cgen, checker: fix comptimeselector passing to generic argument (vlan…

…g#20985)

vlib/v/checker/check_types.v

@@ -234,7 +234,8 @@ fn (mut c Checker) check_expected_call_arg(got ast.Type, expected_ ast.Type, lan
 			return error('cannot use `${got_typ_str}` as `${expected_typ_str}`')
 		}
 
-		if !expected.has_flag(.option) && got.has_flag(.option) && (arg.expr !is ast.Ident
+		if !expected.has_flag(.option) && got.has_flag(.option)
+			&& (!(arg.expr is ast.Ident || arg.expr is ast.ComptimeSelector)
 			|| (arg.expr is ast.Ident && c.comptime.get_ct_type_var(arg.expr) != .field_var)) {
 			got_typ_str, expected_typ_str := c.get_string_names_of(got, expected)
 			return error('cannot use `${got_typ_str}` as `${expected_typ_str}`, it must be unwrapped first')

vlib/v/gen/c/fn.v

@@ -2578,6 +2578,14 @@ fn (mut g Gen) ref_or_deref_arg(arg ast.CallArg, expected_type ast.Type, lang as
 				g.write('(${g.typ(arg.expr.typ)})')
 			}
 		}
+	} else if arg.expr is ast.ComptimeSelector && arg_typ.has_flag(.option)
+		&& !expected_type.has_flag(.option) {
+		// allow to pass val.$(filed.name) where T is expected, doing automatic unwrap in this case
+		styp := g.base_type(arg_typ)
+		g.write('*(${styp}*)')
+		g.expr_with_cast(arg.expr, arg_typ, expected_type)
+		g.write('.data')
+		return
 	}
 	// check if the argument must be dereferenced or not
 	g.arg_no_auto_deref = is_smartcast && !arg_is_ptr && !exp_is_ptr && arg.should_be_ptr

vlib/v/tests/comptime_selector_generic_arg_test.v

@@ -0,0 +1,40 @@
+struct Struc {
+	a string
+	b int
+	c ?string
+	d ?int
+}
+
+pub struct Count {
+mut:
+	total int
+}
+
+// count_chars count json sizen whithout new encode
+pub fn (mut count Count) count_chars[T](val T) {
+	$if val is $option {
+		workaround := val
+		if workaround != none {
+			count.count_chars(val)
+		}
+	} $else $if T is $struct {
+		count.chars_in_struct(val)
+	} $else {
+	}
+}
+
+// chars_in_struct
+fn (mut count Count) chars_in_struct[T](val T) {
+	$for field in T.fields {
+		va := val.$(field.name)
+		count.count_chars(va) // works
+		count.count_chars(val.$(field.name)) // Not works
+	}
+}
+
+fn test_main() {
+	struc := Struc{}
+	mut count := Count{}
+	count.count_chars(struc)
+	assert true
+}