Enable -Wstrict-aliasing warning (#2816)

* Enable -Wstrict-aliasing warning

* Narrow down the scope of -Wstrict-aliasing

* Go home, MSVC, you're drunk

* Make sure "pragma GCC" is not executed on ICC
Co-authored-by: Yannick Jadoul <yannick.jadoul@belgacom.net>

include/pybind11/pybind11.h

@@ -35,7 +35,6 @@
 #  pragma GCC diagnostic ignored "-Wunused-but-set-parameter"
 #  pragma GCC diagnostic ignored "-Wunused-but-set-variable"
 #  pragma GCC diagnostic ignored "-Wmissing-field-initializers"
-#  pragma GCC diagnostic ignored "-Wstrict-aliasing"
 #  pragma GCC diagnostic ignored "-Wattributes"
 #  if __GNUC__ >= 7
 #    pragma GCC diagnostic ignored "-Wnoexcept-type"
@@ -49,10 +48,18 @@
 #include "detail/init.h"
 
 #include <memory>
+#include <new>
 #include <vector>
 #include <string>
 #include <utility>
 
+#if defined(__cpp_lib_launder) && !(defined(_MSC_VER) && (_MSC_VER < 1914))
+#  define PYBIND11_STD_LAUNDER std::launder
+#  define PYBIND11_HAS_STD_LAUNDER 1
+#else
+#  define PYBIND11_STD_LAUNDER
+#  define PYBIND11_HAS_STD_LAUNDER 0
+#endif
 #if defined(__GNUG__) && !defined(__clang__)
 #  include <cxxabi.h>
 #endif
@@ -151,8 +158,21 @@ protected:
 #if defined(__GNUG__) && __GNUC__ >= 6 && !defined(__clang__) && !defined(__INTEL_COMPILER)
 #  pragma GCC diagnostic pop
 #endif
+#if defined(__GNUG__) && !PYBIND11_HAS_STD_LAUNDER && !defined(__INTEL_COMPILER)
+#  pragma GCC diagnostic push
+#  pragma GCC diagnostic ignored "-Wstrict-aliasing"
+#endif
+            // UB without std::launder, but without breaking ABI and/or
+            // a significant refactoring it's "impossible" to solve.
             if (!std::is_trivially_destructible<Func>::value)
-                rec->free_data = [](function_record *r) { ((capture *) &r->data)->~capture(); };
+                rec->free_data = [](function_record *r) {
+                    auto data = PYBIND11_STD_LAUNDER((capture *) &r->data);
+                    (void) data;
+                    data->~capture();
+                };
+#if defined(__GNUG__) && !PYBIND11_HAS_STD_LAUNDER && !defined(__INTEL_COMPILER)
+#  pragma GCC diagnostic pop
+#endif
         } else {
             rec->data[0] = new capture { std::forward<Func>(f) };
             rec->free_data = [](function_record *r) { delete ((capture *) r->data[0]); };