Allow dot property access on union of objects with undeclared property

src/Bicep.Core.IntegrationTests/ScenarioTests.cs

@@ -6053,4 +6053,93 @@ func genModuleTags(moduleName string) moduleTags => {
             ["name name"] = "name",
         });
     }
+
+    [TestMethod]
+    public void Test_Issue14059_repro1()
+    {
+        // https://github.com/Azure/bicep/issues/14059
+        var result = CompilationHelper.Compile("""
+param foo string
+param index int
+
+var test = [
+  { foo: foo }
+  { }
+][index]
+
+output val string = test.foo
+""");
+
+        result.Should().NotHaveAnyDiagnostics();
+    }
+
+    [TestMethod]
+    public void Test_Issue14059_repro2()
+    {
+        // https://github.com/Azure/bicep/issues/14059
+        var result = CompilationHelper.Compile("""
+var items = [
+  { obj: 1 }
+  { obj: 2 }
+  { obj: 3 }
+  { obj: 4 }
+  { obj: 5, x: 5 }
+]
+var s = sort(
+  filter(
+    map(items, x => x.obj < 2 ? { order: 1, value: x } : x.obj > 4 ? { order: 2, value: x } : {}),
+    x => !(empty(x))
+  ),
+  (arg1, arg2) => arg1.order < arg2.order
+)
+""");
+
+        result.ExcludingLinterDiagnostics().Should().NotHaveAnyDiagnostics();
+    }
+
+    [TestMethod]
+    public void Test_Issue14059_repro3()
+    {
+        // https://github.com/Azure/bicep/issues/14059
+        var result = CompilationHelper.Compile("""
+param isProd bool
+param location string
+
+var appConfigValues = [
+  {
+    Key: 'KEY1'
+    Value: isProd ? 'URL1' : 'URL2'
+  }
+  {
+    Key: 'InvoicingImports:AzureBlobStorageImport:ContainerName'
+    Value: 'ikros'
+  }
+  isProd ? {
+    Key: 'ServiceDiscovery:FunctionApps:OpenApi:Url'
+    Value: 'https://kros-esw-prod-openapi-azfun.azurewebsites.net'
+  } : {}
+]
+
+resource appConfiguration 'Microsoft.AppConfiguration/configurationStores@2022-05-01' = {
+  name: 'settings-config'
+  location: location
+  sku: {
+    name: 'standard'
+  }
+  properties: {
+    softDeleteRetentionInDays: 7
+  }
+}
+
+resource addAppConfigValues 'Microsoft.AppConfiguration/configurationStores/keyValues@2022-05-01' = [for i in range(0, length(appConfigValues)) : {
+  name: !empty(appConfigValues[i]) ? '${appConfigValues[i].Key}' : 'notExisting'
+  parent: appConfiguration
+  properties: {
+    value: appConfigValues[i].Value
+  }
+}]
+""");
+
+        result.ExcludingLinterDiagnostics().Should().NotHaveAnyDiagnostics();
+    }
 }

src/Bicep.Core/TypeSystem/TypeAssignmentVisitor.cs

@@ -1785,6 +1785,40 @@ private TypeSymbol GetAccessedType(AccessExpressionSyntax syntax, IDiagnosticWri
                 : baseType;
         }
 
+        private static TypeSymbol? TryGetReadablePropertyType(ObjectType objectType, string propertyName)
+        {
+            if (objectType.Properties.TryGetValue(propertyName) is {} property && !property.Flags.HasFlag(TypePropertyFlags.WriteOnly))
+            {
+                return property.TypeReference.Type;
+            }
+
+            if (objectType.AdditionalPropertiesType is {} additionalPropertiesType && !objectType.AdditionalPropertiesFlags.HasFlag(TypePropertyFlags.WriteOnly))
+            {
+                return additionalPropertiesType.Type;
+            }
+
+            return null;
+        }
+
+        private static TypeSymbol GetNamedPropertyType(UnionType unionType, string propertyName)
+        {
+            var members = new List<TypeSymbol>();
+            foreach (var member in unionType.Members)
+            {
+                if (member is not ObjectType objectType ||
+                    TryGetReadablePropertyType(objectType, propertyName) is not {} propertyType)
+                {
+                    // fall back to any if we can't definitively obtain the property type.
+                    // this may give some false positives - we can further refine this if desired.
+                    return LanguageConstants.Any;
+                }
+
+                members.Add(propertyType);
+            }
+
+            return TypeHelper.CreateTypeUnion(members);
+        }
+
         private static TypeSymbol GetNamedPropertyType(PropertyAccessSyntax syntax, TypeSymbol baseType, IDiagnosticWriter diagnostics) => UnwrapType(baseType) switch
         {
             ErrorType error => error,
@@ -1802,8 +1836,7 @@ TypeSymbol original when TypeHelper.TryRemoveNullability(original) is TypeSymbol
                 syntax.IsSafeAccess || TypeValidator.ShouldWarnForPropertyMismatch(objectType),
                 diagnostics),
 
-            UnionType unionType when unionType.Members.All(x => x is ObjectType)
-                => TypeHelper.CreateTypeUnion(unionType.Members.Select(member => GetNamedPropertyType(syntax, member.Type, diagnostics))),
+            UnionType unionType when syntax.PropertyName.IsValid => GetNamedPropertyType(unionType, syntax.PropertyName.IdentifierName),
 
             // TODO: We might be able use the declared type here to resolve discriminator to improve the assigned type
             DiscriminatedObjectType => LanguageConstants.Any,