diff --git a/mypyc/irbuild/function.py b/mypyc/irbuild/function.py
@@ -261,7 +261,7 @@ def c() -> None:
         )
 
         # Re-enter the FuncItem and visit the body of the function this time.
-        gen_generator_func_body(builder, fn_info, sig, func_reg)
+        gen_generator_func_body(builder, fn_info, func_reg)
     else:
         func_ir, func_reg = gen_func_body(builder, sig, cdef, is_singledispatch)
 

diff --git a/mypyc/irbuild/generator.py b/mypyc/irbuild/generator.py
@@ -13,9 +13,9 @@
 from typing import Callable
 
 from mypy.nodes import ARG_OPT, FuncDef, Var
-from mypyc.common import ENV_ATTR_NAME, NEXT_LABEL_ATTR_NAME, SELF_NAME
+from mypyc.common import ENV_ATTR_NAME, NEXT_LABEL_ATTR_NAME
 from mypyc.ir.class_ir import ClassIR
-from mypyc.ir.func_ir import FuncDecl, FuncIR, FuncSignature, RuntimeArg
+from mypyc.ir.func_ir import FuncDecl, FuncIR
 from mypyc.ir.ops import (
     NO_TRACEBACK_LINE_NO,
     BasicBlock,
@@ -78,17 +78,15 @@ def gen_generator_func(
     return func_ir, func_reg
 
 
-def gen_generator_func_body(
-    builder: IRBuilder, fn_info: FuncInfo, sig: FuncSignature, func_reg: Value | None
-) -> None:
+def gen_generator_func_body(builder: IRBuilder, fn_info: FuncInfo, func_reg: Value | None) -> None:
     """Generate IR based on the body of a generator function.
 
     Add "__next__", "__iter__" and other generator methods to the generator
     class that implements the function (each function gets a separate class).
 
     Return the symbol table for the body.
     """
-    builder.enter(fn_info, ret_type=sig.ret_type)
+    builder.enter(fn_info, ret_type=object_rprimitive)
     setup_env_for_generator_class(builder)
 
     load_outer_envs(builder, builder.fn_info.generator_class)
@@ -117,7 +115,7 @@ class that implements the function (each function gets a separate class).
 
     args, _, blocks, ret_type, fn_info = builder.leave()
 
-    add_methods_to_generator_class(builder, fn_info, sig, args, blocks, fitem.is_coroutine)
+    add_methods_to_generator_class(builder, fn_info, args, blocks, fitem.is_coroutine)
 
     # Evaluate argument defaults in the surrounding scope, since we
     # calculate them *once* when the function definition is evaluated.
@@ -153,10 +151,9 @@ def instantiate_generator_class(builder: IRBuilder) -> Value:
 
 
 def setup_generator_class(builder: IRBuilder) -> ClassIR:
-    name = f"{builder.fn_info.namespaced_name()}_gen"
-
-    generator_class_ir = ClassIR(name, builder.module_name, is_generated=True, is_final_class=True)
-    generator_class_ir.reuse_freed_instance = True
+    mapper = builder.mapper
+    assert isinstance(builder.fn_info.fitem, FuncDef)
+    generator_class_ir = mapper.fdef_to_generator[builder.fn_info.fitem]
     if builder.fn_info.can_merge_generator_and_env_classes():
         builder.fn_info.env_class = generator_class_ir
     else:
@@ -216,46 +213,25 @@ def add_raise_exception_blocks_to_generator_class(builder: IRBuilder, line: int)
 def add_methods_to_generator_class(
     builder: IRBuilder,
     fn_info: FuncInfo,
-    sig: FuncSignature,
     arg_regs: list[Register],
     blocks: list[BasicBlock],
     is_coroutine: bool,
 ) -> None:
-    helper_fn_decl = add_helper_to_generator_class(builder, arg_regs, blocks, sig, fn_info)
-    add_next_to_generator_class(builder, fn_info, helper_fn_decl, sig)
-    add_send_to_generator_class(builder, fn_info, helper_fn_decl, sig)
+    helper_fn_decl = add_helper_to_generator_class(builder, arg_regs, blocks, fn_info)
+    add_next_to_generator_class(builder, fn_info, helper_fn_decl)
+    add_send_to_generator_class(builder, fn_info, helper_fn_decl)
     add_iter_to_generator_class(builder, fn_info)
-    add_throw_to_generator_class(builder, fn_info, helper_fn_decl, sig)
+    add_throw_to_generator_class(builder, fn_info, helper_fn_decl)
     add_close_to_generator_class(builder, fn_info)
     if is_coroutine:
         add_await_to_generator_class(builder, fn_info)
 
