[Macros] Disallow expression macro as default argument

[ApolloZhu]

Add error diagnostic when an expression macro is used as the default argument per SE-0382. Using a macro as a sub expression in default argument position is still allowed as expression macros behave the same as built-in magic literals.

rdar://115674594

[ApolloZhu]

@swift-ci please test

[tevelee]

Can't we fix the behavior (by propagating source location information to the callsite) instead of turning it off? It would be absolutely great to have this working.

Holly solved the expansion of magic literals to accurately represent values in the outermost source file. Can we take this a step further and process default arguments as well?

[ApolloZhu]

Hi @tevelee, I agree that allowing non-built-in macro as default argument could be an interesting future direction. But as reasoned in the original proposal, we should disable it right now as specified, and go through Swift Evolution process to make sure that developers won't be surprised by when the macros expand:

With the exception of the built-in macro declarations for source locations (e.g., #fileID, #line), a macro cannot be used as the default argument of a parameter. The existing features for source locations have special behavior when they appear as a default argument, wherein they are expanded by the caller using the source-location information at the call site rather than in the function declaration where they appear. This is useful, existing behavior that we cannot change, but it might not make sense for all macros, and could be surprising. Therefore, we prohibit such default argument that are (non-built-in) macros to avoid confusion, and are open to revisiting this restriction in the future.

[ApolloZhu]

@swift-ci please smoke test

[tevelee]

Thanks for the context, @ApolloZhu! I missed this during the review of the proposal, your PR makes perfect sense now.