C#: Special handling of unknown types in isMatchingConstant.

csharp/ql/lib/semmle/code/csharp/controlflow/internal/Completion.qll

@@ -293,6 +293,8 @@ private predicate isMatchingConstant(PatternExpr pe, boolean value) {
   value = true
   or
   exists(Type t, Type strippedType |
+    not t instanceof UnknownType and
+    not strippedType instanceof UnknownType and
     typePatternMustHaveMatchingCompletion(pe, t, strippedType) and
     not typePatternCommonSubType(t, strippedType) and
     value = false

csharp/ql/src/change-notes/2025-03-10-unknown-type-matching.md

@@ -0,0 +1,4 @@
+---
+category: minorAnalysis
+---
+* Increase query precision for `cs/useless-assignment-to-local` and `cs/constant-condition` when *unknown* types are involved (mostly relevant for build mode none databases).

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

@@ -0,0 +1,27 @@
+using System;
+
+partial class C1
+{
+    public C2 Prop { get; set; }
+}
+
+class C2 { }
+
+class ConstantMatching
+{
+    void M1()
+    {
+        var c1 = new C1();
+        if (c1.Prop is int) // $ Alert
+        {
+        }
+
+        // Should not be considered a constant condition as
+        // we don't know anything about D.
+        var d = new D();
+        if (d.Prop is C2)
+        {
+        }
+    }
+}
+

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

@@ -0,0 +1,2 @@
+| ConstantCondition.cs:15:13:15:26 | ... is ... | Condition always evaluates to 'false'. |
+| ConstantCondition.cs:15:24:15:26 | access to type Int32 | Pattern never matches. |

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

@@ -0,0 +1,2 @@
+query: Bad Practices/Control-Flow/ConstantCondition.ql
+postprocess: utils/test/InlineExpectationsTestQuery.ql

csharp/ql/test/query-tests/standalone/Likely Bugs/ObjectComparison/options

@@ -0,0 +1 @@
+semmle-extractor-options: --standalone