 
 def add_helper_to_generator_class(
-    builder: IRBuilder,
-    arg_regs: list[Register],
-    blocks: list[BasicBlock],
-    sig: FuncSignature,
-    fn_info: FuncInfo,
+    builder: IRBuilder, arg_regs: list[Register], blocks: list[BasicBlock], fn_info: FuncInfo
 ) -> FuncDecl:
     """Generates a helper method for a generator class, called by '__next__' and 'throw'."""
-    sig = FuncSignature(
-        (
-            RuntimeArg(SELF_NAME, object_rprimitive),
-            RuntimeArg("type", object_rprimitive),
-            RuntimeArg("value", object_rprimitive),
-            RuntimeArg("traceback", object_rprimitive),
-            RuntimeArg("arg", object_rprimitive),
-        ),
-        sig.ret_type,
-    )
-    helper_fn_decl = FuncDecl(
-        "__mypyc_generator_helper__",
-        fn_info.generator_class.ir.name,
-        builder.module_name,
-        sig,
-        internal=True,
-    )
+    helper_fn_decl = fn_info.generator_class.ir.method_decls["__mypyc_generator_helper__"]
     helper_fn_ir = FuncIR(
         helper_fn_decl, arg_regs, blocks, fn_info.fitem.line, traceback_name=fn_info.fitem.name
     )
@@ -272,9 +248,7 @@ def add_iter_to_generator_class(builder: IRBuilder, fn_info: FuncInfo) -> None:
         builder.add(Return(builder.self()))
 
 
-def add_next_to_generator_class(
-    builder: IRBuilder, fn_info: FuncInfo, fn_decl: FuncDecl, sig: FuncSignature
-) -> None:
+def add_next_to_generator_class(builder: IRBuilder, fn_info: FuncInfo, fn_decl: FuncDecl) -> None:
     """Generates the '__next__' method for a generator class."""
     with builder.enter_method(fn_info.generator_class.ir, "__next__", object_rprimitive, fn_info):
         none_reg = builder.none_object()
@@ -289,9 +263,7 @@ def add_next_to_generator_class(
         builder.add(Return(result))
 
 
-def add_send_to_generator_class(
-    builder: IRBuilder, fn_info: FuncInfo, fn_decl: FuncDecl, sig: FuncSignature
-) -> None:
+def add_send_to_generator_class(builder: IRBuilder, fn_info: FuncInfo, fn_decl: FuncDecl) -> None:
     """Generates the 'send' method for a generator class."""
     with builder.enter_method(fn_info.generator_class.ir, "send", object_rprimitive, fn_info):
         arg = builder.add_argument("arg", object_rprimitive)
@@ -307,9 +279,7 @@ def add_send_to_generator_class(
         builder.add(Return(result))
 
 
-def add_throw_to_generator_class(
-    builder: IRBuilder, fn_info: FuncInfo, fn_decl: FuncDecl, sig: FuncSignature
-) -> None:
+def add_throw_to_generator_class(builder: IRBuilder, fn_info: FuncInfo, fn_decl: FuncDecl) -> None:
     """Generates the 'throw' method for a generator class."""
     with builder.enter_method(fn_info.generator_class.ir, "throw", object_rprimitive, fn_info):
         typ = builder.add_argument("type", object_rprimitive)

diff --git a/mypyc/irbuild/main.py b/mypyc/irbuild/main.py
@@ -25,7 +25,7 @@ def f(x: int) -> int:
 from typing import Any, Callable, TypeVar, cast
 
 from mypy.build import Graph
-from mypy.nodes import ClassDef, Expression, MypyFile
+from mypy.nodes import ClassDef, Expression, FuncDef, MypyFile
 from mypy.state import state
 from mypy.types import Type
 from mypyc.analysis.attrdefined import analyze_always_defined_attrs
@@ -37,7 +37,11 @@ def f(x: int) -> int:
 from mypyc.irbuild.builder import IRBuilder
 from mypyc.irbuild.mapper import Mapper
 from mypyc.irbuild.prebuildvisitor import PreBuildVisitor
-from mypyc.irbuild.prepare import build_type_map, find_singledispatch_register_impls
+from mypyc.irbuild.prepare import (
+    build_type_map,
+    create_generator_class_if_needed,
+    find_singledispatch_register_impls,
+)
 from mypyc.irbuild.visitor import IRBuilderVisitor
 from mypyc.irbuild.vtable import compute_vtable
 from mypyc.options import CompilerOptions
@@ -76,6 +80,15 @@ def build_ir(
         pbv = PreBuildVisitor(errors, module, singledispatch_info.decorators_to_remove, types)
         module.accept(pbv)
 
