[cfe] Allow unassigned late local variables

This is a partial fix for http://dartbug.com/40601.  It will be followed
by a CL disallowing definitely unassigned late local variables.

Bug: http://dartbug.com/40601
Change-Id: I2cdf95f8cc1cc6fbbb6a5de8d6fa0aa53c6e477d
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/135309
Commit-Queue: Dmitry Stefantsov <[email protected]>
Reviewed-by: Johnni Winther <[email protected]>

Author: Dmitry Stefantsov

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

@@ -5488,11 +5488,15 @@ class InferenceVisitor
     if (isUnassigned) {
       inferrer.dataForTesting?.flowAnalysisResult?.unassignedNodes?.add(node);
       if (inferrer.isNonNullableByDefault && inferrer.performNnbdChecks) {
-        // Synthetic variables aren't checked.
+        // Synthetic variables, local functions, and variables with
+        // invalid types aren't checked.
+        // TODO(dmitryas): Report errors on definitely unassigned late
+        // local variables with potentially non-nullable types.
         if (variable.name != null &&
             !variable.isLocalFunction &&
             variable.type is! InvalidType &&
-            variable.type.isPotentiallyNonNullable) {
+            variable.type.isPotentiallyNonNullable &&
+            !variable.isLate) {
           if (inferrer.nnbdStrongMode) {
             return new ExpressionInferenceResult(
                 new InvalidType(),

pkg/front_end/testcases/nnbd/definitely_unassigned_late_local_variables.dart

@@ -0,0 +1,20 @@
+// Copyright (c) 2020, 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.
+
+// The test checks that it's a compile-time error to read a late local
+// variables of a potentially non-nullable type if it is definitely
+// unassigned.
+
+abstract class A<T> {
+  bar(T value) {}
+  barInt(int value) {}
+  foo() {
+    late T value;
+    late int intValue;
+    bar(value); // Error.
+    barInt(intValue); // Error.
+  }
+}
+
+main() {}

pkg/front_end/testcases/nnbd/definitely_unassigned_late_local_variables.dart.outline.expect

@@ -0,0 +1,16 @@
+library /*isNonNullableByDefault*/;
+import self as self;
+import "dart:core" as core;
+
+abstract class A<T extends core::Object? = dynamic> extends core::Object {
+  synthetic constructor •() → self::A<self::A::T%>
+    ;
+  method bar(generic-covariant-impl self::A::T% value) → dynamic
+    ;
+  method barInt(core::int value) → dynamic
+    ;
+  method foo() → dynamic
+    ;
+}
+static method main() → dynamic
+  ;

pkg/front_end/testcases/nnbd/definitely_unassigned_late_local_variables.dart.strong.expect

@@ -0,0 +1,18 @@
+library /*isNonNullableByDefault*/;
+import self as self;
+import "dart:core" as core;
+
+abstract class A<T extends core::Object? = dynamic> extends core::Object {
+  synthetic constructor •() → self::A<self::A::T%>
+    : super core::Object::•()
+    ;
+  method bar(generic-covariant-impl self::A::T% value) → dynamic {}
+  method barInt(core::int value) → dynamic {}
+  method foo() → dynamic {
+    late self::A::T% value;
+    late core::int intValue;
+    this.{self::A::bar}(value);
+    this.{self::A::barInt}(intValue);
+  }
+}
+static method main() → dynamic {}

pkg/front_end/testcases/nnbd/definitely_unassigned_late_local_variables.dart.strong.transformed.expect

@@ -0,0 +1,18 @@
+library /*isNonNullableByDefault*/;
+import self as self;
+import "dart:core" as core;
+
+abstract class A<T extends core::Object? = dynamic> extends core::Object {
+  synthetic constructor •() → self::A<self::A::T%>
+    : super core::Object::•()
+    ;
+  method bar(generic-covariant-impl self::A::T% value) → dynamic {}
+  method barInt(core::int value) → dynamic {}
+  method foo() → dynamic {
+    late self::A::T% value;
+    late core::int intValue;
+    this.{self::A::bar}(value);
+    this.{self::A::barInt}(intValue);
+  }
+}
+static method main() → dynamic {}

pkg/front_end/testcases/nnbd/definitely_unassigned_late_local_variables.dart.weak.expect

