E2392 Template instance 'template' is already instantiated (C++)
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There are two ways to trigger this error. If -A is enabled (ANSI compliant mode), then attempting to explicitly instantiate a template specialization which has already been instantiated (either implicitly or explicitly) will cause this error. Regardless of -A, attempting to explicitly specialize a template specialization which has already been either implicit or explicitly instantiated will always trigger this error. For example:
template<class T> class foo; foo<char> x; // causes implicit instantiation of "foo<char>" template<> class foo<char> { }; // error: "foo<char>" already instantiated template class foo<char>; // error in -A mode, otherwise a warning