dmd.expressionsem: Call copy() on default argument from CallExp semantic

[ibuclaw]

With this, the only entrypoint into the front-end inliner would be dmd.mars.

As of #3885, CallExp semantic no longer does early lowering of struct literal constructors into a DeclarationExp, which triggered the backend ICE in bugzilla 2935 - if there is any lowering required, that is appropriately dealt with by functionParameters() after a copy of defaultArg has been made.

So it is now safe to do a plain copy, instead of implicitly calling the front-end inliner to perform a deep copy of the node.

As noted in the original bugzilla issue, a visitor/walker would also suffice to do a deep copy of any nested DeclarationExp's, but I don't see that as necessary anymore as that possibility looks to be no longer reachable.

CC: @WalterBright - because you were the one who cried "Danger, Will Robinson!"

This partiallly reverts commit b5ea872.

[dlang-bot]

Thanks for your pull request, @ibuclaw!

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Testing this PR locally

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dub run digger -- build "master + dmd#14214"

[ibuclaw]

Superseded by #14309.