diff --git a/vlib/v/gen/c/cgen.v b/vlib/v/gen/c/cgen.v
index 028b96607f..66be4430f1 100644
--- a/vlib/v/gen/c/cgen.v
+++ b/vlib/v/gen/c/cgen.v
@@ -2805,9 +2805,10 @@ fn (mut g Gen) write_sumtype_casting_fn(fun SumtypeCastingFn) {
 	} else {
 		// g.definitions.writeln('${g.static_modifier} inline ${exp_cname} ${fun.fn_name}(${got_cname}* x);')
 		// sb.writeln('${g.static_modifier} inline ${exp_cname} ${fun.fn_name}(${got_cname}* x) {')
-		g.definitions.writeln('${exp_cname} ${fun.fn_name}(${got_cname}* x);')
-		sb.writeln('${exp_cname} ${fun.fn_name}(${got_cname}* x) {')
-		sb.writeln('\t${got_cname}* ptr = memdup(x, sizeof(${got_cname}));')
+		g.definitions.writeln('${exp_cname} ${fun.fn_name}(${got_cname}* x, bool is_mut);')
+		sb.writeln('${exp_cname} ${fun.fn_name}(${got_cname}* x, bool is_mut) {')
+		sb.writeln('\t${got_cname}* ptr = x;')
+		sb.writeln('\tif (!is_mut) { ptr = memdup(x, sizeof(${got_cname})); }')
 	}
 	for embed_hierarchy in g.table.get_embeds(got_sym) {
 		// last embed in the hierarchy
@@ -2827,14 +2828,8 @@ fn (mut g Gen) write_sumtype_casting_fn(fun SumtypeCastingFn) {
 		sb.writeln('\t${embed_cname}* ${embed_name}_ptr = memdup(${accessor}, sizeof(${embed_cname}));')
 	}
 	sb.write_string('\treturn (${exp_cname}){ ._${variant_name} = ptr, ._typ = ${type_idx}')
-	g.write_sumtype_casting_fn_common_fields(mut sb, exp_sym, got_cname, got_sym, 'ptr')
-	sb.writeln('};\n}')
-	g.auto_fn_definitions << sb.str()
-}
-
-fn (mut g Gen) write_sumtype_casting_fn_common_fields(mut sb strings.Builder, exp_sym ast.TypeSymbol, got_cname string, got_sym ast.TypeSymbol, ptr_name string) {
 	for field in (exp_sym.info as ast.SumType).fields {
-		mut ptr := ptr_name
+		mut ptr := 'ptr'
 		mut type_cname := got_cname
 		_, embed_types := g.table.find_field_from_embeds(got_sym, field.name) or {
 			ast.StructField{}, []ast.Type{}
@@ -2847,28 +2842,26 @@ fn (mut g Gen) write_sumtype_casting_fn_common_fields(mut sb strings.Builder, ex
 		field_styp := g.styp(field.typ)
 		if got_sym.kind in [.sum_type, .interface] {
 			// the field is already a wrapped pointer; we shouldn't wrap it once again
-			sb.write_string(', .${c_name(field.name)} = ${ptr}->${field.name}')
+			sb.write_string(', .${c_name(field.name)} = ptr->${field.name}')
 		} else {
 			sb.write_string(', .${c_name(field.name)} = (${field_styp}*)((char*)${ptr} + __offsetof_ptr(${ptr}, ${type_cname}, ${c_name(field.name)}))')
 		}
 	}
+	sb.writeln('};\n}')
+	g.auto_fn_definitions << sb.str()
 }
 
 fn (mut g Gen) call_cfn_for_casting_expr(fname string, expr ast.Expr, exp ast.Type, got ast.Type, exp_styp string,
 	got_is_ptr bool, got_is_fn bool, got_styp string) {
 	mut rparen_n := 1
-	mut mutable_idx := 0
+	mut mutable_is_mut_arg_pos := 0
 
 	is_not_ptr_and_fn := !got_is_ptr && !got_is_fn
 	is_sumtype_cast := !got_is_fn && fname.contains('_to_sumtype_')
 	is_comptime_variant := is_not_ptr_and_fn && expr is ast.Ident
 		&& g.comptime.is_comptime_variant_var(expr)
 	if exp.is_ptr() {
-		if is_sumtype_cast && g.expected_arg_mut && expr in [ast.Ident, ast.SelectorExpr] {
-			g.write('&(${exp_styp.trim_right('*')}){._${got_styp.trim_right('*')}=')
-			rparen_n = 0
-			mutable_idx = got.idx()
-		} else if (expr is ast.UnsafeExpr && expr.expr is ast.Nil) || got == ast.nil_type {
