Fixed stack overflow when requesting completion in an object literal

src/Bicep.LangServer.IntegrationTests/CompletionTests.cs

@@ -5134,5 +5134,33 @@ param firstItem object
 output foo string[] = [for item in items: item.bar|]
 """);
         }
+
+        [TestMethod]
+        // https://github.com/azure/bicep/issues/15569
+        public async Task String_literal_union_with_object_value_should_not_cause_stack_overflow()
+        {
+            var serverHelper = new ServerRequestHelper(TestContext, DefaultServer);
+
+            // single parameter of string literal union type
+            var moduleText = "param foo 'foo' | 'bar'";
+            var moduleFile = await serverHelper.OpenFile("/mod.bicep", moduleText);
+
+            var (text, cursor) = ParserHelper.GetFileWithSingleCursor("""
+targetScope = 'resourceGroup'
+
+module mod 'mod.bicep' = {
+  name: ''
+  params: {
+    foo: {
+      |
+    } 
+  }
+}
+""");
+            var mainFile = await serverHelper.OpenFile("/main.bicep", text);
+
+            var completions = await mainFile.RequestCompletion(cursor);
+            completions.Should().BeEmpty();
+        }
     }
 }

src/Bicep.LangServer/Completions/BicepCompletionProvider.cs

@@ -1279,11 +1279,24 @@ private static IEnumerable<TypeProperty> GetProperties(TypeSymbol? type)
                 TestType testType => GetProperties(testType.Body.Type),
                 ObjectType objectType => objectType.Properties.Values,
                 DiscriminatedObjectType discriminated => discriminated.DiscriminatorProperty.AsEnumerable(),
-                UnionType unionType => GetProperties(TypeHelper.TryCollapseTypes(unionType.Members)),
+                UnionType unionType => GetPropertiesFromUnionType(unionType),
                 _ => [],
             }).Where(p => !p.Flags.HasFlag(TypePropertyFlags.FallbackProperty));
         }
 
+        private static IEnumerable<TypeProperty> GetPropertiesFromUnionType(UnionType unionType)
+        {
+            var potentiallyCollapsedType = TypeHelper.TryCollapseTypes(unionType.Members);
+            if(potentiallyCollapsedType is UnionType)
+            {
+                // type collapsed into a new or same union type (may have collapsed into itself)
+                // get properties from each union members
+                return unionType.Members.SelectMany(member => GetProperties(member.Type));
+            }
+
+            return GetProperties(potentiallyCollapsedType);
+        }
+
         private static TypeSymbol? GetAdditionalPropertiesType(TypeSymbol? type) => type switch
         {
             ResourceType resourceType => GetAdditionalPropertiesType(resourceType.Body.Type),