Ensure that null is not used as a context in more places.

This CL changes the handling of writeContext (for complex
assignments), returnOrYieldContext (for closures), expectedType (for
ensureAssignable), and receiverType (for findInterfaceMember) so that
they no longer use `null` to represent a context; they use UnknownType
or DynamicType as appropriate.

It also changes the handling of expectedType (for ensureAssignable) so
that it does not require the type to be known.  That makes this CL
simpler and paves the way for fixing #31792.

Change-Id: Ib1be06182f9a9e9f77a9eb81f5daf2364de0f3a7
Reviewed-on: https://dart-review.googlesource.com/44800
Commit-Queue: Paul Berry <paulberry@google.com>
Reviewed-by: Kevin Millikin <kmillikin@google.com>

Author: stereotype441

pkg/front_end/lib/src/fasta/kernel/body_builder.dart

@@ -604,7 +604,7 @@ class BodyBuilder<Arguments> extends ScopeListener<JumpTarget>
       return member.procedure.function.returnType;
     } else {
       assert(member is KernelConstructorBuilder);
-      return null;
+      return const DynamicType();
     }
   }
 

pkg/front_end/lib/src/fasta/kernel/kernel_shadow_ast.dart

@@ -390,6 +390,7 @@ abstract class ShadowComplexAssignment extends ShadowSyntheticExpression {
 
   _ComplexAssignmentInferenceResult _inferRhs(
       ShadowTypeInferrer inferrer, DartType readType, DartType writeContext) {
+    assert(writeContext != null);
     var writeOffset = write == null ? -1 : write.fileOffset;
     Procedure combinerMember;
     DartType combinedType;
@@ -448,7 +449,8 @@ abstract class ShadowComplexAssignment extends ShadowSyntheticExpression {
       _storeLetType(inferrer, replacedRhs ?? rhs, rhsType);
       if (nullAwareCombiner != null) {
         MethodInvocation equalsInvocation = nullAwareCombiner.condition;
-        inferrer.findMethodInvocationMember(writeContext, equalsInvocation,
+        inferrer.findMethodInvocationMember(
+            greatestClosure(inferrer.coreTypes, writeContext), equalsInvocation,
             silent: true);
         // Note: the case of readType=null only happens for erroneous code.
         combinedType = readType == null
@@ -1079,7 +1081,7 @@ class ShadowIndexAssign extends ShadowComplexAssignmentWithReceiver {
         inferrer.getCalleeFunctionType(writeMember, receiverType, false);
     DartType expectedIndexTypeForWrite;
     DartType indexContext = const UnknownType();
-    DartType writeContext;
+    DartType writeContext = const UnknownType();
     if (calleeType.positionalParameters.length >= 2) {
       // TODO(paulberry): we ought to get a context for the index expression
       // from the index formal parameter, but analyzer doesn't so for now we
@@ -1089,7 +1091,7 @@ class ShadowIndexAssign extends ShadowComplexAssignmentWithReceiver {
     }
     var indexType = inferrer.inferExpression(index, indexContext, true);
     _storeLetType(inferrer, index, indexType);
-    if (writeContext != null) {
+    if (writeContext is! UnknownType) {
       inferrer.ensureAssignable(
           expectedIndexTypeForWrite,
           indexType,
@@ -1660,10 +1662,9 @@ class ShadowReturnStatement extends ReturnStatement implements ShadowStatement {
     var closureContext = inferrer.closureContext;
     var typeContext = !closureContext.isGenerator
         ? closureContext.returnOrYieldContext
-        : null;
+        : const UnknownType();
     var inferredType = expression != null
-        ? inferrer.inferExpression(
-            expression, typeContext ?? const UnknownType(), true)
+        ? inferrer.inferExpression(expression, typeContext, true)
         : const VoidType();
     // Analyzer treats bare `return` statements as having no effect on the
     // inferred type of the closure.  TODO(paulberry): is this what we want
@@ -1695,14 +1696,14 @@ class ShadowStaticAssignment extends ShadowComplexAssignment {
 
