diff --git a/mypyc/ir/func_ir.py b/mypyc/ir/func_ir.py
index 1b82be278df6..dc83de24300a 100644
--- a/mypyc/ir/func_ir.py
+++ b/mypyc/ir/func_ir.py
@@ -70,6 +70,8 @@ class FuncSignature:
     def __init__(self, args: Sequence[RuntimeArg], ret_type: RType) -> None:
         self.args = tuple(args)
         self.ret_type = ret_type
+        # Bitmap arguments are use to mark default values for arguments that
+        # have types with overlapping error values.
         self.num_bitmap_args = num_bitmap_args(self.args)
         if self.num_bitmap_args:
             extra = [
@@ -78,6 +80,12 @@ def __init__(self, args: Sequence[RuntimeArg], ret_type: RType) -> None:
             ]
             self.args = self.args + tuple(reversed(extra))
 
+    def real_args(self) -> tuple[RuntimeArg, ...]:
+        """Return arguments without any synthetic bitmap arguments."""
+        if self.num_bitmap_args:
+            return self.args[: -self.num_bitmap_args]
+        return self.args
+
     def bound_sig(self) -> "FuncSignature":
         if self.num_bitmap_args:
             return FuncSignature(self.args[1 : -self.num_bitmap_args], self.ret_type)
diff --git a/mypyc/irbuild/function.py b/mypyc/irbuild/function.py
index ea8d86ff0468..237088791bc9 100644
--- a/mypyc/irbuild/function.py
+++ b/mypyc/irbuild/function.py
@@ -89,7 +89,7 @@
 from mypyc.primitives.generic_ops import py_setattr_op
 from mypyc.primitives.misc_ops import register_function
 from mypyc.primitives.registry import builtin_names
-from mypyc.sametype import is_same_method_signature
+from mypyc.sametype import is_same_method_signature, is_same_type
 
 # Top-level transform functions
 
@@ -548,7 +548,7 @@ def is_decorated(builder: IRBuilder, fdef: FuncDef) -> bool:
 
 def gen_glue(
     builder: IRBuilder,
-    sig: FuncSignature,
+    base_sig: FuncSignature,
     target: FuncIR,
     cls: ClassIR,
     base: ClassIR,
@@ -566,9 +566,9 @@ def gen_glue(
     "shadow" glue methods that work with interpreted subclasses.
     """
     if fdef.is_property:
-        return gen_glue_property(builder, sig, target, cls, base, fdef.line, do_py_ops)
+        return gen_glue_property(builder, base_sig, target, cls, base, fdef.line, do_py_ops)
     else:
-        return gen_glue_method(builder, sig, target, cls, base, fdef.line, do_py_ops)
+        return gen_glue_method(builder, base_sig, target, cls, base, fdef.line, do_py_ops)
 
 
 class ArgInfo(NamedTuple):
@@ -594,7 +594,7 @@ def get_args(builder: IRBuilder, rt_args: Sequence[RuntimeArg], line: int) -> Ar
 
 def gen_glue_method(
     builder: IRBuilder,
-    sig: FuncSignature,
+    base_sig: FuncSignature,
     target: FuncIR,
     cls: ClassIR,
     base: ClassIR,
@@ -626,16 +626,25 @@ def f(builder: IRBuilder, x: object) -> int: ...
     If do_pycall is True, then make the call using the C API
     instead of a native call.
     """
+    check_native_override(builder, base_sig, target.decl.sig, line)
+
     builder.enter()
-    builder.ret_types[-1] = sig.ret_type
+    builder.ret_types[-1] = base_sig.ret_type
 
-    rt_args = list(sig.args)
+    rt_args = list(base_sig.args)
     if target.decl.kind == FUNC_NORMAL:
-        rt_args[0] = RuntimeArg(sig.args[0].name, RInstance(cls))
+        rt_args[0] = RuntimeArg(base_sig.args[0].name, RInstance(cls))
 
     arg_info = get_args(builder, rt_args, line)
     args, arg_kinds, arg_names = arg_info.args, arg_info.arg_kinds, arg_info.arg_names
 
