[cfe] NullConstant variable declaration value.

Change-Id: I6d5adea4d70d8cd34f565a729034e2636935033e
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/197120
Reviewed-by: Dmitry Stefantsov <dmitryas@google.com>
Reviewed-by: Jake Macdonald <jakemac@google.com>
Commit-Queue: Kallen Tu <kallentu@google.com>

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

@@ -3056,7 +3056,21 @@ class ConstantEvaluator implements ExpressionVisitor<Constant> {
         if (value is AbortConstant) return value;
         env.addVariableValue(parameter, value);
       }
-      return executeBody(function.body);
+
+      final Constant result = executeBody(function.body);
+      if (result is NullConstant &&
+          function.returnType.nullability == Nullability.nonNullable) {
+        // Ensure that the evaluated constant returned is not null if the
+        // function has a non-nullable return type.
+        return createErrorConstant(
+            function,
+            templateConstEvalInvalidType.withArguments(
+                result,
+                function.returnType,
+                result.getType(_staticTypeContext),
+                isNonNullableByDefault));
+      }
+      return result;
     }
 
     if (functionEnvironment != null) {
@@ -3822,6 +3836,8 @@ class StatementConstantEvaluator extends StatementVisitor<ExecutionStatus> {
     if (node.initializer != null) {
       value = evaluate(node.initializer);
       if (value is AbortConstant) return new AbortStatus(value);
+    } else {
+      value = new NullConstant();
     }
     exprEvaluator.env.addVariableValue(node, value);
     return const ProceedStatus();

pkg/front_end/testcases/const_functions/const_functions_variable_declarations.dart

@@ -62,6 +62,12 @@ int function8() {
   return a;
 }
 
+const var9 = function9();
+int? function9() {
+  int? x;
+  return x;
+}
+
 void main() {
   Expect.equals(var1, 4);
   Expect.equals(var1_1, 5);
@@ -72,4 +78,5 @@ void main() {
   Expect.equals(var6, 2);
   Expect.equals(var7, 2);
   Expect.equals(var8, 2);
+  Expect.equals(var9, null);
 }

pkg/front_end/testcases/const_functions/const_functions_variable_declarations.dart.strong.expect

@@ -14,6 +14,7 @@ static const field core::int var5 = #C6;
 static const field core::int var6 = #C5;
 static const field core::int var7 = #C5;
 static const field core::int var8 = #C5;
+static const field core::int? var9 = #C7;
 static method function1(core::int a, core::int b) → core::int {
   core::int x = 1.{core::num::+}(a).{core::num::+}(b);
   return x;
@@ -52,6 +53,10 @@ static method function8() → core::int {
   a = 2;
   return a as{TypeError,ForDynamic,ForNonNullableByDefault} core::int;
 }
+static method function9() → core::int? {
+  core::int? x;
+  return x;
+}
 static method main() → void {
   exp::Expect::equals(#C1, 4);
   exp::Expect::equals(#C2, 5);
@@ -62,6 +67,7 @@ static method main() → void {
   exp::Expect::equals(#C5, 2);
   exp::Expect::equals(#C5, 2);
   exp::Expect::equals(#C5, 2);
+  exp::Expect::equals(#C7, null);
 }
 
 constants  {
@@ -71,4 +77,5 @@ constants  {
   #C4 = 6
   #C5 = 2
   #C6 = -2
+  #C7 = null
 }

pkg/front_end/testcases/const_functions/const_functions_variable_declarations.dart.strong.transformed.expect

@@ -14,6 +14,7 @@ static const field core::int var5 = #C6;
 static const field core::int var6 = #C5;
 static const field core::int var7 = #C5;
 static const field core::int var8 = #C5;
+static const field core::int? var9 = #C7;
 static method function1(core::int a, core::int b) → core::int {
   core::int x = 1.{core::num::+}(a).{core::num::+}(b);
   return x;
@@ -52,6 +53,10 @@ static method function8() → core::int {
   a = 2;
   return a as{TypeError,ForDynamic,ForNonNullableByDefault} core::int;
 }
+static method function9() → core::int? {
+  core::int? x;
+  return x;
+}
 static method main() → void {
   exp::Expect::equals(#C1, 4);
   exp::Expect::equals(#C2, 5);
@@ -62,6 +67,7 @@ static method main() → void {
   exp::Expect::equals(#C5, 2);
   exp::Expect::equals(#C5, 2);
   exp::Expect::equals(#C5, 2);
+  exp::Expect::equals(#C7, null);
 }
 
 constants  {
@@ -71,8 +77,9 @@ constants  {
   #C4 = 6
   #C5 = 2
   #C6 = -2
+  #C7 = null
 }
 