   @override
   DartType _inferExpression(ShadowTypeInferrer inferrer, DartType typeContext) {
-    DartType readType;
+    DartType readType = const DynamicType(); // Only used in error recovery
     var read = this.read;
     if (read is StaticGet) {
       readType = read.target.getterType;
       _storeLetType(inferrer, read, readType);
     }
     Member writeMember;
-    DartType writeContext;
+    DartType writeContext = const UnknownType();
     var write = this.write;
     if (write is StaticSet) {
       writeContext = write.target.setterType;
@@ -2193,7 +2194,7 @@ class ShadowVariableAssignment extends ShadowComplexAssignment {
     if (read is VariableGet) {
       readType = read.promotedType ?? read.variable.type;
     }
-    DartType writeContext;
+    DartType writeContext = const UnknownType();
     var write = this.write;
     if (write is VariableSet) {
       writeContext = write.variable.type;
@@ -2347,17 +2348,21 @@ class ShadowYieldStatement extends YieldStatement implements ShadowStatement {
   @override
   void _inferStatement(ShadowTypeInferrer inferrer) {
     var closureContext = inferrer.closureContext;
-    var typeContext =
-        closureContext.isGenerator ? closureContext.returnOrYieldContext : null;
-    if (isYieldStar && typeContext != null) {
-      typeContext = inferrer.wrapType(
-          typeContext,
-          closureContext.isAsync
-              ? inferrer.coreTypes.streamClass
-              : inferrer.coreTypes.iterableClass);
+    DartType inferredType;
+    if (closureContext.isGenerator) {
+      var typeContext = closureContext.returnOrYieldContext;
+      if (isYieldStar && typeContext != null) {
+        typeContext = inferrer.wrapType(
+            typeContext,
+            closureContext.isAsync
+                ? inferrer.coreTypes.streamClass
+                : inferrer.coreTypes.iterableClass);
+      }
+      inferredType = inferrer.inferExpression(expression, typeContext, true);
+    } else {
+      inferredType =
+          inferrer.inferExpression(expression, const UnknownType(), true);
     }
-    var inferredType = inferrer.inferExpression(
-        expression, typeContext ?? const UnknownType(), true);
     closureContext.handleYield(
         inferrer, isYieldStar, inferredType, expression, fileOffset);
   }

pkg/front_end/lib/src/fasta/type_inference/type_inferrer.dart

@@ -195,7 +195,9 @@ class ClosureContext {
   }
 
   ClosureContext._(this.isAsync, this.isGenerator, this.returnOrYieldContext,
-      this._needToInferReturnType, this._needImplicitDowncasts);
+      this._needToInferReturnType, this._needImplicitDowncasts) {
+    assert(returnOrYieldContext != null);
+  }
 
   /// Updates the inferred return type based on the presence of a return
   /// statement returning the given [type].
@@ -446,12 +448,13 @@ abstract class TypeInferrerImpl extends TypeInferrer {
         typeSchemaEnvironment.isSubtypeOf(actualType, expectedType);
   }
 
-  /// Checks whether [actualType] can be assigned to [expectedType], and inserts
-  /// an implicit downcast if appropriate.
+  /// Checks whether [actualType] can be assigned to the greatest closure of
+  /// [expectedType], and inserts an implicit downcast if appropriate.
   Expression ensureAssignable(DartType expectedType, DartType actualType,
       Expression expression, int fileOffset,
       {bool isReturnFromAsync = false}) {
-    assert(expectedType == null || isKnown(expectedType));
+    assert(expectedType != null);
+    expectedType = greatestClosure(coreTypes, expectedType);
 
     DartType initialExpectedType = expectedType;
     if (isReturnFromAsync && !isAssignable(expectedType, actualType)) {
@@ -549,6 +552,8 @@ abstract class TypeInferrerImpl extends TypeInferrer {
       Expression receiver,
       bool setter: false,
       bool silent: false}) {
+    assert(receiverType != null && isKnown(receiverType));
+
     // Our non-strong golden files currently don't include interface
     // targets, so we can't store the interface target without causing tests
     // to fail.  TODO(paulberry): fix this.
@@ -793,7 +798,7 @@ abstract class TypeInferrerImpl extends TypeInferrer {
     if (type is InterfaceType && identical(type.classNode, class_)) {
       return type.typeArguments[0];
     } else {
-      return null;
+      return const UnknownType();
     }
   }
 
@@ -1135,7 +1140,11 @@ abstract class TypeInferrerImpl extends TypeInferrer {
 
   DartType inferLocalFunction(FunctionNode function, DartType typeContext,
       int fileOffset, DartType returnContext) {
-    bool hasImplicitReturnType = returnContext == null;
+    bool hasImplicitReturnType = false;
+    if (returnContext == null) {
+      hasImplicitReturnType = true;
+      returnContext = const DynamicType();
+    }
     if (!isTopLevel) {
       var positionalParameters = function.positionalParameters;
       for (var i = 0; i < positionalParameters.length; i++) {
@@ -1234,9 +1243,7 @@ abstract class TypeInferrerImpl extends TypeInferrer {
     // Let `N'` be `N[T/S]`.  The [ClosureContext] constructor will adjust
     // accordingly if the closure is declared with `async`, `async*`, or
     // `sync*`.
-    if (returnContext != null) {
-      returnContext = substitution.substituteType(returnContext);
-    }
+    returnContext = substitution.substituteType(returnContext);
 
