Removing GCC -Wunused-but-set-parameter from pragma block at the top of pybind11.h (#3164)

* Cleanup triggered by work on pragma for GCC -Wunused-but-set-parameter.

* Backing out changes to eigen.h (to be worked on later).

* Adding PYBIND11_WORKAROUND_INCORRECT_GCC_UNUSED_BUT_SET_PARAMETER in type_caster_base.h (apparently needed only for older GCCs).

* Apparently older compilers need a simpler overload for silence_unused_warnings().

* clang C++11 compatibility: removing constexpr

* Special case for MSVC 2017: `constexpr void` return

* Trying again without the silence_unused_warnings(const int *) overload.

* Separate macros for ALL_GCC, OLD_GCC_UNUSED_BUT_SET_PARAMETER

* Changing to __GNUC__ <= 2 (turning off)

* Refined condition for PYBIND11_WORKAROUND_INCORRECT_OLD_GCC_UNUSED_BUT_SET_PARAMETER.

* Quick experiment trying out suggestion by @henryiii

* Introducing macro: PYBIND11_INT_ARRAY_WORKING_AROUND_MSVC_CLANG_ISSUES

* Trying henryiii@ (void) expander idea.

* fix: apply simpler expression with fewer workarounds

* Purging new-but-already-obsoleted  macro, made possible by @henryiii's commit.

* Renaming `ALL_GCC` macro back to just `GCC` (because there is no `OLD` anymore, luckily).

* [actions skip] Adding "All GCC versions" to comment, to be clear about it.
Co-authored-by: Henry Schreiner <henryschreineriii@gmail.com>

include/pybind11/attr.h

@@ -517,23 +517,30 @@ template <size_t Nurse, size_t Patient> struct process_attribute<keep_alive<Nurs
 template <typename... Args> struct process_attributes {
     static void init(const Args&... args, function_record *r) {
         PYBIND11_WORKAROUND_INCORRECT_MSVC_C4100(r);
-        int unused[] = { 0, (process_attribute<typename std::decay<Args>::type>::init(args, r), 0) ... };
-        ignore_unused(unused);
+        PYBIND11_WORKAROUND_INCORRECT_GCC_UNUSED_BUT_SET_PARAMETER(r);
+        using expander = int[];
+        (void) expander{
+            0, ((void) process_attribute<typename std::decay<Args>::type>::init(args, r), 0)...};
     }
     static void init(const Args&... args, type_record *r) {
         PYBIND11_WORKAROUND_INCORRECT_MSVC_C4100(r);
-        int unused[] = { 0, (process_attribute<typename std::decay<Args>::type>::init(args, r), 0) ... };
-        ignore_unused(unused);
+        PYBIND11_WORKAROUND_INCORRECT_GCC_UNUSED_BUT_SET_PARAMETER(r);
+        using expander = int[];
+        (void) expander{0,
+                        (process_attribute<typename std::decay<Args>::type>::init(args, r), 0)...};
     }
     static void precall(function_call &call) {
         PYBIND11_WORKAROUND_INCORRECT_MSVC_C4100(call);
-        int unused[] = { 0, (process_attribute<typename std::decay<Args>::type>::precall(call), 0) ... };
-        ignore_unused(unused);
+        using expander = int[];
+        (void) expander{0,
+                        (process_attribute<typename std::decay<Args>::type>::precall(call), 0)...};
     }
     static void postcall(function_call &call, handle fn_ret) {
         PYBIND11_WORKAROUND_INCORRECT_MSVC_C4100(call, fn_ret);
-        int unused[] = { 0, (process_attribute<typename std::decay<Args>::type>::postcall(call, fn_ret), 0) ... };
-        ignore_unused(unused);
+        PYBIND11_WORKAROUND_INCORRECT_GCC_UNUSED_BUT_SET_PARAMETER(fn_ret);
+        using expander = int[];
+        (void) expander{
+            0, (process_attribute<typename std::decay<Args>::type>::postcall(call, fn_ret), 0)...};
     }
 };
 

include/pybind11/cast.h

@@ -609,6 +609,7 @@ protected:
     template <typename T, size_t... Is>
     static handle cast_impl(T &&src, return_value_policy policy, handle parent, index_sequence<Is...>) {
         PYBIND11_WORKAROUND_INCORRECT_MSVC_C4100(src, policy, parent);
+        PYBIND11_WORKAROUND_INCORRECT_GCC_UNUSED_BUT_SET_PARAMETER(policy, parent);
         std::array<object, size> entries{{
             reinterpret_steal<object>(make_caster<Ts>::cast(std::get<Is>(std::forward<T>(src)), policy, parent))...
         }};
@@ -1235,8 +1236,8 @@ public:
         // Tuples aren't (easily) resizable so a list is needed for collection,
         // but the actual function call strictly requires a tuple.
         auto args_list = list();
-        int _[] = { 0, (process(args_list, std::forward<Ts>(values)), 0)... };
-        ignore_unused(_);
+        using expander = int[];
+        (void) expander{0, (process(args_list, std::forward<Ts>(values)), 0)...};
 
         m_args = std::move(args_list);
     }

include/pybind11/detail/common.h

@@ -734,9 +734,6 @@ using function_signature_t = conditional_t<
 template <typename T> using is_lambda = satisfies_none_of<remove_reference_t<T>,
         std::is_function, std::is_pointer, std::is_member_pointer>;
 
