[cfe-dev] Folding constexpr function calls with constant arguments in non constant expression contexts

[Roger Ferrer Ibanez via cfe-dev]

Hi all,

maybe this is just a silly question. But I wondered why clang is not attempting to fold calls to constexpr functions in non-constant expression contexts.

Example:  https://godbolt.org/g/KBaMjp just compare "one" vs "two" in the IR

I understand that clang does not have to do this and I'm not saying it should.

But I'd like to know what is the rationale here.
- Is it a C++ Standard issue?, i.e. doing that would trigger some side-effects that would be non-compliant.
- Is it a "separation of concerns" issue?, i.e. let LLVM optimize these cases and improve if it can't.
- Is it a speed issue?, i.e. even if the original AST is preserved for fidelity, computing an associated constant value to it takes time which is precious for some clients.

Or maybe it is something else that didn't occur to me.

Thank you,
Roger
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