Improve error message for bound typevar in TypeAliasType (#17053)

Follow up to #17038

When a type variable is bound to a class, it cannot be reused in a type
alias. Previously in `TypeAliasType`, this error was reported as "not
included in type_params". However in the following example, the error is
misleading:
```python
from typing import Dict, Generic, TypeVar
from typing_extensions import TypeAliasType

T = TypeVar("T")


class A(Generic[T]):
    Ta11 = TypeAliasType("Ta11", Dict[str, T], type_params=(T,))


x: A.Ta11 = {"a": 1}
reveal_type(x)
```

On the master branch:
```
main.py:8: error: Type variable "T" is not included in type_params  [valid-type]
main.py:8: error: "T" is a type variable and only valid in type context  [misc]
main.py:8: error: Free type variable expected in type_params argument to TypeAliasType  [type-var]
main.py:12: note: Revealed type is "builtins.dict[builtins.str, Any]"
Found 3 errors in 1 file (checked 1 source file)
```

With this PR:
```
typealiastype.py:8: error: Can't use bound type variable "T" to define generic alias  [valid-type]
typealiastype.py:8: error: "T" is a type variable and only valid in type context  [misc]
typealiastype.py:12: note: Revealed type is "builtins.dict[builtins.str, Any]"
Found 2 errors in 1 file (checked 1 source file)
```

This is possible by storing the names of all the declared type_params,
even those that are invalid, and checking if the offending type
variables are in the list.

mypy/semanal.py

@@ -3521,6 +3521,7 @@ def analyze_alias(
         rvalue: Expression,
         allow_placeholder: bool = False,
         declared_type_vars: TypeVarLikeList | None = None,
+        all_declared_type_params_names: list[str] | None = None,
     ) -> tuple[Type | None, list[TypeVarLikeType], set[str], list[str], bool]:
         """Check if 'rvalue' is a valid type allowed for aliasing (e.g. not a type variable).
 
@@ -3573,7 +3574,7 @@ def analyze_alias(
                 in_dynamic_func=dynamic,
                 global_scope=global_scope,
                 allowed_alias_tvars=tvar_defs,
-                has_type_params=declared_type_vars is not None,
+                alias_type_params_names=all_declared_type_params_names,
             )
 
         # There can be only one variadic variable at most, the error is reported elsewhere.
@@ -3622,14 +3623,16 @@ def check_and_set_up_type_alias(self, s: AssignmentStmt) -> bool:
         # It can be `A = TypeAliasType('A', ...)` call, in this case,
         # we just take the second argument and analyze it:
         type_params: TypeVarLikeList | None
+        all_type_params_names: list[str] | None
         if self.check_type_alias_type_call(s.rvalue, name=lvalue.name):
             rvalue = s.rvalue.args[1]
             pep_695 = True
-            type_params = self.analyze_type_alias_type_params(s.rvalue)
+            type_params, all_type_params_names = self.analyze_type_alias_type_params(s.rvalue)
         else:
             rvalue = s.rvalue
             pep_695 = False
             type_params = None
+            all_type_params_names = None
 
         if isinstance(rvalue, CallExpr) and rvalue.analyzed:
             return False
@@ -3686,7 +3689,11 @@ def check_and_set_up_type_alias(self, s: AssignmentStmt) -> bool:
         else:
             tag = self.track_incomplete_refs()
             res, alias_tvars, depends_on, qualified_tvars, empty_tuple_index = self.analyze_alias(
-                lvalue.name, rvalue, allow_placeholder=True, declared_type_vars=type_params
+                lvalue.name,
+                rvalue,
+                allow_placeholder=True,
+                declared_type_vars=type_params,
+                all_declared_type_params_names=all_type_params_names,
             )
             if not res:
                 return False
@@ -3803,20 +3810,28 @@ def check_type_alias_type_call(self, rvalue: Expression, *, name: str) -> TypeGu
 
         return self.check_typevarlike_name(rvalue, name, rvalue)
 
