diff --git a/base/runtime/core.odin b/base/runtime/core.odin
index 5a0b3766c..d8104b1ec 100644
--- a/base/runtime/core.odin
+++ b/base/runtime/core.odin
@@ -166,10 +166,11 @@ Type_Info_Map :: struct {
 	map_info: ^Map_Info,
 }
 Type_Info_Bit_Set :: struct {
-	elem:       ^Type_Info,
-	underlying: ^Type_Info, // Possibly nil
-	lower:      i64,
-	upper:      i64,
+	elem:                ^Type_Info,
+	underlying:          ^Type_Info,
+	explicit_underlying: bool, // false = bit_set[T], true = bit_set[T, U]
+	lower:               i64,
+	upper:               i64,
 }
 Type_Info_Simd_Vector :: struct {
 	elem:       ^Type_Info,
@@ -676,6 +677,26 @@ type_info_core :: proc "contextless" (info: ^Type_Info) -> ^Type_Info {
 	return base
 }
 
+@(require_results)
+type_info_underlying :: proc "contextless" (info: ^Type_Info) -> ^Type_Info {
+	if info == nil {
+		return nil
+	}
+
+	base := info
+	loop: for {
+		#partial switch i in base.variant {
+		case Type_Info_Named:     base = i.base
+		case Type_Info_Enum:      base = i.base
+		case Type_Info_Bit_Set:   base = i.underlying
+		case Type_Info_Bit_Field: base = i.backing_type
+		case:
+			break loop
+		}
+	}
+	return base
+}
+
 // type_info_base_without_enum returns the core-type of a `^Type_Info` stripping the `distinct`ness from the first level AND/OR
 // returns the backing integer type of an enum or bit_set `^Type_Info`.
 // This is also aliased as `type_info_core`
@@ -713,6 +734,11 @@ when !ODIN_NO_RTTI {
 	// returns the backing integer type of an enum or bit_set `typeid`.
 	// This is also aliased as `typeid_core`
 	typeid_base_without_enum :: typeid_core
+
+	typeid_underlying :: proc "contextless" (id: typeid) -> typeid {
+		ti := type_info_underlying(type_info_of(id))
+		return ti.id
+	}
 }
 
 
diff --git a/base/runtime/print.odin b/base/runtime/print.odin
index 2cdde8152..6569ece6c 100644
--- a/base/runtime/print.odin
+++ b/base/runtime/print.odin
@@ -478,7 +478,7 @@ print_type :: #force_no_inline proc "contextless" (ti: ^Type_Info) {
 			print_string("..")
 			print_i64(info.upper)
 		}
-		if info.underlying != nil {
+		if info.explicit_underlying {
 			print_string("; ")
 			print_type(info.underlying)
 		}
@@ -893,7 +893,7 @@ write_write_type :: #force_no_inline proc "contextless" (i: ^int, buf: []byte, t
 			write_string(i, buf, "..")       or_return
 			write_i64   (i, buf, info.upper) or_return
 		}
-		if info.underlying != nil {
+		if info.explicit_underlying {
 			write_string    (i, buf, "; ")            or_return
 			write_write_type(i, buf, info.underlying) or_return
 		}
diff --git a/core/odin/ast/walk.odin b/core/odin/ast/walk.odin
index 24c90c13b..5b9340c62 100644
--- a/core/odin/ast/walk.odin
+++ b/core/odin/ast/walk.odin
@@ -405,9 +405,7 @@ walk :: proc(v: ^Visitor, node: ^Node) {
 		walk_expr_list(v, n.fields)
 	case ^Bit_Set_Type:
 		walk(v, n.elem)
-		if n.underlying != nil {
-			walk(v, n.underlying)
-		}
+		walk(v, n.underlying)
 	case ^Map_Type:
 		walk(v, n.key)
 		walk(v, n.value)
diff --git a/core/reflect/types.odin b/core/reflect/types.odin
index 385edb19b..53561281f 100644
--- a/core/reflect/types.odin
+++ b/core/reflect/types.odin
@@ -442,10 +442,7 @@ is_endian_platform :: proc(info: ^Type_Info) -> bool {
 	case Type_Info_Integer:
 		return v.endianness == .Platform
 	case Type_Info_Bit_Set:
-		if v.underlying != nil {
-			return is_endian_platform(v.underlying)
-		}
-		return true
+		return is_endian_platform(v.underlying)
 	case Type_Info_Pointer:
 		return true
 	}
@@ -467,10 +464,7 @@ is_endian_little :: proc(info: ^Type_Info) -> bool {
 		}
 		return v.endianness == .Little
 	case Type_Info_Bit_Set:
-		if v.underlying != nil {
-			return is_endian_platform(v.underlying)
-		}
-		return ODIN_ENDIAN == .Little
+		return is_endian_little(v.underlying)
 	case Type_Info_Pointer:
 		return ODIN_ENDIAN == .Little
 	}
@@ -492,10 +486,7 @@ is_endian_big :: proc(info: ^Type_Info) -> bool {
 		}
 		return v.endianness == .Big
 	case Type_Info_Bit_Set:
-		if v.underlying != nil {
-			return is_endian_platform(v.underlying)
-		}
-		return ODIN_ENDIAN == .Big
+		return is_endian_big(v.underlying)
 	case Type_Info_Pointer:
 		return ODIN_ENDIAN == .Big
 	}
@@ -754,7 +745,7 @@ write_type_writer :: #force_no_inline proc(w: io.Writer, ti: ^Type_Info, n_writt
 			io.write_string(w, "..=",       &n) or_return
 			io.write_i64(w, info.upper, 10, &n) or_return
 		}
-		if info.underlying != nil {
+		if info.explicit_underlying {
 			io.write_string(w, "; ",       &n) or_return
 			write_type(w, info.underlying, &n) or_return
 		}
diff --git a/src/llvm_backend_type.cpp b/src/llvm_backend_type.cpp
index 382304a4e..89c671f7d 100644
--- a/src/llvm_backend_type.cpp
+++ b/src/llvm_backend_type.cpp
@@ -923,15 +923,25 @@ gb_internal void lb_setup_type_info_data_giant_array(lbModule *m, i64 global_typ
 				GB_ASSERT(is_type_typed(t->BitSet.elem));
 
 
-				LLVMValueRef vals[4] = {
-					get_type_info_ptr(m, t->BitSet.elem),
-					LLVMConstNull(lb_type(m, t_type_info_ptr)),
-					lb_const_int(m, t_i64, t->BitSet.lower).value,
-					lb_const_int(m, t_i64, t->BitSet.upper).value,
+				LLVMValueRef vals[5] = {
+					get_type_info_ptr(m, t->BitSet.elem),          // ^Type_Info
+					nullptr,                                       // ^Type_Info
+					nullptr,                                       // bool
+					lb_const_int(m, t_i64, t->BitSet.lower).value, // i64
+					lb_const_int(m, t_i64, t->BitSet.upper).value, // i64
 				};
+				Type *underlying = nullptr;
+				bool explicit_underlying = false;
 				if (t->BitSet.underlying != nullptr) {
+					underlying = t->BitSet.underlying;
+					explicit_underlying = true;
 					vals[1] = get_type_info_ptr(m, t->BitSet.underlying);
+					vals[2] = lb_const_bool(m, t_bool, true).value;
+				} else {
+					underlying = bit_set_to_int(t);
 				}
+				vals[1] = get_type_info_ptr(m, underlying);
+				vals[2] = lb_const_bool(m, t_bool, explicit_underlying).value;
 
 				variant_value = llvm_const_named_struct(m, tag_type, vals, gb_count_of(vals));
 			}