[Dynamic Instrumentation] DEBUG-4758 Fix type mismatch with @duration expression

tracer/src/Datadog.Trace/Debugger/Expressions/ProbeExpressionParser.cs

@@ -441,7 +441,15 @@ private Expression HandleReturnType(Expression finalExpr, List<ParameterExpressi
     {
         if (typeof(T).IsAssignableFrom(finalExpr.Type))
         {
-            return finalExpr;
+            // If the expression type is already exactly T, return as-is.
+            if (finalExpr.Type == typeof(T))
+            {
+                return finalExpr;
+            }
+
+            // When the expression type is assignable to T but different (e.g., value type -> object),
+            // we need an explicit conversion.
+            return Expression.Convert(finalExpr, typeof(T));
         }
 
         if (typeof(T).IsNumeric()

tracer/test/Datadog.Trace.Tests/Debugger/DebuggerExpressionLanguageTests.cs

@@ -111,6 +111,32 @@ public async Task TestMetrics(string expressionTestFilePath)
             await Test(expressionTestFilePath);
         }
 
+        [Fact]
+        public void ProbeExpressionParser_ObjectReturnType_AllowsValueTypeResults()
+        {
+            // Arrange
+            var scopeMembers = CreateScopeMembers();
+            const string json = """
+                                {
+                                  "ref": "@duration"
+                                }
+                                """;
+
+            // Act
+            var compiled = ProbeExpressionParser<object>.ParseExpression(json, scopeMembers);
+            var result = compiled.Delegate(
+                scopeMembers.InvocationTarget,
+                scopeMembers.Return,
+                scopeMembers.Duration,
+                scopeMembers.Exception,
+                scopeMembers.Members);
+
+            // Assert
+            Assert.NotNull(result);
+            Assert.IsType<TimeSpan>(result);
+            Assert.True(compiled.Errors == null || compiled.Errors.Length == 0);
+        }
+
         private async Task Test(string expressionTestFilePath)
         {
             // Arrange