[ianlancetaylor]

In this case the mismatch is between a type argument and the way that a type parameter is used, so it's not a type mismatch/error, it's a meta-type mismatch error. You are suggesting that the meta-type be inferred from the function (and any functions that it calls) rather than being explicitly stated. That is doable--C++ does it--but it means that the user has to understand the requirements of an implicitly inferred meta-type. The Go design draft suggests instead that the user be required to understand an explicitly stated contract.