Believed to come from this test:

TYPED_TEST(SorterTypedTest, DuplicateValues) {    
    KeyList input = makeInputData(kSmallNumberOfKeys, ShuffleMode::kShuffle);
    input.insert(input.end(), input.begin(), input.end());
    this->assertSortAndMerge(this->opts(), input);
}

Inserting to a vector using its own iterators I think breaks with debug iterators. Validating this is the cause and if so, simple fix. Not an issue with the sorter as the issue is in test data generation and not the sorter itself.