checker: fix for..in statement with ComptimeSelector (#16997)

vlib/v/checker/for.v

@@ -31,11 +31,19 @@ fn (mut c Checker) for_c_stmt(node ast.ForCStmt) {
 	c.in_for_count--
 }
 
+fn (mut c Checker) get_compselector_type_from_selector_name(node ast.ComptimeSelector) (bool, ast.Type) {
+	if c.inside_comptime_for_field && node.field_expr is ast.SelectorExpr
+		&& (node.field_expr as ast.SelectorExpr).expr.str() in c.comptime_fields_type
+		&& (node.field_expr as ast.SelectorExpr).field_name == 'name' {
+		return true, c.unwrap_generic(c.comptime_fields_default_type)
+	}
+	return false, ast.void_type
+}
+
 fn (mut c Checker) for_in_stmt(mut node ast.ForInStmt) {
 	c.in_for_count++
 	prev_loop_label := c.loop_label
-	typ := c.expr(node.cond)
+	mut typ := c.expr(node.cond)
-	typ_idx := typ.idx()
 	if node.key_var.len > 0 && node.key_var != '_' {
 		c.check_valid_snake_case(node.key_var, 'variable name', node.pos)
 		if reserved_type_names_chk.matches(node.key_var) {
@@ -49,6 +57,7 @@ fn (mut c Checker) for_in_stmt(mut node ast.ForInStmt) {
 		}
 	}
 	if node.is_range {
+		typ_idx := typ.idx()
 		high_type := c.expr(node.high)
 		high_type_idx := high_type.idx()
 		if typ_idx in ast.integer_type_idxs && high_type_idx !in ast.integer_type_idxs
@@ -75,12 +84,19 @@ fn (mut c Checker) for_in_stmt(mut node ast.ForInStmt) {
 		node.high_type = high_type
 		node.scope.update_var_type(node.val_var, node.val_type)
 	} else {
-		sym := c.table.final_sym(typ)
+		mut sym := c.table.final_sym(typ)
 		if sym.kind != .string {
 			match mut node.cond {
 				ast.PrefixExpr {
 					node.val_is_ref = node.cond.op == .amp
 				}
+				ast.ComptimeSelector {
+					found, selector_type := c.get_compselector_type_from_selector_name(node.cond)
+					if found {
+						sym = c.table.final_sym(selector_type)
+						typ = selector_type
+					}
+				}
 				ast.Ident {
 					match mut node.cond.info {
 						ast.IdentVar {

vlib/v/tests/for_comptime_selector_test.v

@@ -0,0 +1,23 @@
+struct Test {
+	b ?[]int
+}
+
+fn call_generic[U](val U) int {
+	mut out := 0
+	$for field in U.fields {
+		variable := val.$(field.name)
+		println(variable)
+		for element in val.$(field.name) {
+			dump(element)
+			out += element
+		}
+	}
+	return out
+}
+
+fn test_main() {
+	test := Test{
+		b: [1, 2, 3]
+	}
+	assert call_generic(test) == 6
+}