[cfe] Invalid empty return statements in const function invocations.

Change-Id: Id1408bff917200f3825846bd6d483e52ca326d5b
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/194840
Reviewed-by: Jake Macdonald <jakemac@google.com>
Reviewed-by: Dmitry Stefantsov <dmitryas@google.com>
Commit-Queue: Kallen Tu <kallentu@google.com>

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

@@ -1036,15 +1036,22 @@ class ConstantEvaluator implements ExpressionVisitor<Constant> {
     return result;
   }
 
-  /// Execute the statement using the [StatementConstantEvaluator].
-  Constant execute(Statement statement) {
+  /// Execute a function body using the [StatementConstantEvaluator].
+  Constant executeBody(Statement statement) {
     StatementConstantEvaluator statementEvaluator =
         new StatementConstantEvaluator(this);
     ExecutionStatus status = statement.accept(statementEvaluator);
     if (status is ReturnStatus) {
+      if (status.value == null) {
+        // Void return type from executing the function body.
+        return new NullConstant();
+      }
       return status.value;
     } else if (status is AbortStatus) {
       return status.error;
+    } else if (status is ProceedStatus) {
+      // No return statement in function body with void return type.
+      return new NullConstant();
     }
     return createInvalidExpressionConstant(statement,
         'No valid constant returned from the execution of $statement.');
@@ -2870,7 +2877,7 @@ class ConstantEvaluator implements ExpressionVisitor<Constant> {
         if (value is AbortConstant) return value;
         env.addVariableValue(parameter, value);
       }
-      return execute(function.body);
+      return executeBody(function.body);
     }
 
     if (functionEnvironment != null) {

pkg/front_end/testcases/const_functions/const_functions_return.dart

@@ -0,0 +1,30 @@
+// 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 function invocation return types.
+
+import "package:expect/expect.dart";
+
+const var1 = fn();
+void fn() {}
+
+const var2 = fn2();
+void fn2() {
+  return;
+}
+
+const var3 = fn3();
+int? fn3() => null;
+
+const var4 = fn4();
+int? fn4() {
+  return null;
+}
+
+void main() {
+  Expect.equals((var1 as dynamic), null);
+  Expect.equals((var2 as dynamic), null);
+  Expect.equals(var3, null);
+  Expect.equals(var4, null);
+}

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

@@ -0,0 +1,30 @@
+library /*isNonNullableByDefault*/;
+import self as self;
+import "dart:core" as core;
+import "package:expect/expect.dart" as exp;
+
+import "package:expect/expect.dart";
+
+static const field void var1 = #C1;
+static const field void var2 = #C1;
+static const field core::int? var3 = #C1;
+static const field core::int? var4 = #C1;
+static method fn() → void {}
+static method fn2() → void {
+  return;
+}
+static method fn3() → core::int?
+  return null;
+static method fn4() → core::int? {
+  return null;
+}
+static method main() → void {
+  exp::Expect::equals((#C1) as{ForNonNullableByDefault} dynamic, null);
+  exp::Expect::equals((#C1) as{ForNonNullableByDefault} dynamic, null);
+  exp::Expect::equals(#C1, null);
+  exp::Expect::equals(#C1, null);
+}
+
+constants  {
+  #C1 = null
+}

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

@@ -0,0 +1,35 @@
+library /*isNonNullableByDefault*/;
+import self as self;
+import "dart:core" as core;
+import "package:expect/expect.dart" as exp;
+
+import "package:expect/expect.dart";
+
+static const field void var1 = #C1;
+static const field void var2 = #C1;
+static const field core::int? var3 = #C1;
+static const field core::int? var4 = #C1;
+static method fn() → void {}
+static method fn2() → void {
+  return;
+}
+static method fn3() → core::int?
+  return null;
+static method fn4() → core::int? {
+  return null;
+}
+static method main() → void {
+  exp::Expect::equals((#C1) as{ForNonNullableByDefault} dynamic, null);
+  exp::Expect::equals((#C1) as{ForNonNullableByDefault} dynamic, null);
+  exp::Expect::equals(#C1, null);
+  exp::Expect::equals(#C1, null);
+}
+
+constants  {
+  #C1 = null
+}
+
+Extra constant evaluation status:
+Evaluated: AsExpression @ org-dartlang-testcase:///const_functions_return.dart:26:23 -> NullConstant(null)
+Evaluated: AsExpression @ org-dartlang-testcase:///const_functions_return.dart:27:23 -> NullConstant(null)
+Extra constant evaluation: evaluated: 6, effectively constant: 2