+        # Declare generator classes for nested async functions and generators.
+        for fdef in pbv.nested_funcs:
+            if isinstance(fdef, FuncDef):
+                # Make generator class name sufficiently unique.
+                suffix = f"___{fdef.line}"
+                create_generator_class_if_needed(
+                    module.fullname, None, fdef, mapper, name_suffix=suffix
+                )
+
         # Construct and configure builder objects (cyclic runtime dependency).
         visitor = IRBuilderVisitor()
         builder = IRBuilder(

diff --git a/mypyc/irbuild/mapper.py b/mypyc/irbuild/mapper.py
@@ -64,6 +64,8 @@ def __init__(self, group_map: dict[str, str | None]) -> None:
         self.type_to_ir: dict[TypeInfo, ClassIR] = {}
         self.func_to_decl: dict[SymbolNode, FuncDecl] = {}
         self.symbol_fullnames: set[str] = set()
+        # The corresponding generator class that implements a generator/async function
+        self.fdef_to_generator: dict[FuncDef, ClassIR] = {}
 
     def type_to_rtype(self, typ: Type | None) -> RType:
         if typ is None:
@@ -171,7 +173,14 @@ def fdef_to_sig(self, fdef: FuncDef, strict_dunders_typing: bool) -> FuncSignatu
                 for typ, kind in zip(fdef.type.arg_types, fdef.type.arg_kinds)
             ]
             arg_pos_onlys = [name is None for name in fdef.type.arg_names]
-            ret = self.type_to_rtype(fdef.type.ret_type)
+            # TODO: We could probably support decorators sometimes (static and class method?)
+            if (fdef.is_coroutine or fdef.is_generator) and not fdef.is_decorated:
+                # Give a more precise type for generators, so that we can optimize
+                # code that uses them. They return a generator object, which has a
+                # specific class. Without this, the type would have to be 'object'.
+                ret: RType = RInstance(self.fdef_to_generator[fdef])
+            else:
+                ret = self.type_to_rtype(fdef.type.ret_type)
         else:
             # Handle unannotated functions
             arg_types = [object_rprimitive for _ in fdef.arguments]

diff --git a/mypyc/irbuild/prepare.py b/mypyc/irbuild/prepare.py
@@ -38,7 +38,7 @@
 from mypy.semanal import refers_to_fullname
 from mypy.traverser import TraverserVisitor
 from mypy.types import Instance, Type, get_proper_type
-from mypyc.common import PROPSET_PREFIX, get_id_from_name
+from mypyc.common import PROPSET_PREFIX, SELF_NAME, get_id_from_name
 from mypyc.crash import catch_errors
 from mypyc.errors import Errors
 from mypyc.ir.class_ir import ClassIR
@@ -51,7 +51,14 @@
     RuntimeArg,
 )
 from mypyc.ir.ops import DeserMaps
-from mypyc.ir.rtypes import RInstance, RType, dict_rprimitive, none_rprimitive, tuple_rprimitive
+from mypyc.ir.rtypes import (
+    RInstance,
+    RType,
+    dict_rprimitive,
+    none_rprimitive,
+    object_rprimitive,
+    tuple_rprimitive,
+)
 from mypyc.irbuild.mapper import Mapper
 from mypyc.irbuild.util import (
     get_func_def,
@@ -115,7 +122,7 @@ def build_type_map(
 
     # Collect all the functions also. We collect from the symbol table
     # so that we can easily pick out the right copy of a function that
-    # is conditionally defined.
+    # is conditionally defined. This doesn't include nested functions!
     for module in modules:
         for func in get_module_func_defs(module):
             prepare_func_def(module.fullname, None, func, mapper, options)
@@ -179,6 +186,8 @@ def prepare_func_def(
     mapper: Mapper,
     options: CompilerOptions,
 ) -> FuncDecl:
+    create_generator_class_if_needed(module_name, class_name, fdef, mapper)
+
     kind = (
         FUNC_STATICMETHOD
         if fdef.is_static
@@ -190,6 +199,38 @@ def prepare_func_def(
     return decl
 
