C#: Remove false positives for cs/useless-assignment-to-local

change-notes/1.24/analysis-csharp.md

@@ -20,6 +20,7 @@ The following changes in version 1.24 affect C# analysis in all applications.
 | Useless assignment to local variable (`cs/useless-assignment-to-local`) | Fewer false positive results | Results have been removed when the variable is named `_` in a `foreach` statement. | 
 | Potentially dangerous use of non-short-circuit logic (`cs/non-short-circuit`) | Fewer false positive results | Results have been removed when the expression contains an `out` parameter. |
 | Dereferenced variable may be null (`cs/dereferenced-value-may-be-null`) | More results | Results are reported from parameters with a default value of `null`. |
+| Useless assignment to local variable (`cs/useless-assignment-to-local`) | Fewer false positive results | Results have been removed when the value assigned is an (implicitly or explicitly) cast default-like value. For example, `var s = (string)null` and `string s = default`. |
 
 ## Removal of old queries
 

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

@@ -114,7 +114,7 @@ class RelevantDefinition extends AssignableDefinition {
    */
   private predicate isDefaultLikeInitializer() {
     this.isInitializer() and
-    exists(Expr e | e = this.getSource() |
+    exists(Expr e | e = this.getSource().stripCasts() |
       exists(string val | val = e.getValue() |
         val = "0" or
         val = "-1" or

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

@@ -389,6 +389,20 @@ string M7(bool b)
             return s;
         return null;
     }
+
+    string M8()
+    {
+        string s = default; // "GOOD"
+        s = "";
+        return s;
+    }
+
+    string M9()
+    {
+        var s = (string)null; // "GOOD"
+        s = "";
+        return s;
+    }
 }
 
 class Anonymous