+    bitmap_args = None
+    if base_sig.num_bitmap_args:
+        args = args[: -base_sig.num_bitmap_args]
+        arg_kinds = arg_kinds[: -base_sig.num_bitmap_args]
+        arg_names = arg_names[: -base_sig.num_bitmap_args]
+        bitmap_args = builder.builder.args[-base_sig.num_bitmap_args :]
+
     # We can do a passthrough *args/**kwargs with a native call, but if the
     # args need to get distributed out to arguments, we just let python handle it
     if any(kind.is_star() for kind in arg_kinds) and any(
@@ -655,11 +664,15 @@ def f(builder: IRBuilder, x: object) -> int: ...
             first, target.name, args[st:], line, arg_kinds[st:], arg_names[st:]
         )
     else:
-        retval = builder.builder.call(target.decl, args, arg_kinds, arg_names, line)
-    retval = builder.coerce(retval, sig.ret_type, line)
+        retval = builder.builder.call(
+            target.decl, args, arg_kinds, arg_names, line, bitmap_args=bitmap_args
+        )
+    retval = builder.coerce(retval, base_sig.ret_type, line)
     builder.add(Return(retval))
 
     arg_regs, _, blocks, ret_type, _ = builder.leave()
+    if base_sig.num_bitmap_args:
+        rt_args = rt_args[: -base_sig.num_bitmap_args]
     return FuncIR(
         FuncDecl(
             target.name + "__" + base.name + "_glue",
@@ -673,6 +686,35 @@ def f(builder: IRBuilder, x: object) -> int: ...
     )
 
 
+def check_native_override(
+    builder: IRBuilder, base_sig: FuncSignature, sub_sig: FuncSignature, line: int
+) -> None:
+    """Report an error if an override changes signature in unsupported ways.
+
+    Glue methods can work around many signature changes but not all of them.
+    """
+    for base_arg, sub_arg in zip(base_sig.real_args(), sub_sig.real_args()):
+        if base_arg.type.error_overlap:
+            if not base_arg.optional and sub_arg.optional and base_sig.num_bitmap_args:
+                # This would change the meanings of bits in the argument defaults
+                # bitmap, which we don't support. We'd need to do tricky bit
+                # manipulations to support this generally.
+                builder.error(
+                    "An argument with type "
+                    + f'"{base_arg.type}" cannot be given a default value in a method override',
+                    line,
+                )
+        if base_arg.type.error_overlap or sub_arg.type.error_overlap:
+            if not is_same_type(base_arg.type, sub_arg.type):
+                # This would change from signaling a default via an error value to
+                # signaling a default via bitmap, which we don't support.
+                builder.error(
+                    "Incompatible argument type "
+                    + f'"{sub_arg.type}" (base class has type "{base_arg.type}")',
+                    line,
+                )
+
+
 def gen_glue_property(
     builder: IRBuilder,
     sig: FuncSignature,
diff --git a/mypyc/irbuild/ll_builder.py b/mypyc/irbuild/ll_builder.py
index 25561382fdec..fe0af5b13a73 100644
--- a/mypyc/irbuild/ll_builder.py
+++ b/mypyc/irbuild/ll_builder.py
@@ -935,10 +935,19 @@ def call(
         arg_kinds: list[ArgKind],
         arg_names: Sequence[str | None],
         line: int,
+        *,
+        bitmap_args: list[Register] | None = None,
     ) -> Value:
-        """Call a native function."""
+        """Call a native function.
+
+        If bitmap_args is given, they override the values of (some) of the bitmap
+        arguments used to track the presence of values for certain arguments. By
+        default, the values of the bitmap arguments are inferred from args.
+        """
         # Normalize args to positionals.
-        args = self.native_args_to_positional(args, arg_kinds, arg_names, decl.sig, line)
+        args = self.native_args_to_positional(
+            args, arg_kinds, arg_names, decl.sig, line, bitmap_args=bitmap_args
+        )
         return self.add(Call(decl, args, line))
 
     def native_args_to_positional(
@@ -948,6 +957,8 @@ def native_args_to_positional(
         arg_names: Sequence[str | None],
         sig: FuncSignature,
         line: int,
+        *,
+        bitmap_args: list[Register] | None = None,
     ) -> list[Value]:
         """Prepare arguments for a native call.
 
