Flow analysis: promote to non-nullable on initialization

When flow analysis encounters a variable declaration of the form `T? x
= expr;`, if the type of `expr` is `T`, then the variable is
immediately promoted to type `T`.

Fixes #43099.

Change-Id: Ia206fe0d50e2fdd9bdf637e13c85633d8490dbcc
Bug: dart-lang/sdk#43099
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/163841
Commit-Queue: Paul Berry <[email protected]>
Reviewed-by: Bob Nystrom <[email protected]>
Reviewed-by: Johnni Winther <[email protected]>
Reviewed-by: Konstantin Shcheglov <[email protected]>

Author: stereotype441

pkg/_fe_analyzer_shared/test/flow_analysis/type_promotion/data/initialization.dart

@@ -55,6 +55,18 @@ localVariable_ifElse_sameTypes(bool a) {
   x;
 }
 
+localVariable_initialized_nonNull() {
+  num? x = 0;
+  /*num*/ x;
+  x = null;
+  x;
+}
+
+localVariable_initialized_nonNull_final() {
+  final num? x = 0;
+  x;
+}
+
 localVariable_initialized_promoted_type_var<T>(T t) {
   if (t is num) {
     var x = /*T & num*/ t;
@@ -94,13 +106,13 @@ localVariable_initialized_unpromoted_type_var_with_bound<T extends num?>(T t) {
 
 localVariable_initialized_promoted_type_var_typed<T>(T t) {
   if (t is num) {
-    // TODO(paulberry): This should promote to `T & Object`, because that's the
-    // non-nullable version of T, but it shouldn't promote to `T & num`.
+    // This should promote to `T & Object`, because that's the non-nullable
+    // version of T, but it shouldn't promote to `T & num`.
     T x = /*T & num*/ t;
-    x;
+    /*T & Object*/ x;
     // Check that `T & Object` is a type of interest by promoting and then
     // writing to it
-    if (x is int) {
+    if (/*T & Object*/ x is int) {
       /*T & int*/ x;
       x = /*T & num*/ t;
       /*T & Object*/ x;

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

@@ -1994,12 +1994,17 @@ class ResolverVisitor extends ScopedVisitor {
     }
 
     var initializer = node.initializer;
-    var parent = node.parent;
-    TypeAnnotation declaredType = (parent as VariableDeclarationList).type;
-    if (declaredType == null && initializer != null) {
+    var parent = node.parent as VariableDeclarationList;
+    TypeAnnotation declaredType = parent.type;
+    if (initializer != null) {
       var initializerStaticType = initializer.staticType;
-      if (initializerStaticType is TypeParameterType) {
-        _flowAnalysis?.flow?.promote(declaredElement, initializerStaticType);
+      if (declaredType == null) {
+        if (initializerStaticType is TypeParameterType) {
+          _flowAnalysis?.flow?.promote(declaredElement, initializerStaticType);
+        }
+      } else if (!parent.isFinal) {
+        _flowAnalysis?.flow?.write(declaredElement, initializerStaticType,
+            viaInitializer: true);
       }
     }
   }

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

@@ -5740,8 +5740,17 @@ class InferenceVisitor
     }
     if (initializerResult != null) {
       DartType initializerType = initializerResult.inferredType;
-      if (node.isImplicitlyTyped && initializerType is TypeParameterType) {
-        inferrer.flowAnalysis.promote(node, initializerType);
+      if (node.isImplicitlyTyped) {
+        if (initializerType is TypeParameterType) {
+          inferrer.flowAnalysis.promote(node, initializerType);
+        }
+      } else if (!node.isFinal) {
+        // TODO(paulberry): `initializerType` is sometimes `null` during top
+        // level inference.  Figure out how to prevent this.
+        if (initializerType != null) {
+          inferrer.flowAnalysis
+              .write(node, initializerType, viaInitializer: true);
+        }
       }
       Expression initializer = inferrer.ensureAssignableResult(
           node.type, initializerResult,

pkg/front_end/testcases/incremental_initialize_from_dill/experiments_enabled_1.yaml