@@ -0,0 +1,18 @@
+library /*isNonNullableByDefault*/;
+import self as self;
+import "dart:core" as core;
+
+abstract class A<T extends core::Object? = dynamic> extends core::Object {
+  synthetic constructor •() → self::A<self::A::T%>
+    : super core::Object::•()
+    ;
+  method bar(generic-covariant-impl self::A::T% value) → dynamic {}
+  method barInt(core::int value) → dynamic {}
+  method foo() → dynamic {
+    late self::A::T% value;
+    late core::int intValue;
+    this.{self::A::bar}(value);
+    this.{self::A::barInt}(intValue);
+  }
+}
+static method main() → dynamic {}

pkg/front_end/testcases/nnbd/definitely_unassigned_late_local_variables.dart.weak.transformed.expect

@@ -0,0 +1,18 @@
+library /*isNonNullableByDefault*/;
+import self as self;
+import "dart:core" as core;
+
+abstract class A<T extends core::Object? = dynamic> extends core::Object {
+  synthetic constructor •() → self::A<self::A::T%>
+    : super core::Object::•()
+    ;
+  method bar(generic-covariant-impl self::A::T% value) → dynamic {}
+  method barInt(core::int value) → dynamic {}
+  method foo() → dynamic {
+    late self::A::T% value;
+    late core::int intValue;
+    this.{self::A::bar}(value);
+    this.{self::A::barInt}(intValue);
+  }
+}
+static method main() → dynamic {}

pkg/front_end/testcases/nnbd/not_definitely_unassigned_late_local_variables.dart

@@ -0,0 +1,28 @@
+// Copyright (c) 2020, 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.
+
+// This is a regression test to http://dartbug.com/40601.
+
+abstract class A<T> {
+  T baz();
+  bar(T value) {}
+  barInt(int value) {}
+  foo() {
+    late T value;
+    late int intValue;
+    // The use of variable "value" below shouldn't be a compile-time
+    // error because it's late and not definitely unassigned.
+    var result = () {
+      bar(value);
+      barInt(intValue);
+    };
+    (() {
+      value = baz();
+      intValue = 42;
+    })();
+    return result;
+  }
+}
+
+main() {}

pkg/front_end/testcases/nnbd/not_definitely_unassigned_late_local_variables.dart.outline.expect

@@ -0,0 +1,17 @@
+library /*isNonNullableByDefault*/;
+import self as self;
+import "dart:core" as core;
+
+abstract class A<T extends core::Object? = dynamic> extends core::Object {
+  synthetic constructor •() → self::A<self::A::T%>
+    ;
+  abstract method baz() → self::A::T%;
+  method bar(generic-covariant-impl self::A::T% value) → dynamic
+    ;
+  method barInt(core::int value) → dynamic
+    ;
+  method foo() → dynamic
+    ;
+}
+static method main() → dynamic
+  ;

pkg/front_end/testcases/nnbd/not_definitely_unassigned_late_local_variables.dart.strong.expect

@@ -0,0 +1,26 @@
+library /*isNonNullableByDefault*/;
+import self as self;
+import "dart:core" as core;
+
+abstract class A<T extends core::Object? = dynamic> extends core::Object {
+  synthetic constructor •() → self::A<self::A::T%>
+    : super core::Object::•()
+    ;
+  abstract method baz() → self::A::T%;
+  method bar(generic-covariant-impl self::A::T% value) → dynamic {}
+  method barInt(core::int value) → dynamic {}
+  method foo() → dynamic {
+    late self::A::T% value;
+    late core::int intValue;
+    () → core::Null? result = () → core::Null? {
+      this.{self::A::bar}(value);
+      this.{self::A::barInt}(intValue);
+    };
+    (() → core::Null? {
+      value = this.{self::A::baz}();
+      intValue = 42;
+    }).call();
+    return result;
+  }
+}
+static method main() → dynamic {}