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

@@ -0,0 +1,11 @@
+import "package:expect/expect.dart";
+
+const var1 = fn();
+void fn() {}
+const var2 = fn2();
+void fn2() {}
+const var3 = fn3();
+int? fn3() => null;
+const var4 = fn4();
+int? fn4() {}
+void main() {}

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

@@ -0,0 +1,11 @@
+import "package:expect/expect.dart";
+
+const var1 = fn();
+const var2 = fn2();
+const var3 = fn3();
+const var4 = fn4();
+int? fn3() => null;
+int? fn4() {}
+void fn() {}
+void fn2() {}
+void main() {}

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

@@ -0,0 +1,30 @@
+library /*isNonNullableByDefault*/;
+import self as self;
+import "dart:core" as core;
+import "package:expect/expect.dart" as exp;
+
+import "package:expect/expect.dart";
+
+static const field void var1 = #C1;
+static const field void var2 = #C1;
+static const field core::int? var3 = #C1;
+static const field core::int? var4 = #C1;
+static method fn() → void {}
+static method fn2() → void {
+  return;
+}
+static method fn3() → core::int?
+  return null;
+static method fn4() → core::int? {
+  return null;
+}
+static method main() → void {
+  exp::Expect::equals((#C1) as{ForNonNullableByDefault} dynamic, null);
+  exp::Expect::equals((#C1) as{ForNonNullableByDefault} dynamic, null);
+  exp::Expect::equals(#C1, null);
+  exp::Expect::equals(#C1, null);
+}
+
+constants  {
+  #C1 = null
+}

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

@@ -0,0 +1,20 @@
+library /*isNonNullableByDefault*/;
+import self as self;
+import "dart:core" as core;
+
+import "package:expect/expect.dart";
+
+static const field void var1 = self::fn();
+static const field void var2 = self::fn2();
+static const field core::int? var3 = self::fn3();
+static const field core::int? var4 = self::fn4();
+static method fn() → void
+  ;
+static method fn2() → void
+  ;
+static method fn3() → core::int?
+  ;
+static method fn4() → core::int?
+  ;
+static method main() → void
+  ;

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

@@ -0,0 +1,35 @@
+library /*isNonNullableByDefault*/;
+import self as self;
+import "dart:core" as core;
+import "package:expect/expect.dart" as exp;
+
+import "package:expect/expect.dart";
+
+static const field void var1 = #C1;
+static const field void var2 = #C1;
+static const field core::int? var3 = #C1;
+static const field core::int? var4 = #C1;
+static method fn() → void {}
+static method fn2() → void {
+  return;
+}
+static method fn3() → core::int?
+  return null;
+static method fn4() → core::int? {
+  return null;
+}
+static method main() → void {
+  exp::Expect::equals((#C1) as{ForNonNullableByDefault} dynamic, null);
+  exp::Expect::equals((#C1) as{ForNonNullableByDefault} dynamic, null);
+  exp::Expect::equals(#C1, null);
+  exp::Expect::equals(#C1, null);
+}
+
+constants  {
+  #C1 = null
+}
+
+Extra constant evaluation status:
+Evaluated: AsExpression @ org-dartlang-testcase:///const_functions_return.dart:26:23 -> NullConstant(null)
+Evaluated: AsExpression @ org-dartlang-testcase:///const_functions_return.dart:27:23 -> NullConstant(null)
+Extra constant evaluation: evaluated: 6, effectively constant: 2

tests/language/const_functions/const_functions_return_test.dart

@@ -0,0 +1,40 @@
+// 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 function invocation return types.
+
+// SharedOptions=--enable-experiment=const-functions
+
+import "package:expect/expect.dart";
+
+const var1 = fn();
+//           ^^^^
+// [analyzer] COMPILE_TIME_ERROR.CONST_INITIALIZED_WITH_NON_CONSTANT_VALUE
+void fn() {}
+
+const var2 = fn2();
+//           ^^^^^
+// [analyzer] COMPILE_TIME_ERROR.CONST_INITIALIZED_WITH_NON_CONSTANT_VALUE
+void fn2() {
+  return;
+}
+
+const var3 = fn3();
+//           ^^^^^
+// [analyzer] COMPILE_TIME_ERROR.CONST_INITIALIZED_WITH_NON_CONSTANT_VALUE
+int? fn3() => null;
+
+const var4 = fn4();
+//           ^^^^^
+// [analyzer] COMPILE_TIME_ERROR.CONST_INITIALIZED_WITH_NON_CONSTANT_VALUE
+int? fn4() {
+  return null;
+}
+
+void main() {
+  Expect.equals((var1 as dynamic), null);
+  Expect.equals((var2 as dynamic), null);
+  Expect.equals(var3, null);
+  Expect.equals(var4, null);
+}