Fix pair::swap(const pair&) and tuple::swap(const tuple&) errors with __declspec(dllexport)
#3045
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strega-nil-ms
approved these changes
Aug 19, 2022
CaseyCarter
approved these changes
Aug 19, 2022
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I'm mirroring this to the MSVC-internal repo - please notify me if any further changes are pushed. |
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I think this looks good. |
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Thanks for fixing this pair of issues - I'm sure our customers were about to pull their hair out! |
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Fixes #3013, a high-priority customer-reported bug (see DevCom-10109884).
Normally, member functions of class templates are instantiated "on demand". (I like to remember this rule by thinking of
list<T>, which does not requireTto be less-than comparable untillist<T>::sort()is called.) However, if a user derives from a Standard Library class (which we don't encourage, but which is widely done) and then dllexports their derived class, that attempts to instantiate all member functions (that are not templated themselves).By adding member functions (as required by the Standard) that won't compile when force-instantiated like this, #2687 regressed this scenario. The fix is simple (and something we've used elsewhere): add
template <int = 0>to make instantiation truly on-demand. Conveniently, we already have a test we can expand.(Note: I explored using
requiresto fix this, but that's more verbose and ran into other compiler errors.)