) return $builder(args[0]);
-    }
-    return $builder(T);
-  }
-  return flatten;
-})()''');
-
 /// Memoize a generic type constructor function.
 generic(typeConstructor, setBaseClass) => JS('', '''(() => {
   let length = $typeConstructor.length;

tests/compiler/dart2js/old_frontend/mock_compiler.dart

@@ -282,7 +282,7 @@ class MockCompiler extends Compiler {
   }
 
   /// Create a new [MockCompiler] and apply it asynchronously to [f].
-  static Future<T> create<T>(T f(MockCompiler compiler)) {
+  static Future<T> create<T>(FutureOr<T> f(MockCompiler compiler)) {
     MockCompiler compiler = new MockCompiler.internal();
     return compiler.init().then((_) => f(compiler));
   }

tests/language_2/language_2_analyzer.status

@@ -8,7 +8,6 @@ abstract_override_adds_optional_args_concrete_subclass_test: MissingCompileTimeE
 abstract_override_adds_optional_args_concrete_test: MissingCompileTimeError # Issue #30568
 abstract_override_adds_optional_args_supercall_test: MissingCompileTimeError # Issue #30568
 assertion_initializer_const_function_test/01: MissingCompileTimeError
-async_return_types_test/nestedFuture: MissingCompileTimeError
 bad_initializer2_negative_test: Fail # Issue 14880
 black_listed_test/none: Fail # Issue 14228
 built_in_identifier_prefix_test: CompileTimeError
@@ -1228,7 +1227,6 @@ accessor_conflict_import_test: CompileTimeError # Issue 25626
 additional_interface_adds_optional_args_test: CompileTimeError # Issue #30568
 assertion_initializer_const_function_test/01: MissingStaticWarning
 assertion_initializer_test: CompileTimeError
-async_return_types_test/nestedFuture: MissingCompileTimeError
 cascaded_forwarding_stubs_test: CompileTimeError
 config_import_corelib_test: CompileTimeError
 const_types_test/07: MissingCompileTimeError # Incorrectly allows using type parameter in const expression.

tests/language_2/language_2_dartdevc.status

@@ -343,6 +343,7 @@ async_await_syntax_test/d10a: MissingCompileTimeError
 async_or_generator_return_type_stacktrace_test/01: MissingCompileTimeError
 async_or_generator_return_type_stacktrace_test/02: MissingCompileTimeError
 async_or_generator_return_type_stacktrace_test/03: MissingCompileTimeError
+async_return_types_test/nestedFuture: MissingCompileTimeError
 async_return_types_test/tooManyTypeParameters: MissingCompileTimeError
 async_return_types_test/wrongReturnType: MissingCompileTimeError
 bad_named_parameters2_test/01: MissingCompileTimeError
@@ -1030,7 +1031,6 @@ truncdiv_test: RuntimeError # Issue 29920; Expect.throws fails: Did not throw
 abstract_override_adds_optional_args_concrete_subclass_test: MissingCompileTimeError # Issue #30568
 abstract_override_adds_optional_args_concrete_test: MissingCompileTimeError # Issue #30568
 abstract_override_adds_optional_args_supercall_test: MissingCompileTimeError # Issue #30568
-async_return_types_test/nestedFuture: MissingCompileTimeError
 bit_operations_test/01: MissingCompileTimeError
 bit_operations_test/02: MissingCompileTimeError
 built_in_identifier_prefix_test: CompileTimeError