Make variables assigned in a loop not definitely unassigned anywhere in the loop.

Bug: https://github.com/dart-lang/sdk/issues/42301
Change-Id: I75962b976427cef7f40025f1dcd9f27400306825
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/151047
Reviewed-by: Paul Berry <paulberry@google.com>
Commit-Queue: Konstantin Shcheglov <scheglov@google.com>

pkg/_fe_analyzer_shared/lib/src/flow_analysis/flow_analysis.dart

@@ -1270,6 +1270,7 @@ class FlowModel<Variable, Type> {
       Iterable<Variable> writtenVariables,
       Iterable<Variable> capturedVariables) {
     Map<Variable, VariableModel<Variable, Type>> newVariableInfo;
+
     for (Variable variable in writtenVariables) {
       VariableModel<Variable, Type> info = infoFor(variable);
       if (info.promotedTypes != null) {
@@ -1278,6 +1279,7 @@ class FlowModel<Variable, Type> {
                 variableInfo))[variable] = info.discardPromotions();
       }
     }
+
     for (Variable variable in capturedVariables) {
       VariableModel<Variable, Type> info = variableInfo[variable];
       if (info == null) {
@@ -1291,8 +1293,14 @@ class FlowModel<Variable, Type> {
                 variableInfo))[variable] = info.writeCapture();
       }
     }
-    if (newVariableInfo == null) return this;
-    return new FlowModel<Variable, Type>._(reachable, newVariableInfo);
+
+    FlowModel<Variable, Type> result = newVariableInfo == null
+        ? this
+        : new FlowModel<Variable, Type>._(reachable, newVariableInfo);
+
+    result = result.joinUnassigned(written: writtenVariables);
+
+    return result;
   }
 
   /// Updates the state to reflect a control path that is known to have
@@ -2433,8 +2441,6 @@ class _FlowAnalysisImpl<Node, Statement extends Node, Expression, Variable,
 
   @override
   void for_end() {
-    FlowModel<Variable, Type> afterUpdate = _current;
-
     _WhileContext<Variable, Type> context =
         _stack.removeLast() as _WhileContext<Variable, Type>;
     // Tail of the stack: falseCondition, break
@@ -2442,7 +2448,6 @@ class _FlowAnalysisImpl<Node, Statement extends Node, Expression, Variable,
     FlowModel<Variable, Type> falseCondition = context._conditionInfo.ifFalse;
 
     _current = _join(falseCondition, breakState);
-    _current = _current.joinUnassigned(other: afterUpdate);
   }
 
   @override
@@ -2481,9 +2486,6 @@ class _FlowAnalysisImpl<Node, Statement extends Node, Expression, Variable,
     _stack.add(new _SimpleContext(_current));
     _current = _current.removePromotedAll(_assignedVariables._anywhere._written,
         _assignedVariables._anywhere._captured);
-    _current = _current.joinUnassigned(
-      written: _assignedVariables._anywhere._written,
-    );
   }
 
   @override
@@ -2752,9 +2754,8 @@ class _FlowAnalysisImpl<Node, Statement extends Node, Expression, Variable,
 
     AssignedVariablesNodeInfo<Variable> info =
         _assignedVariables._getInfoForNode(body);
-    FlowModel<Variable, Type> beforeCatch = beforeBody
-        .removePromotedAll(info._written, info._captured)
-        .joinUnassigned(other: afterBody);
+    FlowModel<Variable, Type> beforeCatch =
+        beforeBody.removePromotedAll(info._written, info._captured);
 
     context._beforeCatch = beforeCatch;
     context._afterBodyAndCatches = afterBody;
@@ -2843,9 +2844,7 @@ class _FlowAnalysisImpl<Node, Statement extends Node, Expression, Variable,
   void whileStatement_end() {
     _WhileContext<Variable, Type> context =
         _stack.removeLast() as _WhileContext<Variable, Type>;
-    FlowModel<Variable, Type> afterBody = _current;
     _current = _join(context._conditionInfo.ifFalse, context._breakModel);
-    _current = _current.joinUnassigned(other: afterBody);
   }
 
   @override

pkg/_fe_analyzer_shared/test/flow_analysis/definite_unassignment/data/do.dart

@@ -40,7 +40,7 @@ breakOuterFromInner(bool c) {
 condition() {
   late int v1, v2;
   do {
-    /*unassigned*/ v1; // assigned in the condition, but not yet
+    v1;
   } while ((v1 = 0) + (v2 = 0) >= 0);
   v2;
 }

pkg/_fe_analyzer_shared/test/flow_analysis/definite_unassignment/data/for.dart

@@ -2,6 +2,56 @@
 // 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.
 
+for_assignedInBody_body(bool b) {
+  late int v;
+  for (;;) {
+    if (b) {
+      v = 0;
+    } else {
+      v;
+    }
+    v;
+  }
+  v;
+}
+
+for_assignedInBody_condition() {
+  bool firstTime = true;
+  late int v;
+  for (; firstTime || v > 0;) {
+    firstTime = false;
+    v = 5;
+  }
+  v;
+}
+
+for_assignedInBody_initializer() {
+  bool firstTime = true;
+  late int v;
+  for (var x = /*unassigned*/ v;;) {
+    v = 5;
+  }
+  v;
+}
+
+for_assignedInCondition() {
+  bool firstTime = true;
+  late int v;
+  for (var x = /*unassigned*/ v; (v = 0) > 0;) {
+    v;
+  }
+  v;
+}
+
+for_assignedInUpdater() {
+  bool firstTime = true;
+  late int v;
+  for (var x = /*unassigned*/ v;; v = 0) {
+    v;
+  }
+  v;
+}
+
 for_body(bool c) {
   late int v;
   for (; c;) {
@@ -77,7 +127,7 @@ for_initializer_variable() {
 for_updaters(bool c) {
   late int v1, v2, v3, v4;
   for (; c; v1 = 0, v2 = 0, v3 = 0, /*unassigned*/ v4) {
-    /*unassigned*/ v1;
+    v1;
   }
   v2;
 }
@@ -115,7 +165,7 @@ collection_for_initializer_variable() {
 
 collection_for_updaters(bool c) {
   late int v1, v2, v3, v4;
-  [for (; c; v1 = 0, v2 = 0, v3 = 0, /*unassigned*/ v4) /*unassigned*/ v1 ];
+  [for (; c; v1 = 0, v2 = 0, v3 = 0, /*unassigned*/ v4) v1];
   v2;
 }
 

pkg/_fe_analyzer_shared/test/flow_analysis/definite_unassignment/data/issue41284.dart

@@ -8,7 +8,7 @@ void method1() {
     local = 0;
     return;
   } finally {
-    print(/*unassigned*/ local);
+    print(local);
   }
   local;
 }
@@ -22,7 +22,7 @@ void method2() {
     local = 42;
     rethrow;
   } finally {
-    print(/*unassigned*/ local);
+    print(local);
   }
   local;
 }

pkg/_fe_analyzer_shared/test/flow_analysis/definite_unassignment/data/while.dart

@@ -2,6 +2,29 @@
 // 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.
 
+assignedInBody_body(bool b) {
+  late int v;
+  while (true) {
+    if (b) {
+      v = 0;
+    } else {
+      v;
+    }
+    v;
+  }
+  v;
+}
+
+assignedInBody_condition() {
+  bool firstTime = true;
+  late int v;
+  while (firstTime || v > 0) {
+    firstTime = false;
+    v = 5;
+  }
+  v;
+}
+
 condition() {
   late int v;
   while ((v = 0) >= 0) {