@@ -1015,6 +1026,11 @@ def native_args_to_positional(
             output_args.append(output_arg)
 
         for i in reversed(range(n)):
+            if bitmap_args and i < len(bitmap_args):
+                # Use override provided by caller
+                output_args.append(bitmap_args[i])
+                continue
+            # Infer values of bitmap args
             bitmap = 0
             c = 0
             for lst, arg in zip(formal_to_actual, sig_args):
diff --git a/mypyc/sametype.py b/mypyc/sametype.py
index a3cfd5c08059..056ed683e5b8 100644
--- a/mypyc/sametype.py
+++ b/mypyc/sametype.py
@@ -35,7 +35,7 @@ def is_same_method_signature(a: FuncSignature, b: FuncSignature) -> bool:
         len(a.args) == len(b.args)
         and is_same_type(a.ret_type, b.ret_type)
         and all(
-            is_same_type(t1.type, t2.type) and t1.name == t2.name
+            is_same_type(t1.type, t2.type) and t1.name == t2.name and t1.optional == t2.optional
             for t1, t2 in zip(a.args[1:], b.args[1:])
         )
     )
diff --git a/mypyc/test-data/irbuild-glue-methods.test b/mypyc/test-data/irbuild-glue-methods.test
index 031b7082b787..6d749bf5dd84 100644
--- a/mypyc/test-data/irbuild-glue-methods.test
+++ b/mypyc/test-data/irbuild-glue-methods.test
@@ -327,3 +327,111 @@ L0:
     r0 = __mypyc_self__.boxed
     r1 = box(int, r0)
     return r1
+
+[case testI64GlueWithExtraDefaultArg]
+from mypy_extensions import i64
+
+class C:
+    def f(self) -> None: pass
+
+class D(C):
+    def f(self, x: i64 = 44) -> None: pass
+[out]
+def C.f(self):
+    self :: __main__.C
+L0:
+    return 1
+def D.f(self, x, __bitmap):
+    self :: __main__.D
+    x :: int64
+    __bitmap, r0 :: uint32
+    r1 :: bit
+L0:
+    r0 = __bitmap & 1
+    r1 = r0 == 0
+    if r1 goto L1 else goto L2 :: bool
+L1:
+    x = 44
+L2:
+    return 1
+def D.f__C_glue(self):
+    self :: __main__.D
+    r0 :: None
+L0:
+    r0 = D.f(self, 0, 0)
+    return r0
+
+[case testI64GlueWithSecondDefaultArg]
+from mypy_extensions import i64
+
+class C:
+    def f(self, x: i64 = 11) -> None: pass
+class D(C):
+    def f(self, x: i64 = 12, y: i64 = 13) -> None: pass
+[out]
+def C.f(self, x, __bitmap):
+    self :: __main__.C
+    x :: int64
+    __bitmap, r0 :: uint32
+    r1 :: bit
+L0:
+    r0 = __bitmap & 1
+    r1 = r0 == 0
+    if r1 goto L1 else goto L2 :: bool
+L1:
+    x = 11
+L2:
+    return 1
+def D.f(self, x, y, __bitmap):
+    self :: __main__.D
+    x, y :: int64
+    __bitmap, r0 :: uint32
+    r1 :: bit
+    r2 :: uint32
+    r3 :: bit
+L0:
+    r0 = __bitmap & 1
+    r1 = r0 == 0
+    if r1 goto L1 else goto L2 :: bool
+L1:
+    x = 12
+L2:
+    r2 = __bitmap & 2
+    r3 = r2 == 0
+    if r3 goto L3 else goto L4 :: bool
+L3:
+    y = 13
+L4:
+    return 1
+def D.f__C_glue(self, x, __bitmap):
+    self :: __main__.D
+    x :: int64
+    __bitmap :: uint32
+    r0 :: None
+L0:
+    r0 = D.f(self, x, 0, __bitmap)
+    return r0
+
+[case testI64GlueWithInvalidOverride]
