Fix StrictVariables exception to show property name instead of type name (#899)

* Initial plan

* Fix StrictVariables exception to show property name instead of type name

Co-authored-by: sebastienros <1165805+sebastienros@users.noreply.github.com>

* Add StrictVariables check to async Awaited method

Co-authored-by: sebastienros <1165805+sebastienros@users.noreply.github.com>

* Remove unnecessary IdentifierSegment assertions from tests

Co-authored-by: sebastienros <1165805+sebastienros@users.noreply.github.com>

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: sebastienros <1165805+sebastienros@users.noreply.github.com>

Author: Copilot

Fluid.Tests/StrictVariableTests.cs

@@ -494,6 +494,41 @@ public async Task UndefinedDelegate_CanReturnCustomValue()
         Assert.Equal("[missing not found] [another not found]", result);
     }
 
+    [Fact]
+    public async Task StrictVariables_ExceptionMessage_ContainsPropertyName()
+    {
+        // This test verifies that the exception message contains the actual property name
+        // rather than the type name like "Fluid.Ast.IdentifierSegment"
+        _parser.TryParse("{{ event.userId }}", out var template, out var _);
+        var options = new TemplateOptions { StrictVariables = true };
+        var context = new TemplateContext(options);
+        
+        // Set event but without userId property
+        context.SetValue("event", new { email = "test@example.com" });
+        
+        var exception = await Assert.ThrowsAsync<FluidException>(() => template.RenderAsync(context).AsTask());
+        
+        // The exception message should contain "userId"
+        Assert.Contains("userId", exception.Message);
+    }
+
+    [Fact]
+    public async Task StrictVariables_ExceptionMessage_WithNestedProperty()
+    {
+        // Test that nested property access also shows the correct property name
+        _parser.TryParse("{{ user.profile.avatar }}", out var template, out var _);
+        var options = new TemplateOptions { StrictVariables = true };
+        var context = new TemplateContext(options);
+        
+        // Set user with profile but without avatar property
+        context.SetValue("user", new { profile = new { name = "John" } });
+        
+        var exception = await Assert.ThrowsAsync<FluidException>(() => template.RenderAsync(context).AsTask());
+        
+        // The exception message should contain "avatar"
+        Assert.Contains("avatar", exception.Message);
+    }
+
     private (TemplateOptions, List<string>) CreateStrictOptions()
     {
         var missingVariables = new List<string>();

Fluid/Ast/FunctionCallSegment.cs

@@ -53,5 +53,11 @@ public override async ValueTask<FluidValue> ResolveAsync(FluidValue value, Templ
 
             return await value.InvokeAsync(arguments, context);
         }
+
+        public override string GetSegmentName()
+        {
+            // For function call segments, return a generic representation
+            return "()";
+        }
     }
 }

Fluid/Ast/IdentifierSegment.cs

@@ -17,5 +17,10 @@ public override ValueTask<FluidValue> ResolveAsync(FluidValue value, TemplateCon
         {
             return value.GetValueAsync(Identifier, context);
         }
+
+        public override string GetSegmentName()
+        {
+            return Identifier;
+        }
     }
 }

Fluid/Ast/IndexerSegment.cs

@@ -16,5 +16,12 @@ public override async ValueTask<FluidValue> ResolveAsync(FluidValue value, Templ
             var index = await Expression.EvaluateAsync(context);
             return await value.GetIndexAsync(index, context);
         }
+
+        public override string GetSegmentName()
+        {
+            // For indexer segments, return a representation like [index]
+            // Since we don't have the evaluated value here, we return a generic representation
+            return "[index]";
+        }
     }
 }

Fluid/Ast/MemberExpression.cs

@@ -81,7 +81,7 @@ public override ValueTask<FluidValue> EvaluateAsync(TemplateContext context)
                 {
                     if (context.Options.StrictVariables)
                     {
-                        throw new FluidException($"Undefined variable '{_segments[i]}'");
+                        throw new FluidException($"Undefined variable '{s.GetSegmentName()}'");
                     }
                     return value;
                 }
@@ -105,6 +105,10 @@ private static async ValueTask<FluidValue> Awaited(
                 // Stop processing as soon as a member returns nothing
                 if (value.IsNil())
                 {
+                    if (context.Options.StrictVariables)
+                    {
+                        throw new FluidException($"Undefined variable '{s.GetSegmentName()}'");
+                    }
                     return value;
                 }
             }

Fluid/Ast/MemberSegment.cs

@@ -8,5 +8,10 @@ public abstract class MemberSegment
         /// Resolves the member of a <see cref="FluidValue"/> instance.
         /// </summary>
         public abstract ValueTask<FluidValue> ResolveAsync(FluidValue value, TemplateContext context);
+
+        /// <summary>
+        /// Gets a string representation of this segment for use in error messages.
+        /// </summary>
+        public abstract string GetSegmentName();
     }
 }