[LLDB] Add data formatter for std::coroutine_handle

[avogelsgesang]

Summary

This patch adds a formatter for std::coroutine_handle, both for libc++
and libstdc++. For the type-erased coroutine_handle<>, it shows the
resume and destroy function pointers. For a non-type-erased
coroutine_handle<promise_type> it also shows the promise value.

With this change, executing the v t command on the example from
https://clang.llvm.org/docs/DebuggingCoroutines.html now outputs

(task) t = {
  handle = coro frame = 0x55555555b2a0 {
    resume = 0x0000555555555a10 (a.out`coro_task(int, int) at llvm-example.cpp:36)
    destroy = 0x0000555555556090 (a.out`coro_task(int, int) at llvm-example.cpp:36)
  }
}

instead of just

(task) t = {
  handle = {
    __handle_ = 0x55555555b2a0
  }
}

Note, how the symbols for the resume and destroy function pointer
reveal which coroutine is stored inside the std::coroutine_handle.
A follow-up commit will use this fact to infer the coroutine's promise
type and the representation of its internal coroutine state based on
the resume and destroy pointers.

The same formatter is used for both libc++ and libstdc++. It would
also work for MSVC's standard library, however it is not registered
for MSVC, given that lldb does not provide pretty printers for other
MSVC types, either.

The formatter is in a new added Coroutines.{h,cpp} file because there
does not seem to be an already existing place where we could share
formatters across libc++ and libstdc++. Also, I expect this code to grow
as we improve debugging experience for coroutines further.

Testing

• Added API test

[avogelsgesang]

lldb/source/Plugins/Language/CPlusPlus/CPlusPlusLanguage.cpp

1147

I need to remove this. This is a left-over from an earlier implementation sketch

[ChuanqiXu]

LGTM. But I am not familiar with debugger internals. So I'll leave the formal acceptance for other reviewers.

lldb/source/Plugins/Language/CPlusPlus/Coroutines.cpp

138

Add the new line.

lldb/source/Plugins/Language/CPlusPlus/LibCxx.h

62–64 ↗

(On Diff #454622)

Unrelated change? Generally we could do this in a separate NFC patch.

[avogelsgesang]

remove unrelated changes

[avogelsgesang]

clang-format

[aprantl]

lldb/source/Plugins/Language/CPlusPlus/Coroutines.h

2

typo: -*- C++ -*-

21

Can you convert this to a slightly more verbose Doxygen comment? It's not clear to me what exactly this comment means.

22

It would be nice to add Doxygen comments here, too. At least for the class itself.

lldb/test/API/functionalities/data-formatter/data-formatter-stl/generic/coroutine_handle/TestCoroutineHandle.py

16

Nice test!

45

Does this launch a new process? It might be faster to just set an additional breakpoint and run process.Continue().

lldb/test/API/functionalities/data-formatter/data-formatter-stl/generic/coroutine_handle/main.cpp

36

This looks risky, can you add a call to an empty function here, so we can be reasonably sure that there is code at this line?

[avogelsgesang]

addressed comments from @aprantl

[avogelsgesang]

lldb/test/API/functionalities/data-formatter/data-formatter-stl/generic/coroutine_handle/TestCoroutineHandle.py

45

somehow, I assumed run_to_source_breakpoint would actually continue the running process instead of starting a new process. But looking into run_to_source_breakpoint, you are right, this launches a new process...

I changed this to instead continue execution of the already launched process

[jingham]

lldb/test/API/functionalities/data-formatter/data-formatter-stl/generic/coroutine_handle/TestCoroutineHandle.py

16

Would you mind changing moving this into lldbutil.py? This seems generally useful. If you return the thread & breakpoint it would be even more generally useful?

45

There is also lldbutil.continue_to_breakpoint,

[jingham]

lldb/test/API/functionalities/data-formatter/data-formatter-stl/generic/coroutine_handle/TestCoroutineHandle.py

45

Forgot to delete that inline comment before saving...

[avogelsgesang]

lldb/test/API/functionalities/data-formatter/data-formatter-stl/generic/coroutine_handle/TestCoroutineHandle.py

16

moving this into lldbutil.py

done

[shafik]

lldb/source/Plugins/Language/CPlusPlus/Coroutines.cpp

40

/*result_type=*/ see https://clang.llvm.org/extra/clang-tidy/checks/bugprone/argument-comment.html

applies to the rest of them as well.

