Add support for nested C++11 exceptions (#3608)

* Add support for nested C++11 exceptions

* Remove wrong include

* Fix if directive

* Fix missing skipif

* Simplify code and try to work around MSVC bug

* Clarify comment

* Further simplify code

* Remove the last extra throw statement

* Qualify auto

* Fix typo

* Add missing return for consistency

* Fix clang-tidy complaint

* Fix python2 stub

* Make clang-tidy happy

* Fix compile error

* Fix python2 function signature

* Extract C++20 utility and backport

* Cleanup code a bit more

* Improve test case

* Consolidate code and fix signature

* Fix typo

include/pybind11/detail/common.h

@@ -36,6 +36,9 @@
 #    define PYBIND11_CPP14
 #    if __cplusplus >= 201703L
 #      define PYBIND11_CPP17
+#      if __cplusplus >= 202002L
+#        define PYBIND11_CPP20
+#      endif
 #    endif
 #  endif
 #elif defined(_MSC_VER) && __cplusplus == 199711L
@@ -45,6 +48,9 @@
 #    define PYBIND11_CPP14
 #    if _MSVC_LANG > 201402L && _MSC_VER >= 1910
 #      define PYBIND11_CPP17
+#      if _MSVC_LANG >= 202002L
+#        define PYBIND11_CPP20
+#      endif
 #    endif
 #  endif
 #endif
@@ -612,6 +618,18 @@ template <typename T> using remove_cv_t = typename std::remove_cv<T>::type;
 template <typename T> using remove_reference_t = typename std::remove_reference<T>::type;
 #endif
 
+#if defined(PYBIND11_CPP20)
+using std::remove_cvref;
+using std::remove_cvref_t;
+#else
+template <class T>
+struct remove_cvref {
+    using type = remove_cv_t<remove_reference_t<T>>;
+};
+template <class T>
+using remove_cvref_t = typename remove_cvref<T>::type;
+#endif
+
 /// Index sequences
 #if defined(PYBIND11_CPP14)
 using std::index_sequence;

include/pybind11/detail/internals.h

@@ -10,6 +10,7 @@
 #pragma once
 
 #include "../pytypes.h"
+#include <exception>
 
 /// Tracks the `internals` and `type_info` ABI version independent of the main library version.
 ///
@@ -280,21 +281,104 @@ inline internals **&get_internals_pp() {
     return internals_pp;
 }
 