+		if (expr is ast.UnsafeExpr && expr.expr is ast.Nil) || got == ast.nil_type {
 			g.write('(void*)0')
 			return
 		} else {
@@ -2878,6 +2871,7 @@ fn (mut g Gen) call_cfn_for_casting_expr(fname string, expr ast.Expr, exp ast.Ty
 	} else {
 		g.write('${fname}(')
 	}
+	mutable_is_mut_arg_pos = rparen_n
 	if is_not_ptr_and_fn {
 		is_cast_fixed_array_init := expr is ast.CastExpr
 			&& (expr.expr is ast.ArrayInit && expr.expr.is_fixed)
@@ -2918,25 +2912,14 @@ fn (mut g Gen) call_cfn_for_casting_expr(fname string, expr ast.Expr, exp ast.Ty
 		g.expr(expr)
 		g.left_is_opt = old_left_is_opt
 	}
-	if mutable_idx != 0 {
-		g.write(', ._typ=${mutable_idx}')
-
-		// setup other common fields in sumtype
-		mut sb := strings.new_builder(256)
-		got_sym, exp_sym := g.table.sym(got), g.table.sym(exp)
-		got_cname := g.get_sumtype_variant_type_name(got, got_sym)
-		mutable_sumtype_ptr_name := g.expr_string(expr)
-		ptr_name := if got_is_ptr {
-			'(${mutable_sumtype_ptr_name})'
-		} else {
-			'(&(${mutable_sumtype_ptr_name}))'
-		}
-		g.write_sumtype_casting_fn_common_fields(mut sb, exp_sym, got_cname, got_sym,
-			ptr_name)
-		g.write(sb.str())
-		g.write('}')
+	if is_sumtype_cast {
+		// the `_to_sumtype_` family of functions last `is_mut` param
+		g.write(')'.repeat(rparen_n - mutable_is_mut_arg_pos))
+		g.write(', ${exp.is_ptr()}')
+		g.write(')'.repeat(mutable_is_mut_arg_pos))
+	} else {
+		g.write(')'.repeat(rparen_n))
 	}
-	g.write(')'.repeat(rparen_n))
 }
 
 // use instead of expr() when you need a var to use as reference
@@ -3118,7 +3101,7 @@ fn (mut g Gen) expr_with_cast(expr ast.Expr, got_type_raw ast.Type, expected_typ
 					g.expr(expr)
 					g.writeln(';')
 					g.write2(stmt_str, ' ')
-					g.write('${fname}(&${tmp_var})')
+					g.write('${fname}(&${tmp_var}, ${unwrapped_expected_type.is_ptr()})')
 					return
 				} else {
 					g.call_cfn_for_casting_expr(fname, expr, expected_type, got_type,
@@ -3523,7 +3506,7 @@ fn (mut g Gen) gen_clone_assignment(var_type ast.Type, val ast.Expr, typ ast.Typ
 				g.write('}, sizeof(${shared_styp}))')
 			}
 			if is_sumtype {
-				g.write('))')
+				g.write('), false)')
 			}
 		} else if right_sym.kind == .string {
 			// `str1 = str2` => `str1 = str2.clone()`
diff --git a/vlib/v/gen/c/json.v b/vlib/v/gen/c/json.v
index cf40af0899..db30ad49bd 100644
--- a/vlib/v/gen/c/json.v
+++ b/vlib/v/gen/c/json.v
@@ -381,8 +381,8 @@ fn (mut g Gen) gen_sumtype_enc_dec(utyp ast.Type, sym ast.TypeSymbol, mut enc st
 		if fv_sym.kind == .struct && !is_option && field_op != '->' {
 			dec.writeln('\tif (root->type == cJSON_NULL) { ')
 			dec.writeln('\t\tstruct ${first_variant_name} empty = {0};')
-			dec.writeln('res = ${variant_typ}_to_sumtype_${ret_styp}(&empty); } \n else ')
-			// dec.writeln('res = ${variant_typ}_to_sumtype_${sym.cname}(&empty); } \n else ')
+			dec.writeln('res = ${variant_typ}_to_sumtype_${ret_styp}(&empty, false); } \n else ')
+			// dec.writeln('res = ${variant_typ}_to_sumtype_${sym.cname}(&empty, false); } \n else ')
 		}
 		//
 		dec.writeln('if (cJSON_IsObject(root) || (cJSON_IsArray(root) && cJSON_IsObject(root->child))) {')
@@ -409,7 +409,7 @@ fn (mut g Gen) gen_sumtype_enc_dec(utyp ast.Type, sym ast.TypeSymbol, mut enc st
 
 		// Helpers for decoding
 		g.get_sumtype_casting_fn(variant, typ)