[avogelsgesang]

lldb/source/Plugins/Language/CPlusPlus/Coroutines.cpp

40

done

clang/docs/tools/clang-formatted-files.txt

 lldb/source/Plugins/InstrumentationRuntime/TSan/InstrumentationRuntimeTSan.h
 lldb/source/Plugins/InstrumentationRuntime/UBSan/InstrumentationRuntimeUBSan.cpp
 lldb/source/Plugins/InstrumentationRuntime/UBSan/InstrumentationRuntimeUBSan.h
 lldb/source/Plugins/JITLoader/GDB/JITLoaderGDB.h
 lldb/source/Plugins/Language/ClangCommon/ClangHighlighter.cpp
 lldb/source/Plugins/Language/ClangCommon/ClangHighlighter.h
 lldb/source/Plugins/Language/CPlusPlus/BlockPointer.cpp
 lldb/source/Plugins/Language/CPlusPlus/BlockPointer.h
+lldb/source/Plugins/Language/CPlusPlus/Coroutines.cpp
+lldb/source/Plugins/Language/CPlusPlus/Coroutines.h
 lldb/source/Plugins/Language/CPlusPlus/CPlusPlusLanguage.h
 lldb/source/Plugins/Language/CPlusPlus/CPlusPlusNameParser.h
 lldb/source/Plugins/Language/CPlusPlus/CxxStringTypes.h
 lldb/source/Plugins/Language/CPlusPlus/Generic.h
 lldb/source/Plugins/Language/CPlusPlus/GenericBitset.cpp
 lldb/source/Plugins/Language/CPlusPlus/GenericOptional.cpp
 lldb/source/Plugins/Language/CPlusPlus/LibCxx.h
 lldb/source/Plugins/Language/CPlusPlus/LibCxxAtomic.cpp

lldb/packages/Python/lldbsuite/test/lldbtest.py

def check_value(self, test_base, val, error_msg=None):
                                   this_error_msg)
         if self.expect_value:
             test_base.assertEqual(self.expect_value, val.GetValue(),
                                   this_error_msg)
         if self.expect_type:
             test_base.assertEqual(self.expect_type, val.GetDisplayTypeName(),
                                   this_error_msg)
         if self.expect_summary:
+            if isinstance(self.expect_summary, re.Pattern):
+                test_base.assertRegex(val.GetSummary(), self.expect_summary,
+                                      this_error_msg)
+            else:
                 test_base.assertEqual(self.expect_summary, val.GetSummary(),
                                     this_error_msg)
         if self.children is not None:
             self.check_value_children(test_base, val, error_msg)
 
     def check_value_children(self, test_base, val, error_msg=None):
         """
         Checks that the children of a SBValue match a certain structure and
         have certain properties.
 

lldb/source/Plugins/Language/CPlusPlus/CMakeLists.txt

 add_lldb_library(lldbPluginCPlusPlusLanguage PLUGIN
   BlockPointer.cpp
+  Coroutines.cpp
   CPlusPlusLanguage.cpp
   CPlusPlusNameParser.cpp
   CxxStringTypes.cpp
   GenericBitset.cpp
   GenericOptional.cpp
   LibCxx.cpp
   LibCxxAtomic.cpp
   LibCxxInitializerList.cpp

lldb/source/Plugins/Language/CPlusPlus/CPlusPlusLanguage.cpp

 #include "lldb/DataFormatters/VectorType.h"
 #include "lldb/Symbol/SymbolFile.h"
 #include "lldb/Utility/ConstString.h"
 #include "lldb/Utility/LLDBLog.h"
 #include "lldb/Utility/Log.h"
 #include "lldb/Utility/RegularExpression.h"
 
 #include "BlockPointer.h"
+#include "Coroutines.h"
 #include "CPlusPlusNameParser.h"
 #include "CxxStringTypes.h"
 #include "Generic.h"
 #include "LibCxx.h"
 #include "LibCxxAtomic.h"
 #include "LibCxxVariant.h"
 #include "LibStdcpp.h"
 #include "MSVCUndecoratedNameParser.h"
AddCXXSynthetic(
       ConstString("^(std::__[[:alnum:]]+::)weak_ptr<.+>(( )?&)?$"),
       stl_synth_flags, true);
   AddCXXSummary(cpp_category_sp,
                 lldb_private::formatters::LibcxxFunctionSummaryProvider,
                 "libc++ std::function summary provider",
                 ConstString("^std::__[[:alnum:]]+::function<.+>$"),
                 stl_summary_flags, true);
 