-    def analyze_type_alias_type_params(self, rvalue: CallExpr) -> TypeVarLikeList:
+    def analyze_type_alias_type_params(
+        self, rvalue: CallExpr
+    ) -> tuple[TypeVarLikeList, list[str]]:
+        """Analyze type_params of TypeAliasType.
+
+        Returns declared unbound type variable expressions and a list of all decalred type
+        variable names for error reporting.
+        """
         if "type_params" in rvalue.arg_names:
             type_params_arg = rvalue.args[rvalue.arg_names.index("type_params")]
             if not isinstance(type_params_arg, TupleExpr):
                 self.fail(
                     "Tuple literal expected as the type_params argument to TypeAliasType",
                     type_params_arg,
                 )
-                return []
+                return [], []
             type_params = type_params_arg.items
         else:
-            type_params = []
+            return [], []
 
         declared_tvars: TypeVarLikeList = []
+        all_declared_tvar_names: list[str] = []  # includes bound type variables
         have_type_var_tuple = False
         for tp_expr in type_params:
             if isinstance(tp_expr, StarExpr):
@@ -3843,16 +3858,19 @@ def analyze_type_alias_type_params(self, rvalue: CallExpr) -> TypeVarLikeList:
                         continue
                     have_type_var_tuple = True
             elif not self.found_incomplete_ref(tag):
-                self.fail(
-                    "Free type variable expected in type_params argument to TypeAliasType",
-                    base,
-                    code=codes.TYPE_VAR,
-                )
                 sym = self.lookup_qualified(base.name, base)
-                if sym and sym.fullname in ("typing.Unpack", "typing_extensions.Unpack"):
-                    self.note(
-                        "Don't Unpack type variables in type_params", base, code=codes.TYPE_VAR
+                if sym and isinstance(sym.node, TypeVarLikeExpr):
+                    all_declared_tvar_names.append(sym.node.name)  # Error will be reported later
+                else:
+                    self.fail(
+                        "Free type variable expected in type_params argument to TypeAliasType",
+                        base,
+                        code=codes.TYPE_VAR,
                     )
+                    if sym and sym.fullname in ("typing.Unpack", "typing_extensions.Unpack"):
+                        self.note(
+                            "Don't Unpack type variables in type_params", base, code=codes.TYPE_VAR
+                        )
                 continue
             if tvar in declared_tvars:
                 self.fail(
@@ -3862,8 +3880,9 @@ def analyze_type_alias_type_params(self, rvalue: CallExpr) -> TypeVarLikeList:
                 )
                 continue
             if tvar:
+                all_declared_tvar_names.append(tvar[0])
                 declared_tvars.append(tvar)
-        return declared_tvars
+        return declared_tvars, all_declared_tvar_names
 
     def disable_invalid_recursive_aliases(
         self, s: AssignmentStmt, current_node: TypeAlias

mypy/typeanal.py

@@ -141,7 +141,7 @@ def analyze_type_alias(
     in_dynamic_func: bool = False,
     global_scope: bool = True,
     allowed_alias_tvars: list[TypeVarLikeType] | None = None,
-    has_type_params: bool = False,
+    alias_type_params_names: list[str] | None = None,
 ) -> tuple[Type, set[str]]:
     """Analyze r.h.s. of a (potential) type alias definition.
 
@@ -159,7 +159,7 @@ def analyze_type_alias(
         allow_placeholder=allow_placeholder,
         prohibit_self_type="type alias target",
         allowed_alias_tvars=allowed_alias_tvars,
-        has_type_params=has_type_params,
+        alias_type_params_names=alias_type_params_names,
     )
     analyzer.in_dynamic_func = in_dynamic_func
     analyzer.global_scope = global_scope
@@ -212,7 +212,7 @@ def __init__(
         prohibit_self_type: str | None = None,
         allowed_alias_tvars: list[TypeVarLikeType] | None = None,
         allow_type_any: bool = False,
-        has_type_params: bool = False,
+        alias_type_params_names: list[str] | None = None,
     ) -> None:
         self.api = api
         self.fail_func = api.fail
@@ -234,7 +234,7 @@ def __init__(
         if allowed_alias_tvars is None:
             allowed_alias_tvars = []
         self.allowed_alias_tvars = allowed_alias_tvars
-        self.has_type_params = has_type_params
+        self.alias_type_params_names = alias_type_params_names
         # If false, record incomplete ref if we generate PlaceholderType.
         self.allow_placeholder = allow_placeholder
         # Are we in a context where Required[] is allowed?
@@ -275,6 +275,12 @@ def visit_unbound_type(self, t: UnboundType, defining_literal: bool = False) ->
             return make_optional_type(typ)
         return typ
 