-		g.definitions.writeln('static inline ${sym.cname} ${variant_typ}_to_sumtype_${sym.cname}(${variant_typ}* x);')
+		g.definitions.writeln('static inline ${sym.cname} ${variant_typ}_to_sumtype_${sym.cname}(${variant_typ}* x, bool is_mut);')
 
 		// ENCODING
 		enc.writeln('\tif (${var_data}${field_op}_typ == ${int(variant.idx())}) {')
@@ -485,9 +485,9 @@ fn (mut g Gen) gen_sumtype_enc_dec(utyp ast.Type, sym ast.TypeSymbol, mut enc st
 				dec.writeln('\t\t${variant_typ} value = *(${variant_typ}*)(${tmp}.data);')
 			}
 			if is_option {
-				dec.writeln('\t\t\t_option_ok(&(${sym.cname}[]){ ${variant_typ}_to_sumtype_${sym.cname}(&value) }, (${option_name}*)&res, sizeof(${sym.cname}));')
+				dec.writeln('\t\t\t_option_ok(&(${sym.cname}[]){ ${variant_typ}_to_sumtype_${sym.cname}(&value, false) }, (${option_name}*)&res, sizeof(${sym.cname}));')
 			} else {
-				dec.writeln('\t\tres = ${variant_typ}_to_sumtype_${ret_styp}(&value);')
+				dec.writeln('\t\tres = ${variant_typ}_to_sumtype_${ret_styp}(&value, false);')
 			}
 			dec.writeln('\t}')
 		} $else {
@@ -498,10 +498,10 @@ fn (mut g Gen) gen_sumtype_enc_dec(utyp ast.Type, sym ast.TypeSymbol, mut enc st
 				if utyp.has_flag(.option) {
 					dec.writeln('\t\t\t\t${prefix}res.state = 0;')
 					tmp_time_var := g.new_tmp_var()
-					dec.writeln('\t\t\t\t${g.base_type(utyp)} ${tmp_time_var} = ${variant_typ}_to_sumtype_${sym.cname}(&${tmp});')
+					dec.writeln('\t\t\t\t${g.base_type(utyp)} ${tmp_time_var} = ${variant_typ}_to_sumtype_${sym.cname}(&${tmp}, false);')
 					dec.writeln('\t\t\t\tvmemcpy(&${prefix}res.data, ${tmp_time_var}._time__Time, sizeof(${variant_typ}));')
 				} else {
-					dec.writeln('\t\t\t\t${prefix}res = ${variant_typ}_to_sumtype_${sym.cname}(&${tmp});')
+					dec.writeln('\t\t\t\t${prefix}res = ${variant_typ}_to_sumtype_${sym.cname}(&${tmp}, false);')
 				}
 				dec.writeln('\t\t\t}')
 			} else if !is_js_prim(variant_typ) && variant_sym.kind != .enum {
@@ -512,9 +512,9 @@ fn (mut g Gen) gen_sumtype_enc_dec(utyp ast.Type, sym ast.TypeSymbol, mut enc st
 				dec.writeln('\t\t\t\t\treturn (${result_name}_${ret_styp}){ .is_error = true, .err = ${tmp}.err, .data = {0} };')
 				dec.writeln('\t\t\t\t}')
 				if is_option {
-					dec.writeln('\t\t\t\t_option_ok(&(${sym.cname}[]){ ${variant_typ}_to_sumtype_${sym.cname}((${variant_typ}*)${tmp}.data) }, (${option_name}*)&res, sizeof(${sym.cname}));')
+					dec.writeln('\t\t\t\t_option_ok(&(${sym.cname}[]){ ${variant_typ}_to_sumtype_${sym.cname}((${variant_typ}*)${tmp}.data, false) }, (${option_name}*)&res, sizeof(${sym.cname}));')
 				} else {
-					dec.writeln('\t\t\t\t${prefix}res = ${variant_typ}_to_sumtype_${sym.cname}((${variant_typ}*)${tmp}.data);')
+					dec.writeln('\t\t\t\t${prefix}res = ${variant_typ}_to_sumtype_${sym.cname}((${variant_typ}*)${tmp}.data, false);')
 				}
 				dec.writeln('\t\t\t}')
 			}
@@ -536,7 +536,7 @@ fn (mut g Gen) gen_sumtype_enc_dec(utyp ast.Type, sym ast.TypeSymbol, mut enc st
 				var_t := 'bool'
 				dec.writeln('\t\tif (cJSON_IsBool(root)) {')
 				dec.writeln('\t\t\t${var_t} value = ${js_dec_name(var_t)}(root);')
-				dec.writeln('\t\t\t${prefix}res = ${var_t}_to_sumtype_${sym.cname}(&value);')
+				dec.writeln('\t\t\t${prefix}res = ${var_t}_to_sumtype_${sym.cname}(&value, false);')
 				dec.writeln('\t\t}')
 			}
 
