1729643 - Use `if constexpr (...)` for template with enum parameter

Status: NEW

(Core :: JavaScript Engine, enhancement, P5)

Description

[Tooru Fujisawa [:arai]]

XDR functions uses XDRMode template parameter, and uses it as condition for encode/decode branches.

example code from https://phabricator.services.mozilla.com/D121274

template <XDRMode mode>
/* static */
XDRResult StencilXDR::codeSharedData(XDRState<mode>* xdr,
                                     RefPtr<SharedImmutableScriptData>& sisd) {
...
  if constexpr (mode == XDR_ENCODE) {
...
  } else {
...
    const auto& options = static_cast<XDRStencilDecoder*>(xdr)->options();
    if (options.usePinnedBytecode) {

using if constexpr for the branch allows using static_cast there,
(XDRStencilDecoder is subclass of XDRState<XDR_DECODE>),
and similar simplification can be done for some other places.