C#: Remove false positive and update test output

C#: Mark results as GOOD

Author: calumgrant

csharp/ql/src/Dead Code/DeadStoreOfLocal.ql

@@ -83,8 +83,10 @@ class RelevantDefinition extends AssignableDefinition {
     //or
     //this.(AssignableDefinitions::OutRefDefinition).getTargetAccess().isOutArgument()
     this.(AssignableDefinitions::LocalVariableDefinition).getDeclaration() = any(LocalVariableDeclExpr lvde |
-        lvde = any(SpecificCatchClause scc).getVariableDeclExpr() or
-        lvde = any(ForeachStmt fs).getVariableDeclExpr()
+        lvde = any(SpecificCatchClause scc).getVariableDeclExpr()
+        or
+        lvde = any(ForeachStmt fs).getVariableDeclExpr() and
+        not lvde.getName() = "_"
       )
     or
     this instanceof AssignableDefinitions::PatternDefinition

csharp/ql/test/query-tests/Dead Code/DeadStoreOfLocal/DeadStoreOfLocal.cs

@@ -431,7 +431,7 @@ public static class AnonymousVariable
     public static int Count<T>(this IEnumerable<T> items)
     {
         int count = 0;
-        foreach (var _ in items)
+        foreach (var _ in items) // GOOD
             count++;
         return count;
     }

csharp/ql/test/query-tests/Dead Code/DeadStoreOfLocal/DeadStoreOfLocal.expected

@@ -16,7 +16,6 @@
 | DeadStoreOfLocal.cs:320:9:320:32 | ... = ... | This assignment to $@ is useless, since its value is never read. | DeadStoreOfLocal.cs:316:23:316:23 | b | b |
 | DeadStoreOfLocal.cs:361:13:361:20 | String s = ... | This assignment to $@ is useless, since its value is never read. | DeadStoreOfLocal.cs:361:13:361:13 | s | s |
 | DeadStoreOfLocal.cs:387:13:387:21 | ... = ... | This assignment to $@ is useless, since its value is never read. | DeadStoreOfLocal.cs:385:13:385:13 | s | s |
-| DeadStoreOfLocal.cs:434:22:434:22 | T _ | This assignment to $@ is useless, since its value is never read. | DeadStoreOfLocal.cs:434:22:434:22 | _ | _ |
 | DeadStoreOfLocalBad.cs:7:13:7:48 | Boolean success = ... | This assignment to $@ is useless, since its value is never read. | DeadStoreOfLocalBad.cs:7:13:7:19 | success | success |
 | DeadStoreOfLocalBad.cs:23:32:23:32 | FormatException e | This assignment to $@ is useless, since its value is never read. | DeadStoreOfLocalBad.cs:23:32:23:32 | e | e |
 | DeadStoreOfLocalBad.cs:32:22:32:22 | String s | This assignment to $@ is useless, since its value is never read. | DeadStoreOfLocalBad.cs:32:22:32:22 | s | s |