check for odr violations and fix odr violation (#985)

Closes #976 

This fixes an ODR violation and adds more warnings regarding ODR
violations to the standard build flags (which are treated as errors).

CMakeLists.txt

@@ -85,8 +85,20 @@ set(SOURCES_B
 pybind11_add_module(_core MODULE ${SOURCES_A} ${SOURCES_B})
 if(MSVC)
   target_compile_options(_core PRIVATE /std:c++14 /Y-)
+elseif(CMAKE_CXX_COMPILER_ID STREQUAL "GNU")
+  # lots of warnings from gcc, including odr violations
+  target_compile_options(
+    _core
+    PRIVATE -Wall
+            -Wextra
+            -pedantic
+            -Werror
+            -fstrict-aliasing
+            -Werror=odr
+            -Werror=lto-type-mismatch
+            -Werror=strict-aliasing)
 else()
-  # lots of warnings from clang and gcc
+  # lots of warnings from clang
   target_compile_options(_core PRIVATE -Wall -Wextra -pedantic -Werror
                                        -fstrict-aliasing)
 endif()

src/type_caster.hpp

@@ -1,14 +1,13 @@
-#include <algorithm>
 #include <pybind11/numpy.h>
 #include <pybind11/pybind11.h>
 #include <vector>
 
 namespace pybind11 {
 namespace detail {
 
-template <typename Value>
-struct type_caster<std::vector<Value>> {
-  using vec_t = std::vector<Value>;
+template <typename Value, typename Alloc>
+struct type_caster<std::vector<Value, Alloc>> {
+  using vec_t = std::vector<Value, Alloc>;
   using value_conv = make_caster<Value>;
   using size_conv = make_caster<std::size_t>;