+    def not_declared_in_type_params(self, tvar_name: str) -> bool:
+        return (
+            self.alias_type_params_names is not None
+            and tvar_name not in self.alias_type_params_names
+        )
+
     def visit_unbound_type_nonoptional(self, t: UnboundType, defining_literal: bool) -> Type:
         sym = self.lookup_qualified(t.name, t)
         if sym is not None:
@@ -329,7 +335,7 @@ def visit_unbound_type_nonoptional(self, t: UnboundType, defining_literal: bool)
                 if tvar_def is None:
                     if self.allow_unbound_tvars:
                         return t
-                    if self.defining_alias and self.has_type_params:
+                    if self.defining_alias and self.not_declared_in_type_params(t.name):
                         msg = f'ParamSpec "{t.name}" is not included in type_params'
                     else:
                         msg = f'ParamSpec "{t.name}" is unbound'
@@ -357,7 +363,7 @@ def visit_unbound_type_nonoptional(self, t: UnboundType, defining_literal: bool)
                 and not defining_literal
                 and (tvar_def is None or tvar_def not in self.allowed_alias_tvars)
             ):
-                if self.has_type_params:
+                if self.not_declared_in_type_params(t.name):
                     msg = f'Type variable "{t.name}" is not included in type_params'
                 else:
                     msg = f'Can\'t use bound type variable "{t.name}" to define generic alias'
@@ -376,7 +382,7 @@ def visit_unbound_type_nonoptional(self, t: UnboundType, defining_literal: bool)
                 and self.defining_alias
                 and tvar_def not in self.allowed_alias_tvars
             ):
-                if self.has_type_params:
+                if self.not_declared_in_type_params(t.name):
                     msg = f'Type variable "{t.name}" is not included in type_params'
                 else:
                     msg = f'Can\'t use bound type variable "{t.name}" to define generic alias'
@@ -386,7 +392,7 @@ def visit_unbound_type_nonoptional(self, t: UnboundType, defining_literal: bool)
                 if tvar_def is None:
                     if self.allow_unbound_tvars:
                         return t
-                    if self.defining_alias and self.has_type_params:
+                    if self.defining_alias and self.not_declared_in_type_params(t.name):
                         msg = f'TypeVarTuple "{t.name}" is not included in type_params'
                     else:
                         msg = f'TypeVarTuple "{t.name}" is unbound'
@@ -1281,11 +1287,11 @@ def analyze_callable_args_for_paramspec(
             return None
         elif (
             self.defining_alias
-            and self.has_type_params
+            and self.not_declared_in_type_params(tvar_def.name)
             and tvar_def not in self.allowed_alias_tvars
         ):
             self.fail(
-                f'ParamSpec "{callable_args.name}" is not included in type_params',
+                f'ParamSpec "{tvar_def.name}" is not included in type_params',
                 callable_args,
                 code=codes.VALID_TYPE,
             )

test-data/unit/check-type-aliases.test

@@ -1195,6 +1195,11 @@ reveal_type(unbound_ps_alias3)  # N: Revealed type is "def [P] (*Any, **Any) ->
 #unbound_tvt_alias2: Ta10[int]
 #reveal_type(unbound_tvt_alias2)
 
+class A(Generic[T]):
+    Ta11 = TypeAliasType("Ta11", Dict[str, T], type_params=(T,))  # E: Can't use bound type variable "T" to define generic alias \
+                                                                  # E: "T" is a type variable and only valid in type context
+x: A.Ta11 = {"a": 1}
+reveal_type(x)  # N: Revealed type is "builtins.dict[builtins.str, Any]"
 [builtins fixtures/dict.pyi]
 
 [case testTypeAliasTypeNoUnpackInTypeParams311]