handle Type{Union{}} like typeof(Union{}) more (#55508)

We could try to make them both pointers (by setting mayinlinealloc=false
on Core.TypeofBottom), but let's try to make them both equivalent
representations of the typeof Union{} as a singleton value.

Fixes #55208

src/cgutils.cpp

@@ -650,7 +650,7 @@ static Type *_julia_type_to_llvm(jl_codegen_params_t *ctx, LLVMContext &ctxt, jl
 {
     // this function converts a Julia Type into the equivalent LLVM type
     if (isboxed) *isboxed = false;
-    if (jt == (jl_value_t*)jl_bottom_type)
+    if (jt == (jl_value_t*)jl_bottom_type || jt == (jl_value_t*)jl_typeofbottom_type || jt == (jl_value_t*)jl_typeofbottom_type->super)
         return getVoidTy(ctxt);
     if (jl_is_concrete_immutable(jt)) {
         if (jl_datatype_nbits(jt) == 0)
@@ -760,7 +760,7 @@ static Type *_julia_struct_to_llvm(jl_codegen_params_t *ctx, LLVMContext &ctxt,
     // use this where C-compatible (unboxed) structs are desired
     // use julia_type_to_llvm directly when you want to preserve Julia's type semantics
     if (isboxed) *isboxed = false;
-    if (jt == (jl_value_t*)jl_bottom_type)
+    if (jt == (jl_value_t*)jl_bottom_type || jt == (jl_value_t*)jl_typeofbottom_type || jt == (jl_value_t*)jl_typeofbottom_type->super)
         return getVoidTy(ctxt);
     if (jl_is_primitivetype(jt))
         return bitstype_to_llvm(jt, ctxt, llvmcall);
@@ -948,6 +948,9 @@ static bool for_each_uniontype_small(
         allunbox &= for_each_uniontype_small(f, ((jl_uniontype_t*)ty)->b, counter);
         return allunbox;
     }
+    else if (ty == (jl_value_t*)jl_typeofbottom_type->super) {
+        f(++counter, jl_typeofbottom_type); // treat Tuple{union{}} as identical to typeof(Union{})
+    }
     else if (jl_is_pointerfree(ty)) {
         f(++counter, (jl_datatype_t*)ty);
         return true;
@@ -1691,6 +1694,8 @@ static std::pair<Value*, bool> emit_isa(jl_codectx_t &ctx, const jl_cgval_t &x,
         if (intersected_type == (jl_value_t*)jl_bottom_type)
             known_isa = false;
     }
+    if (intersected_type == (jl_value_t*)jl_typeofbottom_type->super)
+        intersected_type = (jl_value_t*)jl_typeofbottom_type; // swap abstract Type{Union{}} for concrete typeof(Union{})
     if (known_isa) {
         if (!*known_isa && !msg.isTriviallyEmpty()) {
             emit_type_error(ctx, x, literal_pointer_val(ctx, type), msg);

src/codegen.cpp

@@ -2240,6 +2240,8 @@ static inline jl_cgval_t ghostValue(jl_codectx_t &ctx, jl_value_t *typ)
         // replace T::Type{T} with T, by assuming that T must be a leaftype of some sort
         jl_cgval_t constant(NULL, true, typ, NULL, best_tbaa(ctx.tbaa(), typ));
         constant.constant = jl_tparam0(typ);
+        if (typ == (jl_value_t*)jl_typeofbottom_type->super)
+            constant.isghost = true;
         return constant;
     }
     return jl_cgval_t(typ);
@@ -2252,7 +2254,7 @@ static inline jl_cgval_t ghostValue(jl_codectx_t &ctx, jl_datatype_t *typ)
 static inline jl_cgval_t mark_julia_const(jl_codectx_t &ctx, jl_value_t *jv)
 {
     jl_value_t *typ;
-    if (jl_is_type(jv)) {
+    if (jl_is_type(jv) && jv != jl_bottom_type) {
         typ = (jl_value_t*)jl_wrap_Type(jv); // TODO: gc-root this?
     }
     else {
@@ -3619,8 +3621,8 @@ static Value *emit_f_is(jl_codectx_t &ctx, const jl_cgval_t &arg1, const jl_cgva
     if (arg1.constant && arg2.constant)
         return ConstantInt::get(getInt1Ty(ctx.builder.getContext()), jl_egal(arg1.constant, arg2.constant));
 