+  ConstString libcxx_std_coroutine_handle_regex(
+      "^std::__[[:alnum:]]+::coroutine_handle<.+>(( )?&)?$");
+  AddCXXSynthetic(
+      cpp_category_sp,
+      lldb_private::formatters::StdlibCoroutineHandleSyntheticFrontEndCreator,
+      "coroutine_handle synthetic children", libcxx_std_coroutine_handle_regex,
+      stl_deref_flags, true);
+
   stl_summary_flags.SetDontShowChildren(false);
   stl_summary_flags.SetSkipPointers(false);
   AddCXXSummary(cpp_category_sp,
                 lldb_private::formatters::LibcxxContainerSummaryProvider,
                 "libc++ std::bitset summary provider",
                 ConstString("^std::__[[:alnum:]]+::bitset<.+>(( )?&)?$"),
                 stl_summary_flags, true);
   AddCXXSummary(cpp_category_sp,
AddCXXSummary(cpp_category_sp,
                 "libc++ std::weak_ptr summary provider",
                 ConstString("^std::__[[:alnum:]]+::weak_ptr<.+>(( )?&)?$"),
                 stl_summary_flags, true);
   AddCXXSummary(cpp_category_sp,
                 lldb_private::formatters::LibcxxUniquePointerSummaryProvider,
                 "libc++ std::unique_ptr summary provider",
                 libcxx_std_unique_ptr_regex, stl_summary_flags, true);
 
+  AddCXXSummary(cpp_category_sp,
+                lldb_private::formatters::StdlibCoroutineHandleSummaryProvider,
+                "libc++ std::coroutine_handle summary provider",
+                libcxx_std_coroutine_handle_regex, stl_summary_flags, true);
+
   AddCXXSynthetic(
       cpp_category_sp,
       lldb_private::formatters::LibCxxVectorIteratorSyntheticFrontEndCreator,
       "std::vector iterator synthetic children",
       ConstString("^std::__[[:alnum:]]+::__wrap_iter<.+>$"), stl_synth_flags,
       true);
 
   AddCXXSynthetic(
AddCXXSynthetic(
       lldb_private::formatters::LibStdcppSharedPtrSyntheticFrontEndCreator,
       "std::weak_ptr synthetic children",
       ConstString("^std::weak_ptr<.+>(( )?&)?$"), stl_synth_flags, true);
   AddCXXSynthetic(
       cpp_category_sp,
       lldb_private::formatters::LibStdcppTupleSyntheticFrontEndCreator,
       "std::tuple synthetic children", ConstString("^std::tuple<.+>(( )?&)?$"),
       stl_synth_flags, true);
   
+  ConstString libstdcpp_std_coroutine_handle_regex(
+      "^std::coroutine_handle<.+>(( )?&)?$");
+  AddCXXSynthetic(
+      cpp_category_sp,
+      lldb_private::formatters::StdlibCoroutineHandleSyntheticFrontEndCreator,
+      "std::coroutine_handle synthetic children", libstdcpp_std_coroutine_handle_regex,
+      stl_deref_flags, true);
+
   AddCXXSynthetic(
       cpp_category_sp,
       lldb_private::formatters::LibStdcppBitsetSyntheticFrontEndCreator,
       "std::bitset synthetic child", ConstString("^std::bitset<.+>(( )?&)?$"),
       stl_deref_flags, true);
 
   AddCXXSynthetic(
       cpp_category_sp,
       lldb_private::formatters::LibStdcppOptionalSyntheticFrontEndCreator,
AddCXXSummary(cpp_category_sp,
                 "libstdc++ std::shared_ptr summary provider",
                 ConstString("^std::shared_ptr<.+>(( )?&)?$"), stl_summary_flags,
                 true);
   AddCXXSummary(cpp_category_sp,
                 lldb_private::formatters::LibStdcppSmartPointerSummaryProvider,
                 "libstdc++ std::weak_ptr summary provider",
                 ConstString("^std::weak_ptr<.+>(( )?&)?$"), stl_summary_flags,
                 true);
+  AddCXXSummary(cpp_category_sp,
+                lldb_private::formatters::StdlibCoroutineHandleSummaryProvider,
+                "libstdc++ std::coroutine_handle summary provider",
+                libstdcpp_std_coroutine_handle_regex, stl_summary_flags,
+                true);
   AddCXXSummary(
       cpp_category_sp, lldb_private::formatters::GenericOptionalSummaryProvider,
       "libstd++ std::optional summary provider",
       ConstString("^std::optional<.+>(( )?&)?$"), stl_summary_flags, true);
 }
 
