[cfe] Infer type of x! as Never if x's type is Null

Closes flutter#39822.

Bug: http://dartbug.com/39822
Change-Id: I9600efcd4e69e331de5d9fa8f2b676c402df6502
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/128661
Reviewed-by: Johnni Winther <johnniwinther@google.com>
Commit-Queue: Dmitry Stefantsov <dmitryas@google.com>

Author: Dmitry Stefantsov

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

@@ -2370,10 +2370,11 @@ class InferenceVisitor
     // TODO(johnniwinther): Check that the inferred type is potentially
     //  nullable.
     inferrer.flowAnalysis.nonNullAssert_end(node.operand);
-    // TODO(johnniwinther): Return `NonNull(inferredType)`.
-    return new ExpressionInferenceResult(
-        operandResult.inferredType.withNullability(Nullability.nonNullable),
-        node);
+    DartType nonNullableResultType = operandResult.inferredType ==
+            inferrer.coreTypes.nullType
+        ? const NeverType(Nullability.nonNullable)
+        : operandResult.inferredType.withNullability(Nullability.nonNullable);
+    return new ExpressionInferenceResult(nonNullableResultType, node);
   }
 
   ExpressionInferenceResult visitNullAwareMethodInvocation(

pkg/front_end/testcases/nnbd/issue39822.dart

@@ -0,0 +1,11 @@
+// Copyright (c) 2019, 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.
+
+foo(Null x) {
+  bar(x!);
+}
+
+bar(int y) {}
+
+main() {}

pkg/front_end/testcases/nnbd/issue39822.dart.outline.expect

@@ -0,0 +1,10 @@
+library;
+import self as self;
+import "dart:core" as core;
+
+static method foo(core::Null? x) → dynamic
+  ;
+static method bar(core::int y) → dynamic
+  ;
+static method main() → dynamic
+  ;

pkg/front_end/testcases/nnbd/issue39822.dart.strong.expect

@@ -0,0 +1,9 @@
+library;
+import self as self;
+import "dart:core" as core;
+
+static method foo(core::Null? x) → dynamic {
+  self::bar(x!);
+}
+static method bar(core::int y) → dynamic {}
+static method main() → dynamic {}

pkg/front_end/testcases/nnbd/issue39822.dart.strong.transformed.expect

@@ -0,0 +1,9 @@
+library;
+import self as self;
+import "dart:core" as core;
+
+static method foo(core::Null? x) → dynamic {
+  self::bar(x!);
+}
+static method bar(core::int y) → dynamic {}
+static method main() → dynamic {}

pkg/front_end/testcases/nnbd/issue39822.dart.weak.expect

@@ -0,0 +1,9 @@
+library;
+import self as self;
+import "dart:core" as core;
+
+static method foo(core::Null? x) → dynamic {
+  self::bar(x!);
+}
+static method bar(core::int y) → dynamic {}
+static method main() → dynamic {}

pkg/front_end/testcases/nnbd/issue39822.dart.weak.transformed.expect

@@ -0,0 +1,9 @@
+library;
+import self as self;
+import "dart:core" as core;
+
+static method foo(core::Null? x) → dynamic {
+  self::bar(x!);
+}
+static method bar(core::int y) → dynamic {}
+static method main() → dynamic {}

pkg/front_end/testcases/text_serialization.status

@@ -930,6 +930,7 @@ nnbd/function_types: TextSerializationFailure
 nnbd/inheritance_from_opt_in: TypeCheckError
 nnbd/inheritance_from_opt_out: TextSerializationFailure
 nnbd/intersection_types: TextSerializationFailure
+nnbd/issue39822: TextSerializationFailure
 nnbd/issue_39286: TextSerializationFailure
 nnbd/issue_39286_2: TextSerializationFailure
 nnbd/late: TextSerializationFailure

pkg/kernel/lib/ast.dart

@@ -4013,9 +4013,10 @@ class NullCheck extends Expression {
   }
 
   DartType getStaticType(StaticTypeContext context) {
-    return operand
-        .getStaticType(context)
-        .withNullability(Nullability.nonNullable);
+    DartType operandType = operand.getStaticType(context);
+    return operandType == context.typeEnvironment.nullType
+        ? const NeverType(Nullability.nonNullable)
+        : operandType.withNullability(Nullability.nonNullable);
   }
 
   R accept<R>(ExpressionVisitor<R> v) => v.visitNullCheck(this);