py/obj: Fix mp_obj_is_type compilation with C++.

Fixes issue #12951.

Signed-off-by: stijn <stijn@ignitron.net>

3 files changed, 19 insertions, 7 deletions

diff --git a/examples/usercmodule/cppexample/example.cpp b/examples/usercmodule/cppexample/example.cpp
index 06809732a..2df832baa 100644
--- a/examples/usercmodule/cppexample/example.cpp
+++ b/examples/usercmodule/cppexample/example.cpp
@@ -1,9 +1,16 @@
 extern "C" {
 #include <examplemodule.h>
+#include <py/objstr.h>
 
 // Here we implement the function using C++ code, but since it's
 // declaration has to be compatible with C everything goes in extern "C" scope.
 mp_obj_t cppfunc(mp_obj_t a_obj, mp_obj_t b_obj) {
+    // The following no-ops are just here to verify the static assertions used in
+    // the public API all compile with C++.
+    MP_STATIC_ASSERT_STR_ARRAY_COMPATIBLE;
+    if (mp_obj_is_type(a_obj, &mp_type_BaseException)) {
+    }
+
     // Prove we have (at least) C++11 features.
     const auto a = mp_obj_get_int(a_obj);
     const auto b = mp_obj_get_int(b_obj);
diff --git a/py/misc.h b/py/misc.h
index 352a9f34c..e19a53572 100644
--- a/py/misc.h
+++ b/py/misc.h
@@ -52,10 +52,15 @@ typedef unsigned int uint;
 
 // Static assertion macro
 #define MP_STATIC_ASSERT(cond) ((void)sizeof(char[1 - 2 * !(cond)]))
-#if defined(_MSC_VER)
-#define MP_STATIC_ASSERT_NOT_MSC(cond) (1)
+// In C++ things like comparing extern const pointers are not constant-expressions so cannot be used
+// in MP_STATIC_ASSERT. Note that not all possible compiler versions will reject this. Some gcc versions
+// do, others only with -Werror=vla, msvc always does.
+// The (void) is needed to avoid "left operand of comma operator has no effect [-Werror=unused-value]"
+// when using this macro on the left-hand side of a comma.
+#if defined(_MSC_VER) || defined(__cplusplus)
+#define MP_STATIC_ASSERT_NONCONSTEXPR(cond) ((void)1)
 #else
-#define MP_STATIC_ASSERT_NOT_MSC(cond) MP_STATIC_ASSERT(cond)
+#define MP_STATIC_ASSERT_NONCONSTEXPR(cond) MP_STATIC_ASSERT(cond)
 #endif
 
 // Round-up integer division
diff --git a/py/obj.h b/py/obj.h
index 56ae62172..c7b7db0c3 100644
--- a/py/obj.h
+++ b/py/obj.h
@@ -928,10 +928,10 @@ void *mp_obj_malloc_helper(size_t num_bytes, const mp_obj_type_t *type);
 // optimizations (other tricks like using ({ expr; exper; }) or (exp, expr, expr) in mp_obj_is_type() result
 // in missed optimizations)
 #define mp_type_assert_not_bool_int_str_nonetype(t) (                                     \
-    MP_STATIC_ASSERT_NOT_MSC((t) != &mp_type_bool), assert((t) != &mp_type_bool),         \
-    MP_STATIC_ASSERT_NOT_MSC((t) != &mp_type_int), assert((t) != &mp_type_int),           \
-    MP_STATIC_ASSERT_NOT_MSC((t) != &mp_type_str), assert((t) != &mp_type_str),           \
-    MP_STATIC_ASSERT_NOT_MSC((t) != &mp_type_NoneType), assert((t) != &mp_type_NoneType), \
+    MP_STATIC_ASSERT_NONCONSTEXPR((t) != &mp_type_bool), assert((t) != &mp_type_bool),         \
+    MP_STATIC_ASSERT_NONCONSTEXPR((t) != &mp_type_int), assert((t) != &mp_type_int),           \
+    MP_STATIC_ASSERT_NONCONSTEXPR((t) != &mp_type_str), assert((t) != &mp_type_str),           \
+    MP_STATIC_ASSERT_NONCONSTEXPR((t) != &mp_type_NoneType), assert((t) != &mp_type_NoneType), \
     1)
 
 #define mp_obj_is_type(o, t) (mp_type_assert_not_bool_int_str_nonetype(t) && mp_obj_is_exact_type(o, t))