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    Are all these things prototyped and tested in realistic use cases prior to being finalized in the standard?

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      gcc, clang, and msvc often have these features hidden behind experimental flags before the proposals are even approved, so the features certainly get tested in running code, and often also in production as the link from @tt points out:

      Most major C++20 features have been implemented in essentially their current draft standard form in at least one shipping compiler, and in most cases actually used in production code (i.e., has already been released to customers who are very happy with it). For example, coroutines (adopted only five months ago as of this writing) has been used in production in MSVC for two years and in Clang for at least a year with very happy customers at scale (e.g., Azure, Facebook).