 Extra constant evaluation status:
-Evaluated: MethodInvocation @ org-dartlang-testcase:///const_functions_variable_declarations.dart:71:23 -> IntConstant(-2)
-Extra constant evaluation: evaluated: 33, effectively constant: 1
+Evaluated: MethodInvocation @ org-dartlang-testcase:///const_functions_variable_declarations.dart:77:23 -> IntConstant(-2)
+Extra constant evaluation: evaluated: 35, effectively constant: 1

pkg/front_end/testcases/const_functions/const_functions_variable_declarations.dart.textual_outline.expect

@@ -17,4 +17,6 @@ const var7 = function7();
 int function7() {}
 const var8 = function8();
 int function8() {}
+const var9 = function9();
+int? function9() {}
 void main() {}

pkg/front_end/testcases/const_functions/const_functions_variable_declarations.dart.textual_outline_modelled.expect

@@ -10,6 +10,8 @@ const var5 = function5();
 const var6 = function6();
 const var7 = function7();
 const var8 = function8();
+const var9 = function9();
+int? function9() {}
 int function1(int a, int b) {}
 int function3() {}
 int function4() {}

pkg/front_end/testcases/const_functions/const_functions_variable_declarations.dart.weak.expect

@@ -14,6 +14,7 @@ static const field core::int var5 = #C6;
 static const field core::int var6 = #C5;
 static const field core::int var7 = #C5;
 static const field core::int var8 = #C5;
+static const field core::int? var9 = #C7;
 static method function1(core::int a, core::int b) → core::int {
   core::int x = 1.{core::num::+}(a).{core::num::+}(b);
   return x;
@@ -52,6 +53,10 @@ static method function8() → core::int {
   a = 2;
   return a as{TypeError,ForDynamic,ForNonNullableByDefault} core::int;
 }
+static method function9() → core::int? {
+  core::int? x;
+  return x;
+}
 static method main() → void {
   exp::Expect::equals(#C1, 4);
   exp::Expect::equals(#C2, 5);
@@ -62,6 +67,7 @@ static method main() → void {
   exp::Expect::equals(#C5, 2);
   exp::Expect::equals(#C5, 2);
   exp::Expect::equals(#C5, 2);
+  exp::Expect::equals(#C7, null);
 }
 
 constants  {
@@ -71,4 +77,5 @@ constants  {
   #C4 = 6
   #C5 = 2
   #C6 = -2
+  #C7 = null
 }

pkg/front_end/testcases/const_functions/const_functions_variable_declarations.dart.weak.outline.expect

@@ -13,6 +13,7 @@ static const field core::int var5 = self::function5();
 static const field core::int var6 = self::function6();
 static const field core::int var7 = self::function7();
 static const field core::int var8 = self::function8();
+static const field core::int? var9 = self::function9();
 static method function1(core::int a, core::int b) → core::int
   ;
 static method function2() → core::String
@@ -29,5 +30,7 @@ static method function7() → core::int
   ;
 static method function8() → core::int
   ;
+static method function9() → core::int?
+  ;
 static method main() → void
   ;

pkg/front_end/testcases/const_functions/const_functions_variable_declarations.dart.weak.transformed.expect

