Yep, another serious performance problem (also at google, not in chromium) was caused by inaccurate declaration of lambda arguments in an STL algorithm call … ie std::something(begin, end, [](std::pair<foo, bar> foobar) -> bool {}). The actual type (iterating over an unordered map, I believe) would have been const foo, but the compiler correctly concluded it could implicitly create a pair of foo,bar by copy from pair of const foo,bar. These were the days before such arguments could be declared “auto” which would have avoided the problem.

You have to think very very carefully about every line and character in C++ to figure out what it’s doing. Sometimes the easiest way to review it is to compile it and read the assembly.