fixes inferring () -> <bottom> in strong mode

This fixes dart-archive/dev_compiler#47

[email protected]

Review URL: https://codereview.chromium.org/1649523002 .

Author: John Messerly

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

@@ -466,10 +466,19 @@ class StaticTypeAnalyzer extends SimpleAstVisitor<Object> {
         node.element as ExecutableElementImpl;
     DartType computedType = _computeStaticReturnTypeOfFunctionExpression(node);
     if (_strongMode) {
+      // In strong mode, we don't want to allow the function's return type to
+      // be bottom. If the surrounding context has a more precise type, we
+      // will push it down with inference, below. If not we want to use dynamic.
+      // TODO(jmesserly): should we  do this for the `null` literal always in
+      // strong mode, instead of handling it here?
+      if (computedType.isBottom) {
+        computedType = DynamicTypeImpl.instance;
+      }
+
       DartType functionType = InferenceContext.getType(node);
       if (functionType is FunctionType) {
         DartType returnType = functionType.returnType;
-        if ((computedType.isDynamic || computedType.isBottom) &&
+        if (computedType.isDynamic &&
             !(returnType.isDynamic || returnType.isBottom)) {
           computedType = returnType;
           _resolver.inferenceContext.recordInference(node, functionType);

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

@@ -1584,10 +1584,32 @@ main() {
     '''
     });
 
+    // Regression test for https://github.com/dart-lang/dev_compiler/issues/47
+    testChecker('null literal should not infer as bottom', {
+      '/main.dart': '''
+        var h = null;
+        void foo(int f(Object _)) {}
+
+        main() {
+          var f = (x) => null;
+          f = (x) => 'hello';
+
+          var g = null;
+          g = 'hello';
+          (/*info:DYNAMIC_INVOKE*/g.foo());
+
+          h = 'hello';
+          (/*info:DYNAMIC_INVOKE*/h.foo());
+
+          foo(/*info:INFERRED_TYPE_CLOSURE,info:INFERRED_TYPE_CLOSURE*/(x) => null);
+          foo(/*info:INFERRED_TYPE_CLOSURE,info:INFERRED_TYPE_CLOSURE*/(x) => throw "not implemented");
+        }
+    '''
+    });
+
     // TODO(jmesserly): we should change how this inference works.
     // For now this test will cover what we use.
-    testChecker(
-        'infer from RHS only if it wont conflict with overridden fields 2', {
+    testChecker('infer JS builtin', {
       '/main.dart': '''
         import 'dart:_foreign_helper' show JS;
         main() {
@@ -1598,6 +1620,5 @@ main() {
         }
     '''
     });
-
   });
 }