fix: @partial-block usable in #if and as block partial (issue #519)

source/Handlebars.Test/IssueTests.cs

@@ -734,6 +734,23 @@ public void UnrecognisedExpressionThrowsOutOfMemoryException()
             Assert.Throws<HandlebarsCompilerException>(()=> Handlebars.Compile(source));
         }
 
+        // Issue: https://github.com/Handlebars-Net/Handlebars.Net/issues/519
+        [Fact]
+        public void Issue519_PartialBlockUsableAsBlockAndInIf()
+        {
+            var handlebars = Handlebars.Create();
+            handlebars.RegisterTemplate("myPartial",
+                @"Conditional:{{#if @partial-block}} {{> @partial-block}}{{/if}}
+Plain: {{> @partial-block}}
+Block:{{#> @partial-block }}{{/@partial-block}}");
+
+            var render = handlebars.Compile("{{#> myPartial}}Block content{{/myPartial}}");
+            var actual = render(new { });
+            Assert.Contains("Conditional: Block content", actual);
+            Assert.Contains("Plain: Block content", actual);
+            Assert.Contains("Block:Block content", actual);
+        }
+
         // Issue: https://github.com/Handlebars-Net/Handlebars.Net/issues/458
         // System.NotImplementedException: byref delegate on Xamarin.iOS / Mono
         // The Mono runtime does not support delegates with byref (in/ref) parameters.

source/Handlebars/BindingContext.cs

@@ -156,7 +156,20 @@ internal bool TryGetContextVariable(ChainSegment segment, out object value)
                 return BlockParamsObject.TryGetValue(segment, out value)
                    || ContextDataObject.TryGetValue(wellKnownVariable, out value);
             }
-
+
+            // Special handling for @partial-block: return the PartialBlockTemplate delegate
+            // so that {{#if @partial-block}} is truthy when a block partial is provided.
+            if (segment.LowerInvariant == "partial-block")
+            {
+                if (PartialBlockTemplate != null)
+                {
+                    value = PartialBlockTemplate;
+                    return true;
+                }
+                value = UndefinedBindingResult.Create(segment.TrimmedValue);
+                return false;
+            }
+
             return BlockParamsObject.TryGetValue(segment, out value)
                    || ContextDataObject.TryGetValue(segment, out value);
         }

source/Handlebars/Compiler/Lexer/Converter/BlockAccumulators/PartialBlockAccumulatorContext.cs

@@ -24,10 +24,16 @@
 
         public override Expression GetAccumulatedBlock()
         {
+            Expression fallback = _body.Count == 0
+                ? null
+                : _body.Count == 1
+                    ? _body.First()
+                    : Expression.Block(_body);
+
             return HandlebarsExpression.Partial(
                 _startingNode.PartialName,
                 _startingNode.Argument,
-                _body.Count > 1 ? Expression.Block(_body) : _body.First());
+                fallback);
         }
 
         public override bool IsClosingElement(Expression item)