@@ -14,8 +14,8 @@ worlds:
         main() {
           dynamic x;
           print(x >>> 2);
-          Class? c = new Class();
+          Class? c = new Class() as Class?;
           print(c!);
         }
         class Class {}
-    expectedLibraryCount: 1
+    expectedLibraryCount: 1

pkg/front_end/testcases/nnbd/issue41114.dart

@@ -3,7 +3,7 @@
 // BSD-style license that can be found in the LICENSE file.
 
 main() async {
-  List<String>? a = <String>[];
+  List<String>? a = <String>[] as List<String>?;
   Iterable<String>? b = a?.map((e) => e);
   Iterable<String>? i = b ?? a;
   print(i);

pkg/front_end/testcases/nnbd/issue41114.dart.strong.expect

@@ -3,7 +3,7 @@ import self as self;
 import "dart:core" as core;
 
 static method main() → dynamic async {
-  core::List<core::String>? a = <core::String>[];
+  core::List<core::String>? a = <core::String>[] as{ForNonNullableByDefault} core::List<core::String>?;
   core::Iterable<core::String>? b = let final core::List<core::String>? #t1 = a in #t1.{core::List::==}(null) ?{core::Iterable<core::String>?} null : #t1{core::List<core::String>}.{core::Iterable::map}<core::String>((core::String e) → core::String => e);
   core::Iterable<core::String>? i = let final core::Iterable<core::String>? #t2 = b in #t2.{core::Object::==}(null) ?{core::Iterable<core::String>?} a : #t2{core::Iterable<core::String>};
   core::print(i);

pkg/front_end/testcases/nnbd/issue41114.dart.weak.expect

@@ -3,7 +3,7 @@ import self as self;
 import "dart:core" as core;
 
 static method main() → dynamic async {
-  core::List<core::String>? a = <core::String>[];
+  core::List<core::String>? a = <core::String>[] as{ForNonNullableByDefault} core::List<core::String>?;
   core::Iterable<core::String>? b = let final core::List<core::String>? #t1 = a in #t1.{core::List::==}(null) ?{core::Iterable<core::String>?} null : #t1{core::List<core::String>}.{core::Iterable::map}<core::String>((core::String e) → core::String => e);
   core::Iterable<core::String>? i = let final core::Iterable<core::String>? #t2 = b in #t2.{core::Object::==}(null) ?{core::Iterable<core::String>?} a : #t2{core::Iterable<core::String>};
   core::print(i);

pkg/front_end/testcases/nnbd/issue41495.dart

@@ -10,7 +10,7 @@ class A {
 main() {}
 
 errors() {
-  A? a1 = new A();
+  A? a1 = new A() as A?;
   a1.c1;
   a1.test;
 }

pkg/front_end/testcases/nnbd/issue41495.dart.strong.expect

@@ -27,7 +27,7 @@ class A extends core::Object {
 }
 static method main() → dynamic {}
 static method errors() → dynamic {
-  self::A? a1 = new self::A::•();
+  self::A? a1 = new self::A::•() as{ForNonNullableByDefault} self::A?;
   let final<BottomType> #t1 = invalid-expression "pkg/front_end/testcases/nnbd/issue41495.dart:14:6: Error: Property 'c1' cannot be accessed on 'A?' because it is potentially null.
  - 'A' is from 'pkg/front_end/testcases/nnbd/issue41495.dart'.
 Try accessing using ?. instead.

pkg/front_end/testcases/nnbd/issue41495.dart.weak.expect

@@ -27,7 +27,7 @@ class A extends core::Object {
 }
 static method main() → dynamic {}
 static method errors() → dynamic {
-  self::A? a1 = new self::A::•();
+  self::A? a1 = new self::A::•() as{ForNonNullableByDefault} self::A?;
   let final<BottomType> #t1 = invalid-expression "pkg/front_end/testcases/nnbd/issue41495.dart:14:6: Error: Property 'c1' cannot be accessed on 'A?' because it is potentially null.
  - 'A' is from 'pkg/front_end/testcases/nnbd/issue41495.dart'.
 Try accessing using ?. instead.