SUBCLASSPROC

Undocumented in source but is binding to Windows. You might be able to learn more by searching the web for its name.
nothrow extern (Windows)
static if(_WIN32_WINNT >= 0x501)
alias SUBCLASSPROC = LRESULT function(HWND, UINT, WPARAM, LPARAM, UINT_PTR, DWORD_PTR)

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