     // Apply type inference to `B` in return context `N’`, with any references
     // to `xi` in `B` having type `Pi`.  This produces `B’`.

tests/language_2/language_2_dart2js.status

@@ -2061,15 +2061,12 @@ async_await_syntax_test/c03a: Crash # Interpolated value #1 is not an Expression
 async_await_syntax_test/c05a: Crash # Interpolated value #1 is not an Expression or List of Expressions: [VariableUse(f), Instance of 'LiteralNull', null]
 async_await_syntax_test/c06a: Crash # Interpolated value #1 is not an Expression or List of Expressions: [VariableUse(f), Instance of 'LiteralNull', null]
 async_await_syntax_test/c09a: Crash # Interpolated value #1 is not an Expression or List of Expressions: [VariableUse(f), Instance of 'LiteralNull', null]
-async_await_syntax_test/c10a: Crash # Interpolated value #1 is not an Expression or List of Expressions: [VariableUse(f), Instance of 'LiteralNull', null]
 async_await_syntax_test/d01a: Crash # Interpolated value #1 is not an Expression or List of Expressions: [VariableUse(f), Instance of 'LiteralNull', null]
 async_await_syntax_test/d02a: Crash # Interpolated value #1 is not an Expression or List of Expressions: [VariableUse(f), Instance of 'LiteralNull', null]
 async_await_syntax_test/d03a: Crash # Interpolated value #1 is not an Expression or List of Expressions: [VariableUse(f), Instance of 'LiteralNull', null]
 async_await_syntax_test/d05a: Crash # Interpolated value #1 is not an Expression or List of Expressions: [VariableUse(f), Instance of 'LiteralNull', null]
 async_await_syntax_test/d06a: Crash # Interpolated value #1 is not an Expression or List of Expressions: [VariableUse(f), Instance of 'LiteralNull', null]
-async_await_syntax_test/d08b: MissingCompileTimeError
 async_await_syntax_test/d09a: Crash # Interpolated value #1 is not an Expression or List of Expressions: [VariableUse(f), Instance of 'LiteralNull', null]
-async_await_syntax_test/d10a: Crash # Interpolated value #1 is not an Expression or List of Expressions: [VariableUse(f), Instance of 'LiteralNull', null]
 async_await_test/02: Crash # Assertion failure: Runtime type information not available for type_variable_local(bindCallback.R) in (local(_RootZone.bindCallback#)) for j:closure_call(_RootZone_bindCallback_closure.call).
 async_await_test/03: Crash # Assertion failure: Runtime type information not available for type_variable_local(bindCallback.R) in (local(_RootZone.bindCallback#)) for j:closure_call(_RootZone_bindCallback_closure.call).
 async_await_test/none: Crash # Assertion failure: Runtime type information not available for type_variable_local(bindCallback.R) in (local(_RootZone.bindCallback#)) for j:closure_call(_RootZone_bindCallback_closure.call).

tests/language_2/language_2_dartdevc.status

@@ -323,9 +323,6 @@ abstract_getter_test/01: MissingCompileTimeError
 abstract_syntax_test/00: MissingCompileTimeError
 additional_interface_adds_optional_args_concrete_subclass_test: MissingCompileTimeError
 additional_interface_adds_optional_args_concrete_test: MissingCompileTimeError
-async_await_syntax_test/c10a: MissingCompileTimeError
-async_await_syntax_test/d08b: MissingCompileTimeError
-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

tests/language_2/language_2_kernel.status

@@ -28,9 +28,6 @@ abstract_override_adds_optional_args_concrete_test: MissingCompileTimeError # Is
 abstract_syntax_test/00: MissingCompileTimeError # Issue 32013.
 additional_interface_adds_optional_args_concrete_subclass_test: MissingCompileTimeError # Issue 32014.
 additional_interface_adds_optional_args_concrete_test: MissingCompileTimeError # Issue 32014.
-async_await_syntax_test/c10a: MissingCompileTimeError
-async_await_syntax_test/d08b: MissingCompileTimeError
-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