Do not block on duplicate base classes (#15367)

Continue type checking if a class has two or more identical bases.
Closes #15349

---------

Co-authored-by: hauntsaninja <hauntsaninja@gmail.com>

mypy/semanal.py

@@ -2189,6 +2189,10 @@ def configure_base_classes(
         if not self.verify_base_classes(defn):
             self.set_dummy_mro(defn.info)
             return
+        if not self.verify_duplicate_base_classes(defn):
+            # We don't want to block the typechecking process,
+            # so, we just insert `Any` as the base class and show an error.
+            self.set_any_mro(defn.info)
         self.calculate_class_mro(defn, self.object_type)
 
     def configure_tuple_base_class(self, defn: ClassDef, base: TupleType) -> Instance:
@@ -2218,6 +2222,11 @@ def set_dummy_mro(self, info: TypeInfo) -> None:
         info.mro = [info, self.object_type().type]
         info.bad_mro = True
 
+    def set_any_mro(self, info: TypeInfo) -> None:
+        # Give it an MRO consisting direct `Any` subclass.
+        info.fallback_to_any = True
+        info.mro = [info, self.object_type().type]
+
     def calculate_class_mro(
         self, defn: ClassDef, obj_type: Callable[[], Instance] | None = None
     ) -> None:
@@ -2298,12 +2307,14 @@ def verify_base_classes(self, defn: ClassDef) -> bool:
             if self.is_base_class(info, baseinfo):
                 self.fail("Cycle in inheritance hierarchy", defn)
                 cycle = True
-        dup = find_duplicate(info.direct_base_classes())
-        if dup:
-            self.fail(f'Duplicate base class "{dup.name}"', defn, blocker=True)
-            return False
         return not cycle
 
+    def verify_duplicate_base_classes(self, defn: ClassDef) -> bool:
+        dup = find_duplicate(defn.info.direct_base_classes())
+        if dup:
+            self.fail(f'Duplicate base class "{dup.name}"', defn)
+        return not dup
+
     def is_base_class(self, t: TypeInfo, s: TypeInfo) -> bool:
         """Determine if t is a base class of s (but do not use mro)."""
         # Search the base class graph for t, starting from s.

test-data/unit/check-classes.test

@@ -4124,12 +4124,31 @@ int.__eq__(3, 4)
 main:33: error: Too few arguments for "__eq__" of "int"
 main:33: error: Unsupported operand types for == ("int" and "Type[int]")
 
-[case testMroSetAfterError]
-class C(str, str):
-    foo = 0
-    bar = foo
-[out]
-main:1: error: Duplicate base class "str"
+[case testDupBaseClasses]
+class A:
+    def method(self) -> str: ...
+
+class B(A, A):  # E: Duplicate base class "A"
+    attr: int
+
+b: B
+
+reveal_type(b.method())  # N: Revealed type is "Any"
+reveal_type(b.missing())  # N: Revealed type is "Any"
+reveal_type(b.attr)  # N: Revealed type is "builtins.int"
+
+[case testDupBaseClassesGeneric]
+from typing import Generic, TypeVar
+
+T = TypeVar('T')
+class A(Generic[T]):
+    def method(self) -> T: ...
+
+class B(A[int], A[str]):  # E: Duplicate base class "A"
+    attr: int
+
+reveal_type(B().method())  # N: Revealed type is "Any"
+reveal_type(B().attr)   # N: Revealed type is "builtins.int"
 
 [case testCannotDetermineMro]
 class A: pass