Happens on all recent builds. Seemingly unrelated to any code changes.

Last pass: https://ci.appveyor.com/project/DrahtBot/bitcoin/builds/30240770
First fail: https://ci.appveyor.com/project/DrahtBot/bitcoin/builds/30241275
First fail on master: https://ci.appveyor.com/project/DrahtBot/bitcoin/builds/30241926

The first fail happened on the 0.19 branch. Is the appveyor cache shared across branches?

Otherwise it could be a msvc update or another dependency that broke it. If so, I wish we figure out a way to freeze the dependencies and compileres in some way.


```
C:\projects\bitcoin\src\test\cuckoocache_tests.cpp(330,10): error C2976: 'std::deque': too few template arguments [C:\projects\bitcoin\build_msvc\test_bitcoin\test_bitcoin.vcxproj]
C:\projects\bitcoin\src\test\cuckoocache_tests.cpp(330,1): error C2079: 'last_few' uses undefined class 'std::deque' [C:\projects\bitcoin\build_msvc\test_bitcoin\test_bitcoin.vcxproj]
C:\projects\bitcoin\src\test\cuckoocache_tests.cpp(341,1): error C2672: 'begin': no matching overloaded function found [C:\projects\bitcoin\build_msvc\test_bitcoin\test_bitcoin.vcxproj]
C:\projects\bitcoin\src\test\cuckoocache_tests.cpp(341,1): error C2893: Failed to specialize function template 'unknown-type std::begin(_Container &)' [C:\projects\bitcoin\build_msvc\test_bitcoin\test_bitcoin.vcxproj]
C:\projects\bitcoin\src\test\cuckoocache_tests.cpp(341,1): error C2784: 'const _Elem *std::begin(std::initializer_list<_Elem>) noexcept': could not deduce template argument for 'std::initializer_list<_Elem>' from 'int' [C:\projects\bitcoin\build_msvc\test_bitcoin\test_bitcoin.vcxproj]
C:\projects\bitcoin\src\test\cuckoocache_tests.cpp(341,1): error C2672: 'end': no matching overloaded function found [C:\projects\bitcoin\build_msvc\test_bitcoin\test_bitcoin.vcxproj]
C:\projects\bitcoin\src\test\cuckoocache_tests.cpp(341,1): error C2893: Failed to specialize function template 'unknown-type std::end(_Container &)' [C:\projects\bitcoin\build_msvc\test_bitcoin\test_bitcoin.vcxproj]
C:\projects\bitcoin\src\test\cuckoocache_tests.cpp(341,1): error C2784: 'const _Elem *std::end(std::initializer_list<_Elem>) noexcept': could not deduce template argument for 'std::initializer_list<_Elem>' from 'int' [C:\projects\bitcoin\build_msvc\test_bitcoin\test_bitcoin.vcxproj]
C:\projects\bitcoin\src\test\cuckoocache_tests.cpp(341,1): error C3536: '<begin>$L0': cannot be used before it is initialized [C:\projects\bitcoin\build_msvc\test_bitcoin\test_bitcoin.vcxproj]
C:\projects\bitcoin\src\test\cuckoocache_tests.cpp(341,1): error C3536: '<end>$L0': cannot be used before it is initialized [C:\projects\bitcoin\build_msvc\test_bitcoin\test_bitcoin.vcxproj]
C:\projects\bitcoin\src\test\cuckoocache_tests.cpp(341,1): error C2100: illegal indirection [C:\projects\bitcoin\build_msvc\test_bitcoin\test_bitcoin.vcxproj]
boost::unique_lock<boost::shared_mutex> l(mtx);
        
        for (uint32_t i = 0; i < (n_insert / 2); ++i)
            set.insert(hashes_insert_copy[i]);
    }
    /** Spin up 3 threads to run contains with erase.
     */
    std::vector<std::thread> threads;
    
    for (uint32_t x = 0; x < 3; ++x)
        /** Each thread is emplaced with x copy-by-value
        */
        threads.emplace_back([&, x] {
            boost::shared_lock<boost::shared_mutex> l(mtx);
            size_t ntodo = (n_insert/4)/3;
            size_t start = ntodo*x;
            size_t end = ntodo*(x+1);
            for (uint32_t i = start; i < end; ++i) {
                bool contains = set.contains(hashes[i], true);
                assert(contains);
            }
        });
    /** Wait for all threads to finish
     */
    for (std::thread& t : threads)
        t.join();
    
    boost::unique_lock<boost::shared_mutex> l(mtx);