If both are used, 'else' handles named members and '_' handles
unnamed members. In this case the 'else' prong will be unrolled
to an explicit case containing all remaining named values.
Mainly affects ZIR representation of switch_block[_ref]
and special prong (detection) logic for switch.
Adds a new SpecialProng tag 'absorbing_under' that allows
specifying additional explicit tags in a '_' prong which
are respected when checking that every value is handled
during semantic analysis but are not transformed into AIR
and instead 'absorbed' by the '_' branch.
This reverts commit b461d07a54.
After some discussion in the team, we've decided that this is too disruptive,
especially because the linker errors are less than helpful. That's a fixable
problem, so we might reconsider this in the future, but revert it for now.
Wow, *lots* of backends were reliant on Sema doing the heavy lifting for
them. CBE, Wasm, and SPIR-V have all regressed in places now that they
actually need to, like, initialize unions and such.
This logic predates certain Sema enhancements whose behavior it
essentially tries to emulate in one specific case in a problematic way.
In particular, this logic handled initializing comptime-known `const`s
through RLS, which was reworked a few years back in 644041b to not rely
on this logic, and catching runtime fields in comptime-only
initializers, which has since been *correctly* fixed with better checks
in `Sema.storePtr2`. That made the highly complex logic in
`validateStructInit`, `validateUnionInit`, and `zirValidatePtrArrayInit`
entirely redundant. Worse, it was also causing some tracked bugs, as
well as a bug which I have identified and fixed in this PR (a
corresponding behavior test is added).
This commit simplifies union initialization by bringing the runtime
logic more in line with the comptime logic: the tag is now always
populated by `Sema.unionFieldPtr` based on `initializing`, where this
previously happened only in the comptime case (with `validateUnionInit`
instead handling it in the runtime case). Notably, this means that
backends are now able to consider getting a pointer to an inactive union
field as Illegal Behavior, because the `set_union_tag` instruction now
appears *before* the `struct_field_ptr` instruction as you would
probably expect it to.
Resolves: #24520Resolves: #24595
