Windows shared build broken

[brechtsanders]

Since version 1.39.5 building with -DBUILD_SHARED_LIBS:BOOL=ON on Windows (in my case with GCC+MinGW-w64) fails.

The reason is that in source/MaterialXCore/Value.h there is a template class

template <class T> class TypedValue : public Value

whose functions are defined with MX_CORE_API, which is in that case defined as __declspec(dllexport).

This results in the following build errors:

In file included from R:/winlibs16.1.0msvcrt64/MaterialX-1.39.5/source/MaterialXCore/Element.h:16,
                 from R:/winlibs16.1.0msvcrt64/MaterialX-1.39.5/source/MaterialXCore/Geom.h:14,
                 from R:/winlibs16.1.0msvcrt64/MaterialX-1.39.5/source/MaterialXCore/Interface.h:14,
                 from R:/winlibs16.1.0msvcrt64/MaterialX-1.39.5/source/MaterialXCore/Definition.h:14,
                 from R:/winlibs16.1.0msvcrt64/MaterialX-1.39.5/source/MaterialXCore/Node.h:14,
                 from R:/winlibs16.1.0msvcrt64/MaterialX-1.39.5/source/MaterialXCore/Material.h:14,
                 from R:/winlibs16.1.0msvcrt64/MaterialX-1.39.5/source/MaterialXCore/Look.h:14,
                 from R:/winlibs16.1.0msvcrt64/MaterialX-1.39.5/source/MaterialXCore/Document.h:14:
R:/winlibs16.1.0msvcrt64/MaterialX-1.39.5/source/MaterialXCore/Value.h:130:17: error: function 'MaterialX_v1_39_5::TypedValue<T>::TypedValue()' definition is marked dllimport [-Wtemplate-body]
  130 |     MX_CORE_API TypedValue() :
      |                 ^~~~~~~~~~
R:/winlibs16.1.0msvcrt64/MaterialX-1.39.5/source/MaterialXCore/Value.h:134:26: error: function 'MaterialX_v1_39_5::TypedValue<T>::TypedValue(const T&)' definition is marked dllimport [-Wtemplate-body]
  134 |     MX_CORE_API explicit TypedValue(const T& value) :
      |                          ^~~~~~~~~~
R:/winlibs16.1.0msvcrt64/MaterialX-1.39.5/source/MaterialXCore/Value.h:138:25: error: function 'MaterialX_v1_39_5::TypedValue<T>::~TypedValue()' definition is marked dllimport [-Wtemplate-body]
  138 |     MX_CORE_API virtual ~TypedValue() { }
      |                         ^
R:/winlibs16.1.0msvcrt64/MaterialX-1.39.5/source/MaterialXCore/Value.h:141:26: error: function 'MaterialX_v1_39_5::ValuePtr MaterialX_v1_39_5::TypedValue<T>::copy() const' definition is marked dllimport [-Wtemplate-body]
  141 |     MX_CORE_API ValuePtr copy() const override
      |                          ^~~~
R:/winlibs16.1.0msvcrt64/MaterialX-1.39.5/source/MaterialXCore/Value.h:147:22: error: function 'void MaterialX_v1_39_5::TypedValue<T>::setData(const T&)' definition is marked dllimport [-Wtemplate-body]
  147 |     MX_CORE_API void setData(const T& value)
      |                      ^~~~~~~
R:/winlibs16.1.0msvcrt64/MaterialX-1.39.5/source/MaterialXCore/Value.h:153:22: error: function 'void MaterialX_v1_39_5::TypedValue<T>::setData(const MaterialX_v1_39_5::TypedValue<T>&)' definition is marked dllimport [-Wtemplate-body]
  153 |     MX_CORE_API void setData(const TypedValue<T>& value)
      |                      ^~~~~~~
R:/winlibs16.1.0msvcrt64/MaterialX-1.39.5/source/MaterialXCore/Value.h:159:26: error: function 'const T& MaterialX_v1_39_5::TypedValue<T>::getData() const' definition is marked dllimport [-Wtemplate-body]
  159 |     MX_CORE_API const T& getData() const
      |                          ^~~~~~~
R:/winlibs16.1.0msvcrt64/MaterialX-1.39.5/source/MaterialXCore/Value.h:171:22: error: function 'bool MaterialX_v1_39_5::TypedValue<T>::isEqual(MaterialX_v1_39_5::ConstValuePtr) const' definition is marked dllimport [-Wtemplate-body]
  171 |     MX_CORE_API bool isEqual(ConstValuePtr other) const override
      |                      ^~~~~~~
ninja: build stopped: subcommand failed.

[jstone-lucasfilm]

This sounds like a good catch, @brechtsanders, and although we have a GitHub CI entry for Windows shared library builds using MSVC, we don't have a corresponding GitHub CI entry for Windows shared library builds using MinGW.

It sounds like we should address both issues in a single PR, fixing the underlying problem in MaterialXCore, and adding a new GitHub CI entry that validates the appropriate combination of features in MinGW.