-    jl_value_t *rt1 = arg1.typ;
-    jl_value_t *rt2 = arg2.typ;
+    jl_value_t *rt1 = (arg1.constant ? jl_typeof(arg1.constant) : arg1.typ);
+    jl_value_t *rt2 = (arg2.constant ? jl_typeof(arg2.constant) : arg2.typ);
     if (jl_is_concrete_type(rt1) && jl_is_concrete_type(rt2) && !jl_is_kind(rt1) && !jl_is_kind(rt2) && rt1 != rt2) {
         // disjoint concrete leaf types are never equal (quick test)
         return ConstantInt::get(getInt1Ty(ctx.builder.getContext()), 0);
@@ -9534,7 +9536,7 @@ static jl_llvm_functions_t
                     RTindex = UndefValue::get(getInt8Ty(ctx.builder.getContext()));
                 }
                 else if (jl_is_concrete_type(val.typ) || val.constant) {
-                    size_t tindex = get_box_tindex((jl_datatype_t*)val.typ, phiType);
+                    size_t tindex = get_box_tindex((jl_datatype_t*)(val.constant ? jl_typeof(val.constant) : val.typ), phiType);
                     if (tindex == 0) {
                         if (VN)
                             V = boxed(ctx, val);

src/datatype.c

@@ -357,6 +357,8 @@ int jl_struct_try_layout(jl_datatype_t *dt)
 
 int jl_datatype_isinlinealloc(jl_datatype_t *ty, int pointerfree)
 {
+    if (jl_typeofbottom_type && ty == jl_typeofbottom_type->super)
+        ty = jl_typeofbottom_type;
     if (ty->name->mayinlinealloc && jl_struct_try_layout(ty)) {
         if (ty->layout->npointers > 0) {
             if (pointerfree)
@@ -1656,6 +1658,8 @@ JL_DLLEXPORT jl_value_t *jl_new_struct_uninit(jl_datatype_t *type)
 {
     jl_task_t *ct = jl_current_task;
     if (!jl_is_datatype(type) || !type->isconcretetype || type->layout == NULL || jl_is_layout_opaque(type->layout)) {
+        if (type == jl_typeofbottom_type->super)
+            return jl_bottom_type; // ::Type{Union{}} is an abstract type, but is also a singleton when used as a field type
         jl_type_error("new", (jl_value_t*)jl_datatype_type, (jl_value_t*)type);
     }
     if (type->instance != NULL)

src/jltypes.c

@@ -3024,6 +3024,7 @@ void jl_init_types(void) JL_GC_DISABLED
     jl_anytuple_type->layout = NULL;
 
     jl_typeofbottom_type->super = jl_wrap_Type(jl_bottom_type);
+    jl_typeofbottom_type->super->layout = jl_typeofbottom_type->layout; // the only abstract type with a layout
     jl_emptytuple_type = (jl_datatype_t*)jl_apply_tuple_type(jl_emptysvec, 0);
     jl_emptytuple = jl_gc_permobj(0, jl_emptytuple_type);
     jl_emptytuple_type->instance = jl_emptytuple;

test/compiler/codegen.jl

@@ -969,3 +969,18 @@ end
 
 # Core.getptls() special handling
 @test !occursin("call ptr @jlplt", get_llvm(Core.getptls, Tuple{})) #It should lower to a direct load of the ptls and not a ccall
+
+# issue 55208
+@noinline function f55208(x, i)
+    z = (i == 0 ? x[1] : x[i])
+    return z isa Core.TypeofBottom
+end
+@test f55208((Union{}, 5, 6, 7), 0)
+
+@noinline function g55208(x, i)
+    z = (i == 0 ? x[1] : x[i])
+    typeof(z)
+end
+@test g55208((Union{}, true, true), 0) === typeof(Union{})
+
+@test string((Core.Union{}, true, true, true)) == "(Union{}, true, true, true)"

test/core.jl

@@ -8265,3 +8265,5 @@ end
 @test Tuple{Vararg{Int}} === Union{Tuple{Int}, Tuple{}, Tuple{Int, Int, Vararg{Int}}}
 @test (Tuple{Vararg{T}} where T) === (Union{Tuple{T, T, Vararg{T}}, Tuple{}, Tuple{T}} where T)
 @test_broken (Tuple{Vararg{T}} where T) === Union{Tuple{T, T, Vararg{T}} where T, Tuple{}, Tuple{T} where T}
+
+@test sizeof(Pair{Union{typeof(Union{}),Nothing}, Union{Type{Union{}},Nothing}}(Union{}, Union{})) == 2