Allow .V to be null in emit_unionmove (JuliaLang#28248)

We generally handle the case where the .V of a split union is as small
as the active element of the union. If the active element happens to be
a ghost is thus seems reasonable for the split union to not have any storage
at all. Such a union is generated e.g. if we widen from an all-ghost split
union to one that includes sized elements, e.g. in the following example:
```
function foo()
    x = (1, 2)
    if rand() < 0.5
        if rand() < 0.5
            y = nothing
        else
            y = missing
        end
        x = y
    end
    x
end
```

Fixes JuliaLang#28208

src/cgutils.cpp

@@ -2289,7 +2289,8 @@ static void emit_unionmove(jl_codectx_t &ctx, Value *dest, MDNode *tbaa_dst, con
  Value *tindex = ctx.builder.CreateAnd(src.TIndex, ConstantInt::get(T_int8, 0x7f));
  if (skip)
  tindex = ctx.builder.CreateSelect(skip, ConstantInt::get(T_int8, 0), tindex);
- Value *src_ptr = maybe_bitcast(ctx, data_pointer(ctx, src), T_pint8);
+ Value *src_ptr = data_pointer(ctx, src);
+ src_ptr = src_ptr ? maybe_bitcast(ctx, src_ptr, T_pint8) : src_ptr;
  dest = maybe_bitcast(ctx, dest, T_pint8);
  BasicBlock *defaultBB = BasicBlock::Create(jl_LLVMContext, "union_move_skip", ctx.f);
  SwitchInst *switchInst = ctx.builder.CreateSwitch(tindex, defaultBB);
@@ -2302,8 +2303,18 @@ static void emit_unionmove(jl_codectx_t &ctx, Value *dest, MDNode *tbaa_dst, con
  BasicBlock *tempBB = BasicBlock::Create(jl_LLVMContext, "union_move", ctx.f);
  ctx.builder.SetInsertPoint(tempBB);
  switchInst->addCase(ConstantInt::get(T_int8, idx), tempBB);
- if (nb > 0)
- emit_memcpy(ctx, dest, tbaa_dst, src_ptr, src.tbaa, nb, alignment, isVolatile);
+ if (nb > 0) {
+ if (!src_ptr) {
+ Function *trap_func =
+ Intrinsic::getDeclaration(ctx.f->getParent(), Intrinsic::trap);
+ ctx.builder.CreateCall(trap_func);
+ ctx.builder.CreateUnreachable();
+ return;
+ } else {
+ emit_memcpy(ctx, dest, tbaa_dst, src_ptr,
+ src.tbaa, nb, alignment, isVolatile);
+ }
+ }
  ctx.builder.CreateBr(postBB);
  },
  src.typ,

test/core.jl

@@ -6653,3 +6653,20 @@ function foo28224()
  return z
 end
 @test foo28224() == 5
+
+# Issue #28208
+@noinline function foo28208(a::Bool, b::Bool)
+ x = (1, 2)
+ if a
+ if b
+ y = nothing
+ else
+ y = missing
+ end
+ x = y
+ end
+ x
+end
+@test isa(foo28208(false, true), Tuple)
+@test foo28208(true, false) === missing
+@test foo28208(true, true) === nothing