@@ -14,6 +14,7 @@ static const field core::int var5 = #C6;
 static const field core::int var6 = #C5;
 static const field core::int var7 = #C5;
 static const field core::int var8 = #C5;
+static const field core::int? var9 = #C7;
 static method function1(core::int a, core::int b) → core::int {
   core::int x = 1.{core::num::+}(a).{core::num::+}(b);
   return x;
@@ -52,6 +53,10 @@ static method function8() → core::int {
   a = 2;
   return a as{TypeError,ForDynamic,ForNonNullableByDefault} core::int;
 }
+static method function9() → core::int? {
+  core::int? x;
+  return x;
+}
 static method main() → void {
   exp::Expect::equals(#C1, 4);
   exp::Expect::equals(#C2, 5);
@@ -62,6 +67,7 @@ static method main() → void {
   exp::Expect::equals(#C5, 2);
   exp::Expect::equals(#C5, 2);
   exp::Expect::equals(#C5, 2);
+  exp::Expect::equals(#C7, null);
 }
 
 constants  {
@@ -71,8 +77,9 @@ constants  {
   #C4 = 6
   #C5 = 2
   #C6 = -2
+  #C7 = null
 }
 
 Extra constant evaluation status:
-Evaluated: MethodInvocation @ org-dartlang-testcase:///const_functions_variable_declarations.dart:71:23 -> IntConstant(-2)
-Extra constant evaluation: evaluated: 33, effectively constant: 1
+Evaluated: MethodInvocation @ org-dartlang-testcase:///const_functions_variable_declarations.dart:77:23 -> IntConstant(-2)
+Extra constant evaluation: evaluated: 35, effectively constant: 1

pkg/front_end/testcases/const_functions/const_functions_variable_declarations_error.dart

@@ -0,0 +1,22 @@
+// Copyright (c) 2021, 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.
+
+// Tests erroneous variable declaration usage within const functions.
+
+import "package:expect/expect.dart";
+
+const var1 = fn1();
+int fn1() {
+  var a;
+  return a;
+}
+
+const var2 = fn2();
+int fn2() {
+  var x;
+  x = "string";
+  return x;
+}
+
+void main() {}

pkg/front_end/testcases/const_functions/const_functions_variable_declarations_error.dart.strong.expect

@@ -0,0 +1,41 @@
+library /*isNonNullableByDefault*/;
+//
+// Problems in library:
+//
+// pkg/front_end/testcases/const_functions/const_functions_variable_declarations_error.dart:9:14: Error: Constant evaluation error:
+// const var1 = fn1();
+//              ^
+// pkg/front_end/testcases/const_functions/const_functions_variable_declarations_error.dart:12:10: Context: Expected constant 'null' to be of type 'int', but was of type 'Null'.
+//   return a;
+//          ^
+// pkg/front_end/testcases/const_functions/const_functions_variable_declarations_error.dart:9:7: Context: While analyzing:
+// const var1 = fn1();
+//       ^
+//
+// pkg/front_end/testcases/const_functions/const_functions_variable_declarations_error.dart:15:14: Error: Constant evaluation error:
+// const var2 = fn2();
+//              ^
+// pkg/front_end/testcases/const_functions/const_functions_variable_declarations_error.dart:19:10: Context: Expected constant '"string"' to be of type 'int', but was of type 'String'.
+//   return x;
+//          ^
+// pkg/front_end/testcases/const_functions/const_functions_variable_declarations_error.dart:15:7: Context: While analyzing:
+// const var2 = fn2();
+//       ^
+//
+import self as self;
+import "dart:core" as core;
+
+import "package:expect/expect.dart";
+
+static const field core::int var1 = invalid-expression "Expected constant 'null' to be of type 'int', but was of type 'Null'.";
+static const field core::int var2 = invalid-expression "Expected constant '\"string\"' to be of type 'int', but was of type 'String'.";
+static method fn1() → core::int {
+  dynamic a;
+  return a as{TypeError,ForDynamic,ForNonNullableByDefault} core::int;
+}
+static method fn2() → core::int {
+  dynamic x;
+  x = "string";
+  return x as{TypeError,ForDynamic,ForNonNullableByDefault} core::int;
+}
+static method main() → void {}

pkg/front_end/testcases/const_functions/const_functions_variable_declarations_error.dart.strong.transformed.expect

