Merge pull request #1521 from hvitved/csharp/constant-condition-fp

Approved by calumgrant

change-notes/1.22/analysis-csharp.md

@@ -4,6 +4,7 @@
 
 | **Query**                    | **Expected impact**    | **Change**                        |
 |------------------------------|------------------------|-----------------------------------|
+| Constant condition (`cs/constant-condition`) | Fewer false positive results | Results have been removed for default cases (`_`) in switch expressions. |
 | Dispose may not be called if an exception is thrown during execution (`cs/dispose-not-called-on-throw`) | Fewer false positive results | Results have been removed where an object is disposed both by a `using` statement and a `Dispose` call. |
 
 ## Changes to code extraction

csharp/ql/src/Bad Practices/Control-Flow/ConstantCondition.ql

@@ -101,6 +101,13 @@ class ConstantMatchingCondition extends ConstantCondition {
     )
   }
 
+  override predicate isWhiteListed() {
+    exists(SwitchExpr se, int i |
+      se.getCase(i).getPattern() = this.(DiscardExpr) and
+      i > 0
+    )
+  }
+
   override string getMessage() {
     if b = true then result = "Pattern always matches." else result = "Pattern never matches."
   }

csharp/ql/test/query-tests/Bad Practices/Control-Flow/ConstantCondition/ConstantCondition.cs

@@ -1,4 +1,4 @@
-// semmle-extractor-options: /r:System.Threading.Thread.dll /r:System.Diagnostics.Debug.dll
+// semmle-extractor-options: /r:System.Threading.Thread.dll /r:System.Diagnostics.Debug.dll /langversion:preview
 
 using System;
 using System.Collections;
@@ -89,6 +89,23 @@ class ConstantMatching
               break;
         }
     }
+
+    string M4(object o)
+    {
+        return o switch
+        {
+            _ => o.ToString() // BAD
+        };
+    }
+
+    string M5(object o)
+    {
+        return o switch
+        {
+            "" => " ",
+            _ => o.ToString() // GOOD
+        };
+    }
 }
 
 class Assertions

csharp/ql/test/query-tests/Bad Practices/Control-Flow/ConstantCondition/ConstantCondition.expected

@@ -7,6 +7,7 @@
 | ConstantCondition.cs:64:18:64:18 | 2 | Pattern never matches. |
 | ConstantCondition.cs:66:18:66:18 | 3 | Pattern always matches. |
 | ConstantCondition.cs:77:18:77:20 | access to type Int32 | Pattern never matches. |
+| ConstantCondition.cs:97:13:97:13 | _ | Pattern always matches. |
 | ConstantConditionBad.cs:5:16:5:20 | ... > ... | Condition always evaluates to 'false'. |
 | ConstantConditionalExpressionCondition.cs:11:22:11:34 | ... == ... | Condition always evaluates to 'true'. |
 | ConstantConditionalExpressionCondition.cs:12:21:12:25 | false | Condition always evaluates to 'false'. |