 
+def create_generator_class_if_needed(
+    module_name: str, class_name: str | None, fdef: FuncDef, mapper: Mapper, name_suffix: str = ""
+) -> None:
+    """If function is a generator/async function, declare a generator class.
+
+    Each generator and async function gets a dedicated class that implements the
+    generator protocol with generated methods.
+    """
+    if fdef.is_coroutine or fdef.is_generator:
+        name = "_".join(x for x in [fdef.name, class_name] if x) + "_gen" + name_suffix
+        cir = ClassIR(name, module_name, is_generated=True, is_final_class=True)
+        cir.reuse_freed_instance = True
+        mapper.fdef_to_generator[fdef] = cir
+
+        helper_sig = FuncSignature(
+            (
+                RuntimeArg(SELF_NAME, object_rprimitive),
+                RuntimeArg("type", object_rprimitive),
+                RuntimeArg("value", object_rprimitive),
+                RuntimeArg("traceback", object_rprimitive),
+                RuntimeArg("arg", object_rprimitive),
+            ),
+            object_rprimitive,
+        )
+
+        # The implementation of most generator functionality is behind this magic method.
+        helper_fn_decl = FuncDecl(
+            "__mypyc_generator_helper__", name, module_name, helper_sig, internal=True
+        )
+        cir.method_decls[helper_fn_decl.name] = helper_fn_decl
+
+
 def prepare_method_def(
     ir: ClassIR,
     module_name: str,

diff --git a/mypyc/irbuild/statement.py b/mypyc/irbuild/statement.py
@@ -48,11 +48,13 @@
 )
 from mypyc.common import TEMP_ATTR_NAME
 from mypyc.ir.ops import (
+    ERR_NEVER,
     NAMESPACE_MODULE,
     NO_TRACEBACK_LINE_NO,
     Assign,
     BasicBlock,
     Branch,
+    Call,
     InitStatic,
     Integer,
     LoadAddress,
@@ -930,16 +932,41 @@ def emit_yield_from_or_await(
     to_yield_reg = Register(object_rprimitive)
     received_reg = Register(object_rprimitive)
 
-    get_op = coro_op if is_await else iter_op
-    if isinstance(get_op, PrimitiveDescription):
-        iter_val = builder.primitive_op(get_op, [val], line)
+    helper_method = "__mypyc_generator_helper__"
+    if (
+        isinstance(val, (Call, MethodCall))
+        and isinstance(val.type, RInstance)
+        and val.type.class_ir.has_method(helper_method)
+    ):
+        # This is a generated native generator class, and we can use a fast path.
+        # This allows two optimizations:
+        # 1) No need to call CPy_GetCoro() or iter() since for native generators
+        #    it just returns the generator object (implemented here).
+        # 2) Instead of calling next(), call generator helper method directly,
+        #    since next() just calls __next__ which calls the helper method.
+        iter_val: Value = val
     else:
-        iter_val = builder.call_c(get_op, [val], line)
+        get_op = coro_op if is_await else iter_op
+        if isinstance(get_op, PrimitiveDescription):
+            iter_val = builder.primitive_op(get_op, [val], line)
+        else:
+            iter_val = builder.call_c(get_op, [val], line)
 
     iter_reg = builder.maybe_spill_assignable(iter_val)
 
     stop_block, main_block, done_block = BasicBlock(), BasicBlock(), BasicBlock()
-    _y_init = builder.call_c(next_raw_op, [builder.read(iter_reg)], line)
+
+    if isinstance(iter_reg.type, RInstance) and iter_reg.type.class_ir.has_method(helper_method):
+        # Second fast path optimization: call helper directly (see also comment above).
+        obj = builder.read(iter_reg)
+        nn = builder.none_object()
+        m = MethodCall(obj, helper_method, [nn, nn, nn, nn], line)
+        # Generators have custom error handling, so disable normal error handling.
+        m.error_kind = ERR_NEVER
+        _y_init = builder.add(m)
+    else:
+        _y_init = builder.call_c(next_raw_op, [builder.read(iter_reg)], line)
+
     builder.add(Branch(_y_init, stop_block, main_block, Branch.IS_ERROR))
 
     # Try extracting a return value from a StopIteration and return it.
@@ -948,7 +975,7 @@ def emit_yield_from_or_await(
     builder.assign(result, builder.call_c(check_stop_op, [], line), line)
     # Clear the spilled iterator/coroutine so that it will be freed.
     # Otherwise, the freeing of the spilled register would likely be delayed.
-    err = builder.add(LoadErrorValue(object_rprimitive))
+    err = builder.add(LoadErrorValue(iter_reg.type))
     builder.assign(iter_reg, err, line)
     builder.goto(done_block)