Make nanobind::detail::trampoline constructor constexpr (#1223)

This fixes a bug in MSVC, and allows creating trampolines for classes that
have constexpr constructors that take a parameter.

Author: virtuald

include/nanobind/trampoline.h

@@ -26,7 +26,7 @@ NB_CORE void trampoline_leave(ticket *ticket) noexcept;
 template <size_t Size> struct trampoline {
     mutable void *data[2 * Size + 1];
 
-    NB_INLINE trampoline(void *ptr) { trampoline_new(data, Size, ptr); }
+    NB_INLINE constexpr trampoline(void *ptr) { trampoline_new(data, Size, ptr); }
     NB_INLINE ~trampoline() { trampoline_release(data, Size); }
 
     NB_INLINE handle base() const { return (PyObject *) data[0]; }

tests/test_classes.cpp

@@ -747,4 +747,34 @@ NB_MODULE(test_classes_ext, m) {
         .def_prop_ro_static("x", [](nb::handle /*unused*/) { return 42; });
     nb::class_<StaticPropertyOverride2, StaticPropertyOverride>(m, "StaticPropertyOverride2")
         .def_prop_ro_static("x", [](nb::handle /*unused*/) { return 43; });
+
+
+    // nanobind::detail::trampoline's constructor must be constexpr otherwise
+    // the trampoline will not compile under MSVC
+    struct ConstexprClass {
+        constexpr ConstexprClass(int i) : something(i) {}
+        virtual ~ConstexprClass() = default;
+
+        virtual int getInt() const {
+            return 1;
+        };
+
+        int something;
+    };
+
+    struct PyConstexprClass : ConstexprClass {
+        NB_TRAMPOLINE(ConstexprClass, 1);
+
+        int getInt() const override {
+            NB_OVERRIDE(getInt);
+        }
+    };
+
+    auto constexpr_class = nb::class_<ConstexprClass, PyConstexprClass>(m, "ConstexprClass")
+        .def(nb::init<int>())
+        .def("getInt", &ConstexprClass::getInt);
+
+    m.def("constexpr_call_getInt", [](ConstexprClass *c) {
+        return c->getInt();
+    });
 }

tests/test_classes.py

@@ -981,3 +981,11 @@ class SubSubClass(SubClass):
     # Clean up
     del x
     gc.collect()
+
+def test51_constexpr_trampoline():
+    class PyConstexprClass(t.ConstexprClass):
+        def getInt(self):
+            return 42
+
+    c = PyConstexprClass(4)
+    assert t.constexpr_call_getInt(c) == 42

tests/test_classes_ext.pyi.ref

@@ -371,3 +371,10 @@ class StaticPropertyOverride:
 class StaticPropertyOverride2(StaticPropertyOverride):
     x: int = ...
     """(arg: object, /) -> int"""
+
+class ConstexprClass:
+    def __init__(self, arg: int, /) -> None: ...
+
+    def getInt(self) -> int: ...
+
+def constexpr_call_getInt(arg: ConstexprClass, /) -> int: ...