Add -Wdeprecated to test suite and fix associated warnings (#1191)

This commit turns on `-Wdeprecated` in the test suite and fixes several
associated deprecation warnings that show up as a result:

- in C++17 `static constexpr` members are implicitly inline; our
  redeclaration (needed for C++11/14) is deprecated in C++17.

- various test suite classes have destructors and rely on implicit copy
  constructors, but implicit copy constructor definitions when a
  user-declared destructor is present was deprecated in C++11.

- Eigen also has various implicit copy constructors, so just disable
  `-Wdeprecated` in `eigen.h`.

include/pybind11/complex.h

@@ -25,9 +25,13 @@ template <typename T> struct format_descriptor<std::complex<T>, detail::enable_i
     static std::string format() { return std::string(value); }
 };
 
+#ifndef PYBIND11_CPP17
+
 template <typename T> constexpr const char format_descriptor<
     std::complex<T>, detail::enable_if_t<std::is_floating_point<T>::value>>::value[3];
 
+#endif
+
 NAMESPACE_BEGIN(detail)
 
 template <typename T> struct is_fmt_numeric<std::complex<T>, detail::enable_if_t<std::is_floating_point<T>::value>> {

include/pybind11/detail/common.h

@@ -704,9 +704,13 @@ template <typename T> struct format_descriptor<T, detail::enable_if_t<std::is_ar
     static std::string format() { return std::string(1, c); }
 };
 
+#if !defined(PYBIND11_CPP17)
+
 template <typename T> constexpr const char format_descriptor<
     T, detail::enable_if_t<std::is_arithmetic<T>::value>>::value[2];
 
+#endif
+
 /// RAII wrapper that temporarily clears any Python error state
 struct error_scope {
     PyObject *type, *value, *trace;

include/pybind11/eigen.h

@@ -17,6 +17,11 @@
 #  pragma GCC diagnostic push
 #  pragma GCC diagnostic ignored "-Wconversion"
 #  pragma GCC diagnostic ignored "-Wdeprecated-declarations"
+#  ifdef __clang__
+//   Eigen generates a bunch of implicit-copy-constructor-is-deprecated warnings with -Wdeprecated
+//   under Clang, so disable that warning here:
+#    pragma GCC diagnostic ignored "-Wdeprecated"
+#  endif
 #  if __GNUC__ >= 7
 #    pragma GCC diagnostic ignored "-Wint-in-bool-context"
 #  endif

include/pybind11/pytypes.h

@@ -295,6 +295,9 @@ public:
         PyErr_Fetch(&type.ptr(), &value.ptr(), &trace.ptr());
     }
 
+    error_already_set(const error_already_set &) = default;
+    error_already_set(error_already_set &&) = default;
+
     inline ~error_already_set();
 
     /// Give the currently-held error back to Python, if any.  If there is currently a Python error