@@ -552,9 +552,9 @@ fn (mut g Gen) gen_sumtype_enc_dec(utyp ast.Type, sym ast.TypeSymbol, mut enc st
 					dec.writeln('\t\tif (cJSON_IsNumber(root)) {')
 					dec.writeln('\t\t\t${var_t} value = ${js_dec_name('u64')}(root);')
 					if utyp.has_flag(.option) {
-						dec.writeln('\t\t\t_option_ok(&(${sym.cname}[]){ ${var_t}_to_sumtype_${sym.cname}(&value) }, (${option_name}*)&${prefix}res, sizeof(${sym.cname}));')
+						dec.writeln('\t\t\t_option_ok(&(${sym.cname}[]){ ${var_t}_to_sumtype_${sym.cname}(&value, false) }, (${option_name}*)&${prefix}res, sizeof(${sym.cname}));')
 					} else {
-						dec.writeln('\t\t\t${prefix}res = ${var_t}_to_sumtype_${sym.cname}(&value);')
+						dec.writeln('\t\t\t${prefix}res = ${var_t}_to_sumtype_${sym.cname}(&value, false);')
 					}
 					dec.writeln('\t\t}')
 				}
@@ -568,9 +568,9 @@ fn (mut g Gen) gen_sumtype_enc_dec(utyp ast.Type, sym ast.TypeSymbol, mut enc st
 					dec.writeln('\t\tif (cJSON_IsString(root)) {')
 					dec.writeln('\t\t\t${var_t} value = ${js_dec_name(var_t)}(root);')
 					if utyp.has_flag(.option) {
-						dec.writeln('\t\t\t_option_ok(&(${sym.cname}[]){ ${var_t}_to_sumtype_${sym.cname}(&value) }, (${option_name}*)&${prefix}res, sizeof(${sym.cname}));')
+						dec.writeln('\t\t\t_option_ok(&(${sym.cname}[]){ ${var_t}_to_sumtype_${sym.cname}(&value, false) }, (${option_name}*)&${prefix}res, sizeof(${sym.cname}));')
 					} else {
-						dec.writeln('\t\t\t${prefix}res = ${var_t}_to_sumtype_${sym.cname}(&value);')
+						dec.writeln('\t\t\t${prefix}res = ${var_t}_to_sumtype_${sym.cname}(&value, false);')
 					}
 					dec.writeln('\t\t}')
 				}
@@ -592,9 +592,9 @@ fn (mut g Gen) gen_sumtype_enc_dec(utyp ast.Type, sym ast.TypeSymbol, mut enc st
 					dec.writeln('\t\t\t\treturn (${result_name}_${ret_styp}){ .is_error = true, .err = ${tmp}.err, .data = {0} };')
 					dec.writeln('\t\t\t}')
 					if utyp.has_flag(.option) {
-						dec.writeln('\t\t\t_option_ok(&(${sym.cname}[]){ ${var_t}_to_sumtype_${sym.cname}((${var_t}*)${tmp}.data) }, (${option_name}*)&${prefix}res, sizeof(${sym.cname}));')
+						dec.writeln('\t\t\t_option_ok(&(${sym.cname}[]){ ${var_t}_to_sumtype_${sym.cname}((${var_t}*)${tmp}.data, false) }, (${option_name}*)&${prefix}res, sizeof(${sym.cname}));')
 					} else {
-						dec.writeln('\t\t\t${prefix}res = ${var_t}_to_sumtype_${sym.cname}((${var_t}*)${tmp}.data);')
+						dec.writeln('\t\t\t${prefix}res = ${var_t}_to_sumtype_${sym.cname}((${var_t}*)${tmp}.data, false);')
 					}
 					dec.writeln('\t\t}')
 				}
@@ -611,9 +611,9 @@ fn (mut g Gen) gen_sumtype_enc_dec(utyp ast.Type, sym ast.TypeSymbol, mut enc st
 					dec.writeln('\t\tif (cJSON_IsNumber(root)) {')
 					dec.writeln('\t\t\t${var_t} value = ${js_dec_name(var_t)}(root);')
 					if utyp.has_flag(.option) {
-						dec.writeln('\t\t\t_option_ok(&(${sym.cname}[]){ ${var_t}_to_sumtype_${sym.cname}(&value) }, (${option_name}*)&${prefix}res, sizeof(${sym.cname}));')
+						dec.writeln('\t\t\t_option_ok(&(${sym.cname}[]){ ${var_t}_to_sumtype_${sym.cname}(&value, false) }, (${option_name}*)&${prefix}res, sizeof(${sym.cname}));')
 					} else {
-						dec.writeln('\t\t\t${prefix}res = ${var_t}_to_sumtype_${sym.cname}(&value);')
+						dec.writeln('\t\t\t${prefix}res = ${var_t}_to_sumtype_${sym.cname}(&value, false);')
 					}
 					dec.writeln('\t\t}')
 				}