[nnbd_migration] suppress fewer upstream exact nullable nodes

Change-Id: I1acd6b44c098868127e903d96baafa03bcd34b04
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/127421
Reviewed-by: Janice Collins <jcollins@google.com>
Commit-Queue: Mike Fairhurst <mfairhurst@google.com>

Author: MichaelRFairhurst

pkg/analysis_server/lib/src/edit/nnbd_migration/info_builder.dart

@@ -92,10 +92,12 @@ class InfoBuilder {
   }
 
   Iterable<EdgeInfo> upstreamTriggeredEdges(NullabilityNodeInfo node,
-      [List<RegionDetail> details]) {
+      {bool skipExactNullable = true}) {
     var edges = <EdgeInfo>[];
     for (EdgeInfo edge in node.upstreamEdges) {
-      if (node.isExactNullable && edge.sourceNode.isExactNullable) {
+      if (skipExactNullable &&
+          node.isExactNullable &&
+          edge.sourceNode.isExactNullable) {
         // When an exact nullable points here, the nullability propagated
         // in the other direction.
         continue;
@@ -105,7 +107,14 @@ class InfoBuilder {
       }
     }
     for (final containerNode in node.outerCompoundNodes) {
-      edges.addAll(upstreamTriggeredEdges(containerNode, details));
+      // We must include the exact nullable edges in the upstream triggered
+      // edges of the container node. If this node is in a substitution node,
+      // then it's possible it was marked exact nullable because it's container
+      // was marked nullable. It's container could have been marked nullable by
+      // another exact nullable node. We cannot tell. Err on the side of
+      // surfacing too many reasons.
+      edges.addAll(
+          upstreamTriggeredEdges(containerNode, skipExactNullable: false));
     }
 
     return edges;

pkg/analysis_server/test/src/edit/nnbd_migration/info_builder_test.dart

@@ -227,6 +227,33 @@ void g() {
     assertDetail(detail: regions[0].details[0], offset: 104, length: 1);
   }
 
+  test_exactNullable_exactNullable() async {
+    UnitInfo unit = await buildInfoForSingleTestFile('''
+void g(List<int> list1, List<int> list2) {
+  list1[0] = null;
+  list2[0] = list1[0];
+}
+''', migratedContent: '''
+void g(List<int?> list1, List<int?> list2) {
+  list1[0] = null;
+  list2[0] = list1[0];
+}
+''');
+    List<RegionInfo> regions = unit.regions;
+    expect(regions, hasLength(4));
+    // regions[0] is the hard edge that list1 is unconditionally indexed
+    assertRegion(region: regions[1], offset: 15, details: [
+      "An explicit 'null' is assigned",
+      // TODO(mfairhurst): Fix this bug.
+      'exact nullable node with no info (Substituted(type(32), migrated))'
+    ]);
+    // regions[2] is the hard edge that list2 is unconditionally indexed
+    assertRegion(
+        region: regions[3],
+        offset: 33,
+        details: ["A nullable value is assigned"]);
+  }
+
   test_exactNullable() async {
     UnitInfo unit = await buildInfoForSingleTestFile('''
 void f(List<int> list) {