Flow analysis: additional tests for error types.

Verify that the presence of an error type doesn't accidentally trigger
"unnecessary cast pattern" or "unnecessary wildcard pattern" warnings
(this verifies that `promoteForPattern` returns the proper value when
an error type is encountered).

Verify that the presence of an error type doesn't alter reachability
conclusions that were established by previous subpatterns (this
verifies that `promoteForPattern` uses the proper join to update
`_unmatched` when an error type is encountered).

Change-Id: I351c71574dff31602c40b257c28681b81fad2867
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/303260
Commit-Queue: Paul Berry <paulberry@google.com>
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>

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

@@ -719,7 +719,9 @@ abstract class FlowAnalysis<Node extends Object, Statement extends Node,
   /// If [matchMayFailEvenIfCorrectType] is `true`, flow analysis would always
   /// update the unmatched value.
   ///
-  /// Returns `true` if [matchedType] is a subtype of [knownType].
+  /// Returns `true` if [matchedType] is a subtype of [knownType] (and thus the
+  /// user might need to be warned of an unnecessary cast or unnecessary
+  /// wildcard pattern).
   bool promoteForPattern(
       {required Type matchedType,
       required Type knownType,

pkg/_fe_analyzer_shared/test/flow_analysis/flow_analysis_test.dart

@@ -6690,6 +6690,12 @@ main() {
           ]);
         });
       });
+
+      test('Error type does not trigger unnecessary cast warning', () {
+        h.run([
+          ifCase(expr('int'), wildcard().as_('error'), []),
+        ]);
+      });
     });
 
     group('Constant pattern:', () {
@@ -8240,6 +8246,25 @@ main() {
           ]);
         });
       });
+
+      test('Error type does not alter previous reachability conclusions', () {
+        var x = Var('x');
+        h.run([
+          declare(x, initializer: expr('(Null, Object?)')),
+          ifCase(
+              x.expr,
+              recordPattern([
+                relationalPattern('!=', nullLiteral).recordField(),
+                wildcard(type: 'error').recordField()
+              ]),
+              [
+                checkReachable(false),
+              ],
+              [
+                checkReachable(true),
+              ]),
+        ]);
+      });
     });
 
     group('Relational pattern:', () {
@@ -9670,6 +9695,13 @@ main() {
             ]),
       ]);
     });
+
+    test('Error type does not trigger unnecessary wildcard warning', () {
+      h.run([
+        ifCase(expr('num'), wildcard(type: 'int').and(wildcard(type: 'error')),
+            []),
+      ]);
+    });
   });
 }
 

pkg/front_end/test/spell_checking_list_code.txt

@@ -1749,6 +1749,7 @@ walker
 walking
 wanting
 wants
+warned
 waste
 wasted
 watch