Add workaround for equal lambda parameter symbols with different hashcodes

Author: tamasvajk

csharp/extractor/Semmle.Extraction.CSharp/Entities/Expressions/Lambda.cs

@@ -18,6 +18,7 @@ protected override void PopulateExpression(TextWriter trapFile) { }
         private void VisitParameter(ParameterSyntax p)
         {
             var symbol = Context.GetModel(p).GetDeclaredSymbol(p)!;
+            Context.CacheLambdaParameterSymbol(symbol, p);
             Parameter.Create(Context, symbol, this);
         }
 

csharp/extractor/Semmle.Extraction.CSharp/Entities/Parameter.cs

@@ -55,11 +55,17 @@ private Kind ParamKind
             }
         }
 
-        public static Parameter Create(Context cx, IParameterSymbol param, IEntity parent, Parameter? original = null) =>
-            ParameterFactory.Instance.CreateEntity(cx, param, (param, parent, original));
+        public static Parameter Create(Context cx, IParameterSymbol param, IEntity parent, Parameter? original = null)
+        {
+            var cachedSymbol = cx.GetPossiblyCachedParameterSymbol(param);
+            return ParameterFactory.Instance.CreateEntity(cx, cachedSymbol, (cachedSymbol, parent, original));
+        }
 
-        public static Parameter Create(Context cx, IParameterSymbol param) =>
-            ParameterFactory.Instance.CreateEntity(cx, param, (param, null, null));
+        public static Parameter Create(Context cx, IParameterSymbol param)
+        {
+            var cachedSymbol = cx.GetPossiblyCachedParameterSymbol(param);
+            return ParameterFactory.Instance.CreateEntity(cx, cachedSymbol, (cachedSymbol, null, null));
+        }
 
         public override void WriteId(EscapingTextWriter trapFile)
         {

csharp/extractor/Semmle.Extraction.CSharp/Extractor/Context.cs

@@ -39,6 +39,36 @@ public SemanticModel GetModel(SyntaxNode node)
         private SemanticModel? cachedModelForTree;
         private SemanticModel? cachedModelForOtherTrees;
 
+        // The below is a workaround to the bug reported in https://github.com/dotnet/roslyn/issues/58226
+        // Lambda parameters that are equal according to `SymbolEqualityComparer.Default`, might have different
+        // hash-codes, and as a result might not be found in `symbolEntityCache` by hash-code lookup.
+        internal IParameterSymbol GetPossiblyCachedParameterSymbol(IParameterSymbol param)
+        {
+            if ((param.ContainingSymbol as IMethodSymbol)?.MethodKind != MethodKind.AnonymousFunction)
+            {
+                return param;
+            }
+
+            foreach (var sr in param.DeclaringSyntaxReferences)
+            {
+                var syntax = sr.GetSyntax();
+                if (lambdaParameterCache.TryGetValue(syntax, out var cached) &&
+                    SymbolEqualityComparer.Default.Equals(param, cached))
+                {
+                    return cached;
+                }
+            }
+
+            return param;
+        }
+
+        internal void CacheLambdaParameterSymbol(IParameterSymbol param, SyntaxNode syntax)
+        {
+            lambdaParameterCache[syntax] = param;
+        }
+
+        private readonly Dictionary<SyntaxNode, IParameterSymbol> lambdaParameterCache = new Dictionary<SyntaxNode, IParameterSymbol>();
+
         /// <summary>
         /// The current compilation unit.
         /// </summary>