+from mypy_extensions import i64
+
+class C:
+    def f(self, x: i64, y: i64 = 5) -> None: pass
+    def ff(self, x: int) -> None: pass
+class CC(C):
+    def f(self, x: i64 = 12, y: i64 = 5) -> None: pass  # Line 7
+    def ff(self, x: int = 12) -> None: pass
+
+class D:
+    def f(self, x: int) -> None: pass
+class DD(D):
+    def f(self, x: i64) -> None: pass  # Line 13
+
+class E:
+    def f(self, x: i64) -> None: pass
+class EE(E):
+    def f(self, x: int) -> None: pass  # Line 18
+[out]
+main:7: error: An argument with type "int64" cannot be given a default value in a method override
+main:13: error: Incompatible argument type "int64" (base class has type "int")
+main:18: error: Incompatible argument type "int" (base class has type "int64")
diff --git a/mypyc/test-data/irbuild-i64.test b/mypyc/test-data/irbuild-i64.test
index a04894913c33..ecedab2cd45d 100644
--- a/mypyc/test-data/irbuild-i64.test
+++ b/mypyc/test-data/irbuild-i64.test
@@ -1628,3 +1628,38 @@ L3:
     goto L1
 L4:
     return 1
+
+[case testI64MethodDefaultValueOverride]
+from mypy_extensions import i64
+
+class C:
+    def f(self, x: i64 = 11) -> None: pass
+class D(C):
+    def f(self, x: i64 = 12) -> None: pass
+[out]
+def C.f(self, x, __bitmap):
+    self :: __main__.C
+    x :: int64
+    __bitmap, r0 :: uint32
+    r1 :: bit
+L0:
+    r0 = __bitmap & 1
+    r1 = r0 == 0
+    if r1 goto L1 else goto L2 :: bool
+L1:
+    x = 11
+L2:
+    return 1
+def D.f(self, x, __bitmap):
+    self :: __main__.D
+    x :: int64
+    __bitmap, r0 :: uint32
+    r1 :: bit
+L0:
+    r0 = __bitmap & 1
+    r1 = r0 == 0
+    if r1 goto L1 else goto L2 :: bool
+L1:
+    x = 12
+L2:
+    return 1
diff --git a/mypyc/test-data/run-i64.test b/mypyc/test-data/run-i64.test
index 357a6b0811b6..ee0a09760ab1 100644
--- a/mypyc/test-data/run-i64.test
+++ b/mypyc/test-data/run-i64.test
@@ -1126,3 +1126,47 @@ def test_many_locals() -> None:
     assert a31 == 10
     assert a32 == 55
     assert a33 == 20
+
+[case testI64GlueMethods]
+from typing_extensions import Final
+
+MYPY = False
+if MYPY:
+    from mypy_extensions import i64
+
+MAGIC: Final = -113
+
+class Base:
+    def foo(self) -> i64:
+        return 5
+
+    def bar(self, x: i64 = 2) -> i64:
+        return x + 1
+
+    def hoho(self, x: i64) -> i64:
+        return x - 1
+
+class Derived(Base):
+    def foo(self, x: i64 = 5) -> i64:
+        return x + 10
+
+    def bar(self, x: i64 = 3, y: i64 = 20) -> i64:
+        return x + y + 2
+
+    def hoho(self, x: i64 = 7) -> i64:
+        return x - 2
+
+def test_derived_adds_bitmap() -> None:
+    b: Base = Derived()
+    assert b.foo() == 15
+
+def test_derived_adds_another_default_arg() -> None:
+    b: Base = Derived()
+    assert b.bar() == 25
+    assert b.bar(1) == 23
+    assert b.bar(MAGIC) == MAGIC + 22
+
+def test_derived_switches_arg_to_have_default() -> None:
+    b: Base = Derived()
+    assert b.hoho(5) == 3
+    assert b.hoho(MAGIC) == MAGIC - 2