ast, cgen: fix generic method with variadic generic argument (#14404)

vlib/v/ast/types.v

@@ -1464,7 +1464,6 @@ pub fn (t &TypeSymbol) find_method_with_generic_parent(name string) ?Fn {
 									param.typ = pt
 								}
 							}
-							method.generic_names.clear()
 							return method
 						}
 						else {}

vlib/v/gen/c/fn.v

@@ -1625,10 +1625,23 @@ fn (mut g Gen) call_args(node ast.CallExpr) {
 		arr_sym := g.table.sym(varg_type)
 		mut arr_info := arr_sym.info as ast.Array
 		if varg_type.has_flag(.generic) {
+			if node.is_method {
+				left_sym := g.table.sym(node.left_type)
+				if fn_def := left_sym.find_method_with_generic_parent(node.name) {
+					mut muttable := unsafe { &ast.Table(g.table) }
+					if utyp := muttable.resolve_generic_to_concrete(arr_info.elem_type,
+						fn_def.generic_names, node.concrete_types)
+					{
+						arr_info.elem_type = utyp
+					}
+				} else {
+					g.error('unable to find method $node.name', node.pos)
+				}
+			} else {
 				if fn_def := g.table.find_fn(node.name) {
 					mut muttable := unsafe { &ast.Table(g.table) }
-				if utyp := muttable.resolve_generic_to_concrete(arr_info.elem_type, fn_def.generic_names,
-					node.concrete_types)
+					if utyp := muttable.resolve_generic_to_concrete(arr_info.elem_type,
+						fn_def.generic_names, node.concrete_types)
 					{
 						arr_info.elem_type = utyp
 					}
@@ -1636,6 +1649,7 @@ fn (mut g Gen) call_args(node ast.CallExpr) {
 					g.error('unable to find function $node.name', node.pos)
 				}
 			}
+		}
 		elem_type := g.typ(arr_info.elem_type)
 		if (g.pref.translated || g.file.is_translated) && args.len == 1 {
 			// Handle `foo(c'str')` for `fn foo(args ...&u8)`

vlib/v/tests/generic_method_with_variadic_generic_args_test.v

@@ -0,0 +1,17 @@
+struct Foo<T> {
+mut:
+	arr []T
+}
+
+fn (mut foo Foo<T>) push<T>(items ...T) {
+	for item in items {
+		foo.arr << item
+	}
+}
+
+fn test_generic_method_with_variadic_args() {
+	mut f := Foo<int>{}
+	f.push(1, 2, 3, 5, 8, 13, 21, 34, 55)
+	println(f.arr)
+	assert f.arr == [1, 2, 3, 5, 8, 13, 21, 34, 55]
+}