Factoid: Each class template instantiation costs 1KiB

[AaronBallman]

@vvassilev

Clang was built under the assumption that the AST is immutable. However, that’s limiting in quite a few places (ASTImporter, lldb, other tools that work on high-level and synthesize code). I wonder if we can relax the assumption of immutable AST and make it mutable in the template instantiation area.

Given that this has been the assumption in Clang for 15+ years, I would be pretty wary of trying to walk that back now. That has impacts on tooling, but also, reimplementing the template instantiation engine is something I think we want to avoid for as long as we can due to the risk (silent ABI changes due to template instantiation differences are a serious concern).