[clang] [Clang] Eagerly instantiate used constexpr function upon definition. (PR #73463)

[via cfe-commits]

cor3ntin wrote:

> Does this have any compile-time impact?

Nope, should be ~free. It does add 1 DenseMap lookup per constexpr function definition, which ought to be negligible.

https://github.com/llvm/llvm-project/pull/73463