From 0a2536736b7ae15e836c68d40d754a45f0455250 Mon Sep 17 00:00:00 2001
From: David Peter <mail@david-peter.de>
Date: Tue, 25 Nov 2025 14:54:04 +0100
Subject: [PATCH] Better diagnostic message

---
 .../resources/mdtest/implicit_type_aliases.md       | 11 ++++++++++-
 .../src/types/infer/builder/type_expression.rs      | 13 +++++++------
 2 files changed, 17 insertions(+), 7 deletions(-)

diff --git a/crates/ty_python_semantic/resources/mdtest/implicit_type_aliases.md b/crates/ty_python_semantic/resources/mdtest/implicit_type_aliases.md
index c975b14db2..c2f02be9d5 100644
--- a/crates/ty_python_semantic/resources/mdtest/implicit_type_aliases.md
+++ b/crates/ty_python_semantic/resources/mdtest/implicit_type_aliases.md
@@ -403,7 +403,6 @@ reveal_type(ListOrTuple)  # revealed: types.UnionType
 reveal_type(ListOrTupleLegacy)  # revealed: types.UnionType
 reveal_type(MyCallable)  # revealed: GenericAlias
 reveal_type(AnnotatedType)  # revealed: <typing.Annotated special form>
-# TODO: This should ideally be `T@TransparentAlias`
 reveal_type(TransparentAlias)  # revealed: typing.TypeVar
 reveal_type(MyOptional)  # revealed: types.UnionType
 
@@ -660,6 +659,16 @@ def _(
     reveal_type(specialized)  # revealed: Unknown
 ```
 
+Similarly, if you try to specialize a union type without a binding context, we emit an error:
+
+```py
+# error: [invalid-type-form] "`types.UnionType` is not subscriptable"
+x: (list[T] | set[T])[int]
+
+def _():
+    reveal_type(x)  # revealed: Unknown
+```
+
 ### Multiple definitions
 
 #### Shadowed definitions
diff --git a/crates/ty_python_semantic/src/types/infer/builder/type_expression.rs b/crates/ty_python_semantic/src/types/infer/builder/type_expression.rs
index 3eb26d6574..3256e13dc7 100644
--- a/crates/ty_python_semantic/src/types/infer/builder/type_expression.rs
+++ b/crates/ty_python_semantic/src/types/infer/builder/type_expression.rs
@@ -762,19 +762,20 @@ impl<'db> TypeInferenceBuilder<'db, '_> {
     ) -> Type<'db> {
         let db = self.db();
 
-        let Some(type_alias_definition) = typevar_binding_context else {
-            // TODO
+        let Some(typevar_binding_context) = typevar_binding_context else {
             if let Some(builder) = self.context.report_lint(&INVALID_TYPE_FORM, subscript) {
-                builder.into_diagnostic(
-                    "Cannot specialize implicit type alias with unknown definition",
-                );
+                let mut diag = builder.into_diagnostic(format_args!(
+                    "`{}` is not subscriptable",
+                    value_ty.display(db)
+                ));
+                diag.info("Consider creating a type alias to create a binding context for the type variable(s)");
             }
             return Type::unknown();
         };
 
         let generic_type_alias = value_ty.apply_type_mapping(
             db,
-            &TypeMapping::BindLegacyTypevars(BindingContext::Definition(type_alias_definition)),
+            &TypeMapping::BindLegacyTypevars(BindingContext::Definition(typevar_binding_context)),
             TypeContext::default(),
         );