 static void LoadSystemFormatters(lldb::TypeCategoryImplSP cpp_category_sp) {
   if (!cpp_category_sp)

lldb/source/Plugins/Language/CPlusPlus/Coroutines.h

+//===-- Coroutines.h -------------------------------------------*- C++//-*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLDB_SOURCE_PLUGINS_LANGUAGE_CPLUSPLUS_COROUTINES_H
+#define LLDB_SOURCE_PLUGINS_LANGUAGE_CPLUSPLUS_COROUTINES_H
+
+#include "lldb/Core/ValueObject.h"
+#include "lldb/DataFormatters/TypeSummary.h"
+#include "lldb/DataFormatters/TypeSynthetic.h"
+#include "lldb/Utility/Stream.h"
+
+namespace lldb_private {
+namespace formatters {
+
+// libc++, libstdc++ and MSVC STL: std::coroutine_handle<T>
+bool StdlibCoroutineHandleSummaryProvider(ValueObject &valobj, Stream &stream,
+                                          const TypeSummaryOptions &options);
+
+class StdlibCoroutineHandleSyntheticFrontEnd
+    : public SyntheticChildrenFrontEnd {
+public:
+  StdlibCoroutineHandleSyntheticFrontEnd(lldb::ValueObjectSP valobj_sp);
+
+  ~StdlibCoroutineHandleSyntheticFrontEnd() override;
+
+  size_t CalculateNumChildren() override;
+
+  lldb::ValueObjectSP GetChildAtIndex(size_t idx) override;
+
+  bool Update() override;
+
+  bool MightHaveChildren() override;
+
+  size_t GetIndexOfChildWithName(ConstString name) override;
+
+private:
+  lldb::ValueObjectSP m_frame_ptr_sp;
+};
+
+SyntheticChildrenFrontEnd *
+StdlibCoroutineHandleSyntheticFrontEndCreator(CXXSyntheticChildren *,
+                                              lldb::ValueObjectSP);
+
+} // namespace formatters
+} // namespace lldb_private
+
+#endif // LLDB_SOURCE_PLUGINS_LANGUAGE_CPLUSPLUS_COROUTINES_H