@@ -0,0 +1,41 @@
+library /*isNonNullableByDefault*/;
+//
+// Problems in library:
+//
+// pkg/front_end/testcases/const_functions/const_functions_variable_declarations_error.dart:9:14: Error: Constant evaluation error:
+// const var1 = fn1();
+//              ^
+// pkg/front_end/testcases/const_functions/const_functions_variable_declarations_error.dart:12:10: Context: Expected constant 'null' to be of type 'int', but was of type 'Null'.
+//   return a;
+//          ^
+// pkg/front_end/testcases/const_functions/const_functions_variable_declarations_error.dart:9:7: Context: While analyzing:
+// const var1 = fn1();
+//       ^
+//
+// pkg/front_end/testcases/const_functions/const_functions_variable_declarations_error.dart:15:14: Error: Constant evaluation error:
+// const var2 = fn2();
+//              ^
+// pkg/front_end/testcases/const_functions/const_functions_variable_declarations_error.dart:19:10: Context: Expected constant '"string"' to be of type 'int', but was of type 'String'.
+//   return x;
+//          ^
+// pkg/front_end/testcases/const_functions/const_functions_variable_declarations_error.dart:15:7: Context: While analyzing:
+// const var2 = fn2();
+//       ^
+//
+import self as self;
+import "dart:core" as core;
+
+import "package:expect/expect.dart";
+
+static const field core::int var1 = invalid-expression "Expected constant 'null' to be of type 'int', but was of type 'Null'.";
+static const field core::int var2 = invalid-expression "Expected constant '\"string\"' to be of type 'int', but was of type 'String'.";
+static method fn1() → core::int {
+  dynamic a;
+  return a as{TypeError,ForDynamic,ForNonNullableByDefault} core::int;
+}
+static method fn2() → core::int {
+  dynamic x;
+  x = "string";
+  return x as{TypeError,ForDynamic,ForNonNullableByDefault} core::int;
+}
+static method main() → void {}

pkg/front_end/testcases/const_functions/const_functions_variable_declarations_error.dart.textual_outline.expect

@@ -0,0 +1,7 @@
+import "package:expect/expect.dart";
+
+const var1 = fn1();
+int fn1() {}
+const var2 = fn2();
+int fn2() {}
+void main() {}

pkg/front_end/testcases/const_functions/const_functions_variable_declarations_error.dart.textual_outline_modelled.expect

@@ -0,0 +1,7 @@
+import "package:expect/expect.dart";
+
+const var1 = fn1();
+const var2 = fn2();
+int fn1() {}
+int fn2() {}
+void main() {}

pkg/front_end/testcases/const_functions/const_functions_variable_declarations_error.dart.weak.expect

@@ -0,0 +1,41 @@
+library /*isNonNullableByDefault*/;
+//
+// Problems in library:
+//
+// pkg/front_end/testcases/const_functions/const_functions_variable_declarations_error.dart:9:14: Error: Constant evaluation error:
+// const var1 = fn1();
+//              ^
+// pkg/front_end/testcases/const_functions/const_functions_variable_declarations_error.dart:10:8: Context: Expected constant 'null' to be of type 'int', but was of type 'Null'.
+// int fn1() {
+//        ^
+// pkg/front_end/testcases/const_functions/const_functions_variable_declarations_error.dart:9:7: Context: While analyzing:
+// const var1 = fn1();
+//       ^
+//
+// pkg/front_end/testcases/const_functions/const_functions_variable_declarations_error.dart:15:14: Error: Constant evaluation error:
+// const var2 = fn2();
+//              ^
+// pkg/front_end/testcases/const_functions/const_functions_variable_declarations_error.dart:19:10: Context: Expected constant '"string"' to be of type 'int', but was of type 'String'.
+//   return x;
+//          ^
+// pkg/front_end/testcases/const_functions/const_functions_variable_declarations_error.dart:15:7: Context: While analyzing:
+// const var2 = fn2();
+//       ^
+//
+import self as self;
+import "dart:core" as core;
+
+import "package:expect/expect.dart";
+
+static const field core::int var1 = invalid-expression "Expected constant 'null' to be of type 'int', but was of type 'Null'.";
+static const field core::int var2 = invalid-expression "Expected constant '\"string\"' to be of type 'int', but was of type 'String'.";
+static method fn1() → core::int {
+  dynamic a;
+  return a as{TypeError,ForDynamic,ForNonNullableByDefault} core::int;
+}
+static method fn2() → core::int {
+  dynamic x;
+  x = "string";
+  return x as{TypeError,ForDynamic,ForNonNullableByDefault} core::int;
+}
+static method main() → void {}

pkg/front_end/testcases/const_functions/const_functions_variable_declarations_error.dart.weak.outline.expect