-/// Ignore that a variable is unused in compiler warnings
-inline void ignore_unused(const int *) { }
-
 // [workaround(intel)] Internal error on fold expression
 /// Apply a function over each element of a parameter pack
 #if defined(__cpp_fold_expressions) && !defined(__INTEL_COMPILER)
@@ -927,19 +924,29 @@ inline static std::shared_ptr<T> try_get_shared_from_this(std::enable_shared_fro
 #endif
 }
 
-#if defined(_MSC_VER) && _MSC_VER <= 1916
-
-// warning C4100: Unreferenced formal parameter
+// For silencing "unused" compiler warnings in special situations.
 template <typename... Args>
-inline constexpr void workaround_incorrect_msvc_c4100(Args &&...) {}
+#if defined(_MSC_VER) && _MSC_VER >= 1910 && _MSC_VER < 1920 // MSVC 2017
+constexpr
+#endif
+inline void silence_unused_warnings(Args &&...) {}
 
+// MSVC warning C4100: Unreferenced formal parameter
+#if defined(_MSC_VER) && _MSC_VER <= 1916
 #    define PYBIND11_WORKAROUND_INCORRECT_MSVC_C4100(...)                                         \
-        detail::workaround_incorrect_msvc_c4100(__VA_ARGS__)
-
+        detail::silence_unused_warnings(__VA_ARGS__)
 #else
 #    define PYBIND11_WORKAROUND_INCORRECT_MSVC_C4100(...)
 #endif
 
+// GCC -Wunused-but-set-parameter  All GCC versions (as of July 2021).
+#if defined(__GNUG__) && !defined(__clang__) && !defined(__INTEL_COMPILER)
+#    define PYBIND11_WORKAROUND_INCORRECT_GCC_UNUSED_BUT_SET_PARAMETER(...)                   \
+        detail::silence_unused_warnings(__VA_ARGS__)
+#else
+#    define PYBIND11_WORKAROUND_INCORRECT_GCC_UNUSED_BUT_SET_PARAMETER(...)
+#endif
+
 #if defined(_MSC_VER) // All versions (as of July 2021).
 
 // warning C4127: Conditional expression is constant

include/pybind11/detail/type_caster_base.h

@@ -927,18 +927,17 @@ protected:
     using Constructor = void *(*)(const void *);
 
     /* Only enabled when the types are {copy,move}-constructible *and* when the type
-       does not have a private operator new implementation. */
+       does not have a private operator new implementation. A comma operator is used in the decltype
+       argument to apply SFINAE to the public copy/move constructors.*/
     template <typename T, typename = enable_if_t<is_copy_constructible<T>::value>>
-    static auto make_copy_constructor(const T *x) -> decltype(new T(*x), Constructor{}) {
-        PYBIND11_WORKAROUND_INCORRECT_MSVC_C4100(x);
+    static auto make_copy_constructor(const T *) -> decltype(new T(std::declval<const T>()), Constructor{}) {
         return [](const void *arg) -> void * {
             return new T(*reinterpret_cast<const T *>(arg));
         };
     }
 
     template <typename T, typename = enable_if_t<std::is_move_constructible<T>::value>>
-    static auto make_move_constructor(const T *x) -> decltype(new T(std::move(*const_cast<T *>(x))), Constructor{}) {
-        PYBIND11_WORKAROUND_INCORRECT_MSVC_C4100(x);
+    static auto make_move_constructor(const T *) -> decltype(new T(std::declval<T&&>()), Constructor{}) {
         return [](const void *arg) -> void * {
             return new T(std::move(*const_cast<T *>(reinterpret_cast<const T *>(arg))));
         };

include/pybind11/pybind11.h

@@ -12,7 +12,6 @@
 
 #if defined(__GNUG__) && !defined(__clang__) && !defined(__INTEL_COMPILER)
 #  pragma GCC diagnostic push
-#  pragma GCC diagnostic ignored "-Wunused-but-set-parameter"
 #  pragma GCC diagnostic ignored "-Wattributes"
 #endif