Prevent crash with Unpack of a fixed tuple in PEP695 type alias (#18451)

Fixes #18309.

Add missing `visit_type_alias_stmt()` implementation to
mixedtraverser.py to visit the alias target directly.

mypy/mixedtraverser.py

@@ -10,7 +10,9 @@
     NamedTupleExpr,
     NewTypeExpr,
     PromoteExpr,
+    TypeAlias,
     TypeAliasExpr,
+    TypeAliasStmt,
     TypeApplication,
     TypedDictExpr,
     TypeVarExpr,
@@ -48,9 +50,7 @@ def visit_class_def(self, o: ClassDef, /) -> None:
 
     def visit_type_alias_expr(self, o: TypeAliasExpr, /) -> None:
         super().visit_type_alias_expr(o)
-        self.in_type_alias_expr = True
-        o.node.target.accept(self)
-        self.in_type_alias_expr = False
+        o.node.accept(self)
 
     def visit_type_var_expr(self, o: TypeVarExpr, /) -> None:
         super().visit_type_var_expr(o)
@@ -81,6 +81,17 @@ def visit_assignment_stmt(self, o: AssignmentStmt, /) -> None:
         super().visit_assignment_stmt(o)
         self.visit_optional_type(o.type)
 
+    def visit_type_alias_stmt(self, o: TypeAliasStmt, /) -> None:
+        super().visit_type_alias_stmt(o)
+        if o.alias_node is not None:
+            o.alias_node.accept(self)
+
+    def visit_type_alias(self, o: TypeAlias, /) -> None:
+        super().visit_type_alias(o)
+        self.in_type_alias_expr = True
+        o.target.accept(self)
+        self.in_type_alias_expr = False
+
     def visit_for_stmt(self, o: ForStmt, /) -> None:
         super().visit_for_stmt(o)
         self.visit_optional_type(o.index_type)

test-data/unit/check-python312.test

@@ -1972,3 +1972,19 @@ class D:
             class G[Q]:
                 def g(self, x: Q): ...
             d: G[str]
+
+[case testTypeAliasNormalization]
+from collections.abc import Callable
+from typing import Unpack
+from typing_extensions import TypeAlias
+
+type RK_function_args = tuple[float, int]
+type RK_functionBIS = Callable[[Unpack[RK_function_args], int], int]
+
+def ff(a: float, b: int, c: int) -> int:
+    return 2
+
+bis: RK_functionBIS = ff
+res: int = bis(1.0, 2, 3)
+[builtins fixtures/tuple.pyi]
+[typing fixtures/typing-full.pyi]