@@ -0,0 +1,14 @@
+library /*isNonNullableByDefault*/;
+import self as self;
+import "dart:core" as core;
+
+import "package:expect/expect.dart";
+
+static const field core::int var1 = self::fn1();
+static const field core::int var2 = self::fn2();
+static method fn1() → core::int
+  ;
+static method fn2() → core::int
+  ;
+static method main() → void
+  ;

pkg/front_end/testcases/const_functions/const_functions_variable_declarations_error.dart.weak.transformed.expect

@@ -0,0 +1,41 @@
+library /*isNonNullableByDefault*/;
+//
+// Problems in library:
+//
+// pkg/front_end/testcases/const_functions/const_functions_variable_declarations_error.dart:9:14: Error: Constant evaluation error:
+// const var1 = fn1();
+//              ^
+// pkg/front_end/testcases/const_functions/const_functions_variable_declarations_error.dart:10:8: Context: Expected constant 'null' to be of type 'int', but was of type 'Null'.
+// int fn1() {
+//        ^
+// pkg/front_end/testcases/const_functions/const_functions_variable_declarations_error.dart:9:7: Context: While analyzing:
+// const var1 = fn1();
+//       ^
+//
+// pkg/front_end/testcases/const_functions/const_functions_variable_declarations_error.dart:15:14: Error: Constant evaluation error:
+// const var2 = fn2();
+//              ^
+// pkg/front_end/testcases/const_functions/const_functions_variable_declarations_error.dart:19:10: Context: Expected constant '"string"' to be of type 'int', but was of type 'String'.
+//   return x;
+//          ^
+// pkg/front_end/testcases/const_functions/const_functions_variable_declarations_error.dart:15:7: Context: While analyzing:
+// const var2 = fn2();
+//       ^
+//
+import self as self;
+import "dart:core" as core;
+
+import "package:expect/expect.dart";
+
+static const field core::int var1 = invalid-expression "Expected constant 'null' to be of type 'int', but was of type 'Null'.";
+static const field core::int var2 = invalid-expression "Expected constant '\"string\"' to be of type 'int', but was of type 'String'.";
+static method fn1() → core::int {
+  dynamic a;
+  return a as{TypeError,ForDynamic,ForNonNullableByDefault} core::int;
+}
+static method fn2() → core::int {
+  dynamic x;
+  x = "string";
+  return x as{TypeError,ForDynamic,ForNonNullableByDefault} core::int;
+}
+static method main() → void {}

tests/language/const_functions/const_functions_variable_declarations_error_test.dart

@@ -16,3 +16,13 @@ int fn1() {
   var a;
   return a;
 }
+
+const var2 = fn2();
+//           ^^^^^
+// [analyzer] COMPILE_TIME_ERROR.CONST_INITIALIZED_WITH_NON_CONSTANT_VALUE
+// [cfe] Constant evaluation error:
+int fn2() {
+  var x;
+  x = "string";
+  return x;
+}

tests/language/const_functions/const_functions_variable_declarations_test.dart

@@ -82,6 +82,14 @@ int function8() {
   return a;
 }
 
+const var9 = function9();
+//           ^^^^^^^^^^^
+// [analyzer] COMPILE_TIME_ERROR.CONST_INITIALIZED_WITH_NON_CONSTANT_VALUE
+int? function9() {
+  int? x;
+  return x;
+}
+
 void main() {
   Expect.equals(var1, 4);
   Expect.equals(var1_1, 5);
@@ -92,4 +100,5 @@ void main() {
   Expect.equals(var6, 2);
   Expect.equals(var7, 2);
   Expect.equals(var8, 2);
+  Expect.equals(var9, null);
 }