lldb/source/Plugins/Language/CPlusPlus/Coroutines.cpp

+//===-- Coroutines.cpp ----------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "Coroutines.h"
+
+#include "Plugins/TypeSystem/Clang/TypeSystemClang.h"
+
+using namespace lldb;
+using namespace lldb_private;
+using namespace lldb_private::formatters;
+
+static ValueObjectSP GetCoroFramePtrFromHandle(ValueObject &valobj) {
+  ValueObjectSP valobj_sp(valobj.GetNonSyntheticValue());
+  if (!valobj_sp)
+    return nullptr;
+
+  // We expect a single pointer in the `coroutine_handle` class.
+  // We don't care about its name.
+  if (valobj_sp->GetNumChildren() != 1)
+    return nullptr;
+  ValueObjectSP ptr_sp(valobj_sp->GetChildAtIndex(0, true));
+  if (!ptr_sp)
+    return nullptr;
+  if (!ptr_sp->GetCompilerType().IsPointerType())
+    return nullptr;
+
+  return ptr_sp;
+}
+
+static CompilerType GetCoroutineFrameType(TypeSystemClang &ast_ctx,
+                                          CompilerType promise_type) {
+  CompilerType void_type = ast_ctx.GetBasicType(lldb::eBasicTypeVoid);
+  CompilerType coro_func_type = ast_ctx.CreateFunctionType(
+      /*result_type*/ void_type, /*args*/ &void_type, /*num_args*/ 1,
+      /*is_variadic*/ false, /*qualifiers*/ 0);
+  CompilerType coro_abi_type;
+  if (promise_type.IsVoidType()) {
+    coro_abi_type = ast_ctx.CreateStructForIdentifier(
+        ConstString(), {{"resume", coro_func_type.GetPointerType()},
+                        {"destroy", coro_func_type.GetPointerType()}});
+  } else {
+    coro_abi_type = ast_ctx.CreateStructForIdentifier(
+        ConstString(), {{"resume", coro_func_type.GetPointerType()},
+                        {"destroy", coro_func_type.GetPointerType()},
+                        {"promise", promise_type}});
+  }
+  return coro_abi_type;
+}
+
+bool lldb_private::formatters::StdlibCoroutineHandleSummaryProvider(
+    ValueObject &valobj, Stream &stream, const TypeSummaryOptions &options) {
+  ValueObjectSP ptr_sp(GetCoroFramePtrFromHandle(valobj));
+  if (!ptr_sp)
+    return false;
+
+  stream.Printf("coro frame = 0x%" PRIx64, ptr_sp->GetValueAsUnsigned(0));
+  return true;
+}
+
+lldb_private::formatters::StdlibCoroutineHandleSyntheticFrontEnd::
+    StdlibCoroutineHandleSyntheticFrontEnd(lldb::ValueObjectSP valobj_sp)
+    : SyntheticChildrenFrontEnd(*valobj_sp) {
+  if (valobj_sp)
+    Update();
+}
+
+lldb_private::formatters::StdlibCoroutineHandleSyntheticFrontEnd::
+    ~StdlibCoroutineHandleSyntheticFrontEnd() = default;
+
+size_t lldb_private::formatters::StdlibCoroutineHandleSyntheticFrontEnd::
+    CalculateNumChildren() {
+  if (!m_frame_ptr_sp)
+    return 0;
+
+  return m_frame_ptr_sp->GetNumChildren();
+}
+
+lldb::ValueObjectSP lldb_private::formatters::
+    StdlibCoroutineHandleSyntheticFrontEnd::GetChildAtIndex(size_t idx) {
+  if (!m_frame_ptr_sp)
+    return lldb::ValueObjectSP();
+
+  return m_frame_ptr_sp->GetChildAtIndex(idx, true);
+}
+
+bool lldb_private::formatters::StdlibCoroutineHandleSyntheticFrontEnd::
+    Update() {
+  m_frame_ptr_sp.reset();
+
+  ValueObjectSP valobj_sp = m_backend.GetSP();
+  if (!valobj_sp)
+    return false;
+
+  ValueObjectSP ptr_sp(GetCoroFramePtrFromHandle(m_backend));
+  if (!ptr_sp)
+    return false;
+
+  TypeSystemClang *ast_ctx = llvm::dyn_cast_or_null<TypeSystemClang>(
+      valobj_sp->GetCompilerType().GetTypeSystem());
+  if (!ast_ctx)
+    return false;
+
+  CompilerType promise_type(
+      valobj_sp->GetCompilerType().GetTypeTemplateArgument(0));
+  if (!promise_type)
+    return false;
+  CompilerType coro_frame_type = GetCoroutineFrameType(*ast_ctx, promise_type);
+
+  m_frame_ptr_sp = ptr_sp->Cast(coro_frame_type.GetPointerType());
+
+  return false;
+}
+
+bool lldb_private::formatters::StdlibCoroutineHandleSyntheticFrontEnd::
+    MightHaveChildren() {
+  return true;
+}
+
+size_t StdlibCoroutineHandleSyntheticFrontEnd::GetIndexOfChildWithName(
+    ConstString name) {
+  if (!m_frame_ptr_sp)
+    return UINT32_MAX;
+
+  return m_frame_ptr_sp->GetIndexOfChildWithName(name);
+}
+
+SyntheticChildrenFrontEnd *
+lldb_private::formatters::StdlibCoroutineHandleSyntheticFrontEndCreator(
+    CXXSyntheticChildren *, lldb::ValueObjectSP valobj_sp) {
+  return (valobj_sp ? new StdlibCoroutineHandleSyntheticFrontEnd(valobj_sp)
+                    : nullptr);
+}
 No newline at end of file

lldb/test/API/functionalities/data-formatter/data-formatter-stl/generic/coroutine_handle/Makefile

+CXX_SOURCES := main.cpp
+CFLAGS_EXTRAS := -std=c++20
+
+include Makefile.rules

lldb/test/API/functionalities/data-formatter/data-formatter-stl/generic/coroutine_handle/TestCoroutineHandle.py

