feat: add mapped_type_vars to TypeInfo

mypy/mro.py

@@ -3,7 +3,7 @@
 from typing import Callable
 
 from mypy.nodes import TypeInfo
-from mypy.types import Instance
+from mypy.types import Instance, ProperType, TypeVarLikeType
 from mypy.typestate import type_state
 
 
@@ -15,11 +15,61 @@ def calculate_mro(info: TypeInfo, obj_type: Callable[[], Instance] | None = None
     mro = linearize_hierarchy(info, obj_type)
     assert mro, f"Could not produce a MRO at all for {info}"
     info.mro = mro
+    fill_mapped_type_vars(info)
     # The property of falling back to Any is inherited.
     info.fallback_to_any = any(baseinfo.fallback_to_any for baseinfo in info.mro)
     type_state.reset_all_subtype_caches_for(info)
 
 
+def fill_mapped_type_vars(info: TypeInfo) -> None:
+    """Calculates the final TypeVar value from inheritor to parent.
+
+    class A[T1]:
+        # mapped_type_vars = {T1: str}
+
+    class B[T2]:
+        # mapped_type_vars = {T2: T4}
+
+    class C[T3](B[T3]):
+        # mapped_type_vars = {T3: T4}
+
+    class D[T4](C[T4], A[str]):
+        # mapped_type_vars = {}
+    """
+    bases = {b.type: b for b in info.bases}
+
+    for subinfo in filter(lambda x: x.is_generic, info.mro):
+        if base_info := bases.get(subinfo):
+            subinfo.mapped_type_vars = {
+                tv: actual_type for tv, actual_type in zip(subinfo.defn.type_vars, base_info.args)
+            }
+        info.mapped_type_vars |= subinfo.mapped_type_vars
+
+    final_mapped_type_vars: dict[TypeVarLikeType, ProperType] = {}
+    for k, v in info.mapped_type_vars.items():
+        final_mapped_type_vars[k] = _resolve_mappped_vars(info.mapped_type_vars, v)
+
+    for subinfo in filter(lambda x: x.is_generic, info.mro):
+        _resolve_info_type_vars(subinfo, final_mapped_type_vars)
+
+
+def _resolve_info_type_vars(
+    info: TypeInfo, mapped_type_vars: dict[TypeVarLikeType, ProperType]
+) -> None:
+    final_mapped_type_vars = {}
+    for tv in info.defn.type_vars:
+        final_mapped_type_vars[tv] = _resolve_mappped_vars(mapped_type_vars, tv)
+    info.mapped_type_vars = final_mapped_type_vars
+
+
+def _resolve_mappped_vars(
+    mapped_type_vars: dict[TypeVarLikeType, ProperType], key: ProperType
+) -> ProperType:
+    if key in mapped_type_vars:
+        return _resolve_mappped_vars(mapped_type_vars, mapped_type_vars[key])
+    return key
+
+
 class MroError(Exception):
     """Raised if a consistent mro cannot be determined for a class."""
 

mypy/nodes.py

@@ -2944,6 +2944,7 @@ class is generic then it will be a type constructor of higher kind.
         "fallback_to_any",
         "meta_fallback_to_any",
         "type_vars",
+        "mapped_type_vars",
         "has_param_spec_type",
         "bases",
         "_promote",
@@ -3048,6 +3049,8 @@ class is generic then it will be a type constructor of higher kind.
 
     # Generic type variable names (full names)
     type_vars: list[str]
+    # Map of current class TypeVars and Inheritor specified type to calculate real type in MRO
+    mapped_type_vars: dict[mypy.types.TypeVarLikeType, mypy.types.ProperType]
 
     # Whether this class has a ParamSpec type variable
     has_param_spec_type: bool
@@ -3139,6 +3142,7 @@ def __init__(self, names: SymbolTable, defn: ClassDef, module_name: str) -> None
         self.defn = defn
         self.module_name = module_name
         self.type_vars = []
+        self.mapped_type_vars = {}
         self.has_param_spec_type = False
         self.has_type_var_tuple_type = False
         self.bases = []