+#if PY_VERSION_HEX >= 0x03030000
+// forward decl
+inline void translate_exception(std::exception_ptr);
+
+template <class T,
+          enable_if_t<std::is_same<std::nested_exception, remove_cvref_t<T>>::value, int> = 0>
+bool handle_nested_exception(const T &exc, const std::exception_ptr &p) {
+    std::exception_ptr nested = exc.nested_ptr();
+    if (nested != nullptr && nested != p) {
+        translate_exception(nested);
+        return true;
+    }
+    return false;
+}
+
+template <class T,
+          enable_if_t<!std::is_same<std::nested_exception, remove_cvref_t<T>>::value, int> = 0>
+bool handle_nested_exception(const T &exc, const std::exception_ptr &p) {
+    if (auto *nep = dynamic_cast<const std::nested_exception *>(std::addressof(exc))) {
+        return handle_nested_exception(*nep, p);
+    }
+    return false;
+}
+
+#else
+
+template <class T>
+bool handle_nested_exception(const T &, std::exception_ptr &) {
+    return false;
+}
+#endif
+
+inline bool raise_err(PyObject *exc_type, const char *msg) {
+#if PY_VERSION_HEX >= 0x03030000
+    if (PyErr_Occurred()) {
+        raise_from(exc_type, msg);
+        return true;
+    }
+#endif
+    PyErr_SetString(exc_type, msg);
+    return false;
+};
+
 inline void translate_exception(std::exception_ptr p) {
+    if (!p) {
+        return;
+    }
     try {
-        if (p) std::rethrow_exception(p);
+        std::rethrow_exception(p);
-    } catch (error_already_set &e)           { e.restore();                                    return;
+    } catch (error_already_set &e) {
-    } catch (const builtin_exception &e)     { e.set_error();                                  return;
+        handle_nested_exception(e, p);
-    } catch (const std::bad_alloc &e)        { PyErr_SetString(PyExc_MemoryError,   e.what()); return;
+        e.restore();
-    } catch (const std::domain_error &e)     { PyErr_SetString(PyExc_ValueError,    e.what()); return;
+        return;
-    } catch (const std::invalid_argument &e) { PyErr_SetString(PyExc_ValueError,    e.what()); return;
+    } catch (const builtin_exception &e) {
-    } catch (const std::length_error &e)     { PyErr_SetString(PyExc_ValueError,    e.what()); return;
+        // Could not use template since it's an abstract class.
-    } catch (const std::out_of_range &e)     { PyErr_SetString(PyExc_IndexError,    e.what()); return;
+        if (auto *nep = dynamic_cast<const std::nested_exception *>(std::addressof(e))) {
-    } catch (const std::range_error &e)      { PyErr_SetString(PyExc_ValueError,    e.what()); return;
+            handle_nested_exception(*nep, p);
-    } catch (const std::overflow_error &e)   { PyErr_SetString(PyExc_OverflowError, e.what()); return;
+        }
-    } catch (const std::exception &e)        { PyErr_SetString(PyExc_RuntimeError,  e.what()); return;
+        e.set_error();
+        return;
+    } catch (const std::bad_alloc &e) {
+        handle_nested_exception(e, p);
+        raise_err(PyExc_MemoryError, e.what());
+        return;
+    } catch (const std::domain_error &e) {
+        handle_nested_exception(e, p);
+        raise_err(PyExc_ValueError, e.what());
+        return;
+    } catch (const std::invalid_argument &e) {
+        handle_nested_exception(e, p);
+        raise_err(PyExc_ValueError, e.what());
+        return;
+    } catch (const std::length_error &e) {
+        handle_nested_exception(e, p);
+        raise_err(PyExc_ValueError, e.what());
+        return;
+    } catch (const std::out_of_range &e) {
+        handle_nested_exception(e, p);
+        raise_err(PyExc_IndexError, e.what());
+        return;
+    } catch (const std::range_error &e) {
+        handle_nested_exception(e, p);
+        raise_err(PyExc_ValueError, e.what());
+        return;
+    } catch (const std::overflow_error &e) {
+        handle_nested_exception(e, p);
+        raise_err(PyExc_OverflowError, e.what());
+        return;
+    } catch (const std::exception &e) {
+        handle_nested_exception(e, p);
+        raise_err(PyExc_RuntimeError, e.what());
+        return;
+    } catch (const std::nested_exception &e) {
+        handle_nested_exception(e, p);
+        raise_err(PyExc_RuntimeError, "Caught an unknown nested exception!");
+        return;
     } catch (...) {
-        PyErr_SetString(PyExc_RuntimeError, "Caught an unknown exception!");
+        raise_err(PyExc_RuntimeError, "Caught an unknown exception!");
         return;
     }
 }

tests/test_exceptions.cpp

@@ -11,6 +11,8 @@
 #include "local_bindings.h"
 
 #include "pybind11_tests.h"
+#include <exception>
+#include <stdexcept>
 #include <utility>
 
 // A type that should be raised as an exception in Python
@@ -105,7 +107,6 @@ struct PythonAlreadySetInDestructor {
     py::str s;
 };
 
-
 TEST_SUBMODULE(exceptions, m) {
     m.def("throw_std_exception", []() {
         throw std::runtime_error("This exception was intentionally thrown.");
@@ -281,5 +282,12 @@ TEST_SUBMODULE(exceptions, m) {
         }
     });
 
+    m.def("throw_nested_exception", []() {
+        try {
+            throw std::runtime_error("Inner Exception");
+        } catch (const std::runtime_error &) {
+            std::throw_with_nested(std::runtime_error("Outer Exception"));
+        }
+    });
 #endif
 }

tests/test_exceptions.py

@@ -239,6 +239,14 @@ def test_nested_throws(capture):
     assert str(excinfo.value) == "this is a helper-defined translated exception"
 
 
+@pytest.mark.skipif("env.PY2")
+def test_throw_nested_exception():
+    with pytest.raises(RuntimeError) as excinfo:
+        m.throw_nested_exception()
+    assert str(excinfo.value) == "Outer Exception"
+    assert str(excinfo.value.__cause__) == "Inner Exception"
+
+
 # This can often happen if you wrap a pybind11 class in a Python wrapper
 def test_invalid_repr():
     class MyRepr(object):