Removing MSVC C4100 from pragma block at the top of pybind11.h (#3150)

* Removing pragma for 4100 (to see what is still broken with the latest code).

* Adding --keep-going

* Revert "Adding --keep-going"

This reverts commit 1c844c6ffd07a6111b644811e7e3b0a50b9d44bb.

* Introducing PYBIND11_WORKAROUND_INCORRECT_MSVC_C4100.

* _MSC_VER <= 1916

* Replacing simple variadic function with variadic template (attempt to resolve MSVC 2017 failures).

* Preserving existing comment (moved from pybind11.h to detail/common.h).

* Adding blank lines for readability.

include/pybind11/attr.h

@@ -516,18 +516,22 @@ template <size_t Nurse, size_t Patient> struct process_attribute<keep_alive<Nurs
 /// Recursively iterate over variadic template arguments
 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);
     }
     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);
     }
     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);
     }
     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);
     }
@@ -545,6 +549,7 @@ template <typename... Extra,
           size_t named = constexpr_sum(std::is_base_of<arg, Extra>::value...),
           size_t self  = constexpr_sum(std::is_same<is_method, Extra>::value...)>
 constexpr bool expected_num_args(size_t nargs, bool has_args, bool has_kwargs) {
+    PYBIND11_WORKAROUND_INCORRECT_MSVC_C4100(nargs, has_args, has_kwargs);
     return named == 0 || (self + named + size_t(has_args) + size_t(has_kwargs)) == nargs;
 }
 

include/pybind11/cast.h

@@ -605,6 +605,7 @@ protected:
     /* Implementation: Convert a C++ tuple into a Python tuple */
     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);
         std::array<object, size> entries{{
             reinterpret_steal<object>(make_caster<Ts>::cast(std::get<Is>(std::forward<T>(src)), policy, parent))...
         }};

include/pybind11/detail/common.h

@@ -913,5 +913,18 @@ 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
+template <typename... Args>
+inline constexpr void workaround_incorrect_msvc_c4100(Args &&...) {}
+
+#    define PYBIND11_WORKAROUND_INCORRECT_MSVC_C4100(...)                                         \
+        detail::workaround_incorrect_msvc_c4100(__VA_ARGS__)
+
+#else
+#    define PYBIND11_WORKAROUND_INCORRECT_MSVC_C4100(...)
+#endif
+
 PYBIND11_NAMESPACE_END(detail)
 PYBIND11_NAMESPACE_END(PYBIND11_NAMESPACE)

include/pybind11/detail/descr.h

@@ -42,6 +42,7 @@ struct descr {
 template <size_t N1, size_t N2, typename... Ts1, typename... Ts2, size_t... Is1, size_t... Is2>
 constexpr descr<N1 + N2, Ts1..., Ts2...> plus_impl(const descr<N1, Ts1...> &a, const descr<N2, Ts2...> &b,
                                                    index_sequence<Is1...>, index_sequence<Is2...>) {
+    PYBIND11_WORKAROUND_INCORRECT_MSVC_C4100(b);
     return {a.text[Is1]..., b.text[Is2]...};
 }
 

include/pybind11/detail/init.h

@@ -94,6 +94,7 @@ void construct(...) {
 // construct an Alias from the returned base instance.
 template <typename Class>
 void construct(value_and_holder &v_h, Cpp<Class> *ptr, bool need_alias) {
+    PYBIND11_WORKAROUND_INCORRECT_MSVC_C4100(need_alias);
     no_nullptr(ptr);
     if (Class::has_alias && need_alias && !is_alias<Class>(ptr)) {
         // We're going to try to construct an alias by moving the cpp type.  Whether or not
@@ -131,6 +132,7 @@ void construct(value_and_holder &v_h, Alias<Class> *alias_ptr, bool) {
 // derived type (through those holder's implicit conversion from derived class holder constructors).
 template <typename Class>
 void construct(value_and_holder &v_h, Holder<Class> holder, bool need_alias) {
+    PYBIND11_WORKAROUND_INCORRECT_MSVC_C4100(need_alias);
     auto *ptr = holder_helper<Holder<Class>>::get(holder);
     no_nullptr(ptr);
     // If we need an alias, check that the held pointer is actually an alias instance
@@ -148,6 +150,7 @@ void construct(value_and_holder &v_h, Holder<Class> holder, bool need_alias) {
 // need it, we simply move-construct the cpp value into a new instance.
 template <typename Class>
 void construct(value_and_holder &v_h, Cpp<Class> &&result, bool need_alias) {
+    PYBIND11_WORKAROUND_INCORRECT_MSVC_C4100(need_alias);
     static_assert(std::is_move_constructible<Cpp<Class>>::value,
         "pybind11::init() return-by-value factory function requires a movable class");
     if (Class::has_alias && need_alias)

include/pybind11/detail/type_caster_base.h

@@ -930,6 +930,7 @@ protected:
        does not have a private operator new implementation. */
     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);
         return [](const void *arg) -> void * {
             return new T(*reinterpret_cast<const T *>(arg));
         };
@@ -937,6 +938,7 @@ protected:
 
     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);
         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(_MSC_VER) && !defined(__INTEL_COMPILER)
 #  pragma warning(push)
-#  pragma warning(disable: 4100) // warning C4100: Unreferenced formal parameter
 #  pragma warning(disable: 4127) // warning C4127: Conditional expression is constant
 #  pragma warning(disable: 4505) // warning C4505: 'PySlice_GetIndicesEx': unreferenced local function has been removed (PyPy only)
 #elif defined(__GNUG__) && !defined(__clang__) && !defined(__INTEL_COMPILER)
@@ -1388,12 +1387,14 @@ public:
 
     template <typename... Args, typename... Extra>
     class_ &def(const detail::initimpl::constructor<Args...> &init, const Extra&... extra) {
+        PYBIND11_WORKAROUND_INCORRECT_MSVC_C4100(init);
         init.execute(*this, extra...);
         return *this;
     }
 
     template <typename... Args, typename... Extra>
     class_ &def(const detail::initimpl::alias_constructor<Args...> &init, const Extra&... extra) {
+        PYBIND11_WORKAROUND_INCORRECT_MSVC_C4100(init);
         init.execute(*this, extra...);
         return *this;
     }