cgen: use alias next method if defined in for x in iterator { (#24905)

vlib/v/gen/c/for.v

@@ -422,10 +422,21 @@ fn (mut g Gen) for_in_stmt(node_ ast.ForInStmt) {
 			g.writeln('${field_accessor}str[${i}];')
 		}
 	} else if node.kind in [.struct, .interface] {
-		cond_type_sym := g.table.final_sym(node.cond_type)
+		cond_type_sym := g.table.sym(node.cond_type)
-		next_fn := cond_type_sym.find_method_with_generic_parent('next') or {
+		mut next_fn := ast.Fn{}
-			verror('`next` method not found')
+		// use alias `next` method if exists else use parent type `next` method
-			return
+		if cond_type_sym.kind == .alias {
+			next_fn = cond_type_sym.find_method_with_generic_parent('next') or {
+				g.table.final_sym(node.cond_type).find_method_with_generic_parent('next') or {
+					verror('`next` method not found')
+					return
+				}
+			}
+		} else {
+			next_fn = cond_type_sym.find_method_with_generic_parent('next') or {
+				verror('`next` method not found')
+				return
+			}
 		}
 		ret_typ := next_fn.return_type
 		t_expr := g.new_tmp_var()

vlib/v/tests/for_in_iterator_next_test.v

@@ -35,10 +35,18 @@ fn iterator_as_concrete_type() LineIterator {
 
 fn iterator_as_alias_concrete_type() AliasLineIterator {
 	return AliasLineIterator(LineIterator{
-		lines: ['alias']
+		lines: ['alias', 'iterator', 'next']
 	})
 }
 
+pub fn (mut line_iterator AliasLineIterator) next() ?string {
+	if line_iterator.idx >= line_iterator.lines.len {
+		return none
+	}
+	defer { line_iterator.idx += 2 }
+	return line_iterator.lines[line_iterator.idx]
+}
+
 fn test_main() {
 	mut out := []string{}
 	for line in iterator_as_interface() {
@@ -78,4 +86,5 @@ fn test_main() {
 	}
 
 	assert out[6] == 'LINE: alias'
+	assert out[7] == 'LINE: next'
 }