[ty] Add subdiagnostic hint if a variable with type `Never` is used in a type expression (#21660)

crates/ty_python_semantic/resources/mdtest/snapshots/unreachable.md_-_Unreachable_code_-_`Never`-inferred_var…_(6ce5aa6d2a0ce029).snap

@@ -0,0 +1,45 @@
+---
+source: crates/ty_test/src/lib.rs
+expression: snapshot
+---
+---
+mdtest name: unreachable.md - Unreachable code - `Never`-inferred variables in type expressions
+mdtest path: crates/ty_python_semantic/resources/mdtest/unreachable.md
+---
+
+# Python source files
+
+## module.py
+
+```
+1 | import sys
+2 | 
+3 | if sys.version_info >= (3, 14):
+4 |     raise RuntimeError("this library doesn't support 3.14 yet!!!")
+5 | 
+6 | class AwesomeAPI: ...
+```
+
+## main.py
+
+```
+1 | import module
+2 | 
+3 | def f(x: module.AwesomeAPI): ...  # error: [invalid-type-form]
+```
+
+# Diagnostics
+
+```
+error[invalid-type-form]: Variable of type `Never` is not allowed in a type expression
+ --> src/main.py:3:10
+  |
+1 | import module
+2 |
+3 | def f(x: module.AwesomeAPI): ...  # error: [invalid-type-form]
+  |          ^^^^^^^^^^^^^^^^^
+  |
+help: The variable may have been inferred as `Never` because its definition was inferred as being unreachable
+info: rule `invalid-type-form` is enabled by default
+
+```

crates/ty_python_semantic/resources/mdtest/unreachable.md

@@ -581,3 +581,35 @@ if False:
 
     1 / number
 ```
+
+## `Never`-inferred variables in type expressions
+
+We offer a helpful subdiagnostic if a variable in a type expression is inferred as having type
+`Never`, since this almost certainly resulted in the definition of the type being inferred by ty as
+being unreachable:
+
+<!-- snapshot-diagnostics -->
+
+```toml
+[environment]
+python-version = "3.14"
+```
+
+`module.py`:
+
+```py
+import sys
+
+if sys.version_info >= (3, 14):
+    raise RuntimeError("this library doesn't support 3.14 yet!!!")
+
+class AwesomeAPI: ...
+```
+
+`main.py`:
+
+```py
+import module
+
+def f(x: module.AwesomeAPI): ...  # error: [invalid-type-form]
+```

crates/ty_python_semantic/src/types.rs

@@ -8879,11 +8879,15 @@ impl<'db> InvalidTypeExpression<'db> {
     }
 
     fn add_subdiagnostics(self, db: &'db dyn Db, mut diagnostic: LintDiagnosticGuard) {
-        if let InvalidTypeExpression::InvalidType(ty, scope) = self {
-            let Type::ModuleLiteral(module_type) = ty else {
-                return;
-            };
-            let module = module_type.module(db);
+        if let InvalidTypeExpression::InvalidType(Type::Never, _) = self {
+            diagnostic.help(
+                "The variable may have been inferred as `Never` because \
+                its definition was inferred as being unreachable",
+            );
+        } else if let InvalidTypeExpression::InvalidType(ty @ Type::ModuleLiteral(module), scope) =
+            self
+        {
+            let module = module.module(db);
             let Some(module_name_final_part) = module.name(db).components().next_back() else {
                 return;
             };