Fix for issue 14059

src/Bicep.Core.IntegrationTests/ScenarioTests.cs

@@ -6053,4 +6053,23 @@ func genModuleTags(moduleName string) moduleTags => {
             ["name name"] = "name",
         });
     }
+
+    // https://github.com/Azure/bicep/issues/14059
+    [TestMethod]
+    public void Test_Issue14059()
+    {
+        var result = CompilationHelper.Compile("""
+param foo string
+param index int
+
+var test = [
+  { foo: foo }
+  { }
+][index]
+
+output val string = test.foo
+""");
+
+        result.Should().NotHaveAnyDiagnostics();
+    }
 }

src/Bicep.Core/TypeSystem/TypeAssignmentVisitor.cs

@@ -1785,36 +1785,72 @@ private TypeSymbol GetAccessedType(AccessExpressionSyntax syntax, IDiagnosticWri
                 : baseType;
         }
 
-        private static TypeSymbol GetNamedPropertyType(PropertyAccessSyntax syntax, TypeSymbol baseType, IDiagnosticWriter diagnostics) => UnwrapType(baseType) switch
+        private static TypeSymbol? TryGetReadablePropertyType(ObjectType objectType, string propertyName)
         {
-            ErrorType error => error,
-            TypeSymbol withErrors when withErrors.GetDiagnostics().Any() => ErrorType.Create(withErrors.GetDiagnostics()),
-
-            TypeSymbol original when TypeHelper.TryRemoveNullability(original) is TypeSymbol nonNullable => EmitNullablePropertyAccessDiagnosticAndEraseNullability(syntax, original, nonNullable, diagnostics),
-
-            // the property is not valid
-            // there's already a parse error for it, so we don't need to add a type error as well
-            ObjectType when !syntax.PropertyName.IsValid => ErrorType.Empty(),
+            if (objectType.Properties.TryGetValue(propertyName) is {} property && !property.Flags.HasFlag(TypePropertyFlags.WriteOnly))
+            {
+                return property.TypeReference.Type;
+            }
 
-            ObjectType objectType => TypeHelper.GetNamedPropertyType(objectType,
-                syntax.PropertyName,
-                syntax.PropertyName.IdentifierName,
-                syntax.IsSafeAccess || TypeValidator.ShouldWarnForPropertyMismatch(objectType),
-                diagnostics),
+            if (objectType.AdditionalPropertiesType is {} additionalPropertiesType && !objectType.AdditionalPropertiesFlags.HasFlag(TypePropertyFlags.WriteOnly))
+            {
+                return additionalPropertiesType.Type;
+            }
 
-            UnionType unionType when unionType.Members.All(x => x is ObjectType)
-                => TypeHelper.CreateTypeUnion(unionType.Members.Select(member => GetNamedPropertyType(syntax, member.Type, diagnostics))),
+            // TODO could consider returning the null type here, but need to be careful about downstream impact.
+            // See https://github.com/Azure/bicep/issues/14059 for example
+            return null;
+        }
 
-            // TODO: We might be able use the declared type here to resolve discriminator to improve the assigned type
-            DiscriminatedObjectType => LanguageConstants.Any,
+        private static TypeSymbol GetNamedPropertyType(PropertyAccessSyntax syntax, TypeSymbol baseType, IDiagnosticWriter diagnostics)
+        {
+            var unwrapped = UnwrapType(baseType);
+            switch (unwrapped)
+            {
+                case ErrorType error:
+                    return error;
+                case TypeSymbol withErrors when withErrors.GetDiagnostics().Any():
+                    return ErrorType.Create(withErrors.GetDiagnostics());
+                case TypeSymbol original when TypeHelper.TryRemoveNullability(original) is TypeSymbol nonNullable:
+                    return EmitNullablePropertyAccessDiagnosticAndEraseNullability(syntax, original, nonNullable, diagnostics);
+                case ObjectType when !syntax.PropertyName.IsValid:
+                    // the property is not valid
+                    // there's already a parse error for it, so we don't need to add a type error as well
+                    return ErrorType.Empty();
+                case ObjectType objectType:
+                    return TypeHelper.GetNamedPropertyType(objectType,
+                        syntax.PropertyName,
+                        syntax.PropertyName.IdentifierName,
+                        syntax.IsSafeAccess || TypeValidator.ShouldWarnForPropertyMismatch(objectType),
+                        diagnostics);
+                case UnionType unionType when syntax.PropertyName.IsValid:
+                    var propertyName = syntax.PropertyName.IdentifierName;
+                    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;
+                        }
 
-            // We can assign to an object, but we don't have a type for that object.
-            // The best we can do is allow it and return the 'any' type.
-            TypeSymbol maybeObject when TypeValidator.AreTypesAssignable(maybeObject, LanguageConstants.Object) => LanguageConstants.Any,
+                        members.Add(propertyType);
+                    }
 
-            // can only access properties of objects
-            TypeSymbol otherwise => ErrorType.Create(DiagnosticBuilder.ForPosition(syntax.PropertyName).ObjectRequiredForPropertyAccess(otherwise)),
-        };
+                    return TypeHelper.CreateTypeUnion(members);
+                case DiscriminatedObjectType:
+                    // TODO: We might be able use the declared type here to resolve discriminator to improve the assigned type
+                    return LanguageConstants.Any;
+                case TypeSymbol maybeObject when TypeValidator.AreTypesAssignable(maybeObject, LanguageConstants.Object):
+                    // We can assign to an object, but we don't have a type for that object.
+                    // The best we can do is allow it and return the 'any' type.
+                    return LanguageConstants.Any;
+                default:
+                    return ErrorType.Create(DiagnosticBuilder.ForPosition(syntax.PropertyName).ObjectRequiredForPropertyAccess(unwrapped));
+            }
+        }
 
         private static TypeSymbol EmitNullablePropertyAccessDiagnosticAndEraseNullability(PropertyAccessSyntax syntax, TypeSymbol originalBaseType, TypeSymbol nonNullableBaseType, IDiagnosticWriter diagnostics)
         {