[nnbd_migration] fix hard edges on ??= assignments

Change-Id: Ie2731e723147189591302e411d1dcb7540e09b23
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/119762
Reviewed-by: Paul Berry <paulberry@google.com>
Commit-Queue: Mike Fairhurst <mfairhurst@google.com>

Author: MichaelRFairhurst

pkg/nnbd_migration/lib/src/edge_builder.dart

@@ -1658,7 +1658,8 @@ class EdgeBuilder extends GeneralizingAstVisitor<DecoratedType>
         _checkAssignment(expressionChecksOrigin,
             source: sourceType,
             destination: destinationType,
-            hard: _postDominatedLocals.isReferenceInScope(expression));
+            hard: questionAssignNode == null &&
+                _postDominatedLocals.isReferenceInScope(expression));
       }
       if (questionAssignNode != null) {
         // a ??= b is only nullable if both a and b are nullable.

pkg/nnbd_migration/test/edge_builder_test.dart

@@ -806,9 +806,11 @@ void Function(int) f(void Function(int) x, void Function(int) y) => x ??= y;
 List<int> f(List<int> x, List<int> y) => x ??= y;
 ''');
     var xNullable = decoratedTypeAnnotation('List<int> x').node;
+    var yNullable = decoratedTypeAnnotation('List<int> y').node;
     var xElementNullable = decoratedTypeAnnotation('int> x').node;
     var yElementNullable = decoratedTypeAnnotation('int> y').node;
     var returnElementNullable = decoratedTypeAnnotation('int> f').node;
+    assertEdge(yNullable, xNullable, hard: false, guards: [xNullable]);
     assertEdge(yElementNullable, xElementNullable,
         hard: false, guards: [xNullable]);
     assertEdge(xElementNullable, returnElementNullable, hard: false);
@@ -822,7 +824,7 @@ int f(int x, int y) => (x ??= y);
     var xNullable = decoratedTypeAnnotation('int x').node;
     var returnNullable = decoratedTypeAnnotation('int f').node;
     var glbNode = decoratedExpressionType('(x ??= y)').node;
-    assertEdge(yNullable, xNullable, hard: true, guards: [xNullable]);
+    assertEdge(yNullable, xNullable, hard: false, guards: [xNullable]);
     assertEdge(yNullable, glbNode, hard: false, guards: [xNullable]);
     assertEdge(glbNode, xNullable, hard: false);
     assertEdge(glbNode, yNullable, hard: false);