+"""
+Test lldb data formatter subsystem.
+"""
+
+
+
+import lldb
+from lldbsuite.test.decorators import *
+from lldbsuite.test.lldbtest import *
+from lldbsuite.test import lldbutil
+
+USE_LIBSTDCPP = "USE_LIBSTDCPP"
+USE_LIBCPP = "USE_LIBCPP"
+
+class TestCoroutineHandle(TestBase):
+    def do_test(self, stdlib_type):
+        """Test std::coroutine_handle is displayed correctly."""
+        self.build(dictionary={stdlib_type: "1"})
+
+        test_generator_func_ptr_re = re.compile(
+            r"^\(a.out`my_generator_func\(\) at main.cpp:[0-9]*\)$")
+
+        lldbutil.run_to_source_breakpoint(self, '// Break at initial_suspend',
+                lldb.SBFileSpec("main.cpp", False))
+        # Check that we show the correct function pointers and the `promise`. 
+        self.expect_expr("gen.hdl",
+            result_summary=re.compile("^coro frame = 0x[0-9a-f]*$"),
+            result_children=[
+                ValueCheck(name="resume", summary = test_generator_func_ptr_re),
+                ValueCheck(name="destroy", summary = test_generator_func_ptr_re),
+                ValueCheck(name="promise", children=[
+                    ValueCheck(name="current_value", value = "-1"),
+                ])
+            ])
+        # For type-erased `coroutine_handle<>` we are missing the `promise`
+        # but still show `resume` and `destroy`.
+        self.expect_expr("type_erased_hdl",
+            result_summary=re.compile("^coro frame = 0x[0-9a-f]*$"),
+            result_children=[
+                ValueCheck(name="resume", summary = test_generator_func_ptr_re),
+                ValueCheck(name="destroy", summary = test_generator_func_ptr_re),
+            ])
+
+        lldbutil.run_to_source_breakpoint(self, '// Break after co_yield',
+                lldb.SBFileSpec("main.cpp", False))
+        # We correctly show the updated value inside `prommise.current_value`.
+        self.expect_expr("gen.hdl",
+            result_children=[
+                ValueCheck(name="resume", summary = test_generator_func_ptr_re),
+                ValueCheck(name="destroy", summary = test_generator_func_ptr_re),
+                ValueCheck(name="promise", children=[
+                    ValueCheck(name="current_value", value = "42"),
+                ])
+            ])
+        
+        lldbutil.run_to_source_breakpoint(self, '// Break at final_suspend',
+                lldb.SBFileSpec("main.cpp", False))
+        # At the final suspension point, `resume` is set to a nullptr.
+        # Check that we still show the remaining data correctly.
+        self.expect_expr("gen.hdl",
+            result_children=[
+                ValueCheck(name="resume", value = "0x0000000000000000"),
+                ValueCheck(name="destroy", summary = test_generator_func_ptr_re),
+                ValueCheck(name="promise", children=[
+                    ValueCheck(name="current_value", value = "42"),
+                ])
+            ])
+
+    @add_test_categories(["libstdcxx"])
+    def test_libstdcpp(self):
+        self.do_test(USE_LIBSTDCPP)
+
+    @add_test_categories(["libc++"])
+    def test_libcpp(self):
+        self.do_test(USE_LIBCPP)

lldb/test/API/functionalities/data-formatter/data-formatter-stl/generic/coroutine_handle/main.cpp

+#include <coroutine>
+
+// `int_generator` is a stripped down, minimal coroutine generator
+// type.
+struct int_generator {
+  struct promise_type {
+    int current_value = -1;
+
+    auto get_return_object() {
+      return std::coroutine_handle<promise_type>::from_promise(*this);
+    }
+    auto initial_suspend() { return std::suspend_always(); }
+    auto final_suspend() noexcept { return std::suspend_always(); }
+    auto return_void() { return std::suspend_always(); }
+    void unhandled_exception() { __builtin_unreachable(); }
+    auto yield_value(int v) {
+      current_value = v;
+      return std::suspend_always();
+    }
+  };
+
+  std::coroutine_handle<promise_type> hdl;
+
+  int_generator(std::coroutine_handle<promise_type> h) : hdl(h) {}
+  ~int_generator() { hdl.destroy(); }
+};
+
+int_generator my_generator_func() { co_yield 42; }
+
+int main() {
+  int_generator gen = my_generator_func();
+  std::coroutine_handle<> type_erased_hdl = gen.hdl;
+  gen.hdl.resume(); // Break at initial_suspend
+  gen.hdl.resume(); // Break after co_yield
+  // Break at final_suspend
+}