The mingw-w64-headers/include/oleauto.h
contains
WINOLEAUTAPI VarDecFromStr(OLECHAR *strIn,LCID lcid,ULONG dwFlags,DECIMAL *pdecOut);
but the valid prototype of this function is
HRESULT VarDecFromStr(
[in] LPCOLESTR strIn,
[in] LCID lcid,
[in] ULONG dwFlags,
[out] DECIMAL *pdecOut
);
That is. OLECHAR *strIn
should be changed to LPCOLESTR
to avoid compile errors like
error: invalid conversion
from 'LPCOLESTR' {aka 'const wchar_t*'}
to 'OLECHAR*' {aka 'wchar_t*'} [-fpermissive]
Please send this to the mingw-w64-public mailing list. Very unfortunately, the issue tracker doesn't get much attention.
Apparently, this has been patched and fixed, see
https://sourceforge.net/p/mingw-w64/mailman/message/37670051/