cgen: fix string interliteral of the comptime selector (#12475)

vlib/v/gen/c/str_intp.v

@@ -140,7 +140,13 @@ fn (mut g Gen) str_val(node ast.StringInterLiteral, i int) {
 			g.expr(expr)
 		}
 	} else if node.fmts[i] == `s` || typ.has_flag(.variadic) {
-		g.gen_expr_to_string(expr, typ)
+		mut exp_typ := typ
+		if expr is ast.Ident && g.comptime_var_type_map.len > 0 {
+			if expr.obj is ast.Var {
+				exp_typ = expr.obj.typ
+			}
+		}
+		g.gen_expr_to_string(expr, exp_typ)
 	} else if typ.is_number() || typ.is_pointer() || node.fmts[i] == `d` {
 		if typ.is_signed() && node.fmts[i] in [`x`, `X`, `o`] {
 			// convert to unsigned first befors C's integer propagation strikes

vlib/v/tests/comptime_for_in_field_selector_test.v

@@ -2,6 +2,7 @@ fn print_field_values<T>(s T) {
 	mut value_list := []string{}
 	mut value_type_list := []string{}
 	mut var_value_list := []string{}
+	mut var_intp_value_list := []string{}
 
 	$for field in T.fields {
 		println(s.$(field.name))
@@ -13,6 +14,7 @@ fn print_field_values<T>(s T) {
 		val := s.$(field.name)
 		println(val)
 		var_value_list << val.str()
+		var_intp_value_list << 'field value: $val'
 	}
 	assert value_list.len == 4
 	assert value_list[0] == 'Simon'
@@ -31,6 +33,12 @@ fn print_field_values<T>(s T) {
 	assert var_value_list[1] == 'simon1234'
 	assert var_value_list[2] == 'simon@gmail.com'
 	assert var_value_list[3] == '15'
+
+	assert var_intp_value_list.len == 4
+	assert var_intp_value_list[0] == 'field value: Simon'
+	assert var_intp_value_list[1] == 'field value: simon1234'
+	assert var_intp_value_list[2] == 'field value: simon@gmail.com'
+	assert var_intp_value_list[3] == 'field value: 15'
 }
 
 struct Foo {