Like Ada, C++ is standardised by ISO. With I suspect much the same consequence, ie there is a big gap between the ambition of the language's most important architects and what they can persuade the committee to sign off on for the standard.

It is informative for example to review talks Bjarne gave about C++ Concepts in 2015 compared to the C++ 20 feature Concepts. That's five years to get something similar in scope into the actual language standard, and of course since this isn't exactly what Bjarne designed, you then to have transition compilers and codebases to the actual standard feature...

I'm dubious about the value of conformance testing as a deliberate exercise. In practice real world implementations will definitely be defective and it's important that defects are cured when identified, mostly by changing implementations but sometimes in reality by changing the specification to reflect what is implemented. But I don't know how much value I believe there is in somebody sitting down to try to construct a test suite starting from the standards documents, rather than gathering minimal samples of real world code that did not do what the standard says it should do so that implementations can check they get those right (and where appropriate the standards body can reconsider whether the standard itself is wrong here). It appears to me that ACATS tries to do the former, right?