Fixed stack overflow when requesting completion in an object literal (#…

…15579)

When the user has an object literal value set on a module parameter of
string literal union type, we were recursing infinitely which caused the
stack overflow. We were not correctly collecting the properties from
members of the collapsed union type.

This fixes #15569.
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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),