checker, cgen: fix generics interface method (#10858)

vlib/v/checker/checker.v

@@ -1470,8 +1470,8 @@ pub fn (mut c Checker) infix_expr(mut node ast.InfixExpr) ast.Type {
 				// `array << elm`
 				c.check_expr_opt_call(node.right, right_type)
 				node.auto_locked, _ = c.fail_if_immutable(node.left)
-				left_value_type := c.table.value_type(left_type)
-				left_value_sym := c.table.get_type_symbol(left_value_type)
+				left_value_type := c.table.value_type(c.unwrap_generic(left_type))
+				left_value_sym := c.table.get_type_symbol(c.unwrap_generic(left_value_type))
 				if left_value_sym.kind == .interface_ {
 					if right_final.kind != .array {
 						// []Animal << Cat
@@ -1489,8 +1489,9 @@ pub fn (mut c Checker) infix_expr(mut node ast.InfixExpr) ast.Type {
 					return ast.void_type
 				}
 				// []T << T or []T << []T
-				if c.check_types(right_type, left_value_type)
-					|| c.check_types(right_type, left_type) {
+				unwrapped_right_type := c.unwrap_generic(right_type)
+				if c.check_types(unwrapped_right_type, left_value_type)
+					|| c.check_types(unwrapped_right_type, left_type) {
 					return ast.void_type
 				}
 				c.error('cannot append `$right_sym.name` to `$left_sym.name`', right_pos)

vlib/v/gen/c/fn.v

@@ -548,7 +548,23 @@ fn (mut g Gen) method_call(node ast.CallExpr) {
 	if node.receiver_type == 0 {
 		g.checker_bug('CallExpr.receiver_type is 0 in method_call', node.pos)
 	}
-	unwrapped_rec_type := g.unwrap_generic(node.receiver_type)
+	mut unwrapped_rec_type := node.receiver_type
+	if g.table.cur_fn.generic_names.len > 0 { // in generic fn
+		unwrapped_rec_type = g.unwrap_generic(node.receiver_type)
+	} else { // in non-generic fn
+		sym := g.table.get_type_symbol(node.receiver_type)
+		match sym.info {
+			ast.Struct, ast.Interface, ast.SumType {
+				generic_names := sym.info.generic_types.map(g.table.get_type_symbol(it).name)
+				if utyp := g.table.resolve_generic_to_concrete(node.receiver_type, generic_names,
+					sym.info.concrete_types)
+				{
+					unwrapped_rec_type = utyp
+				}
+			}
+			else {}
+		}
+	}
 	typ_sym := g.table.get_type_symbol(unwrapped_rec_type)
 	rec_cc_type := g.cc_type(unwrapped_rec_type, false)
 	mut receiver_type_name := util.no_dots(rec_cc_type)

vlib/v/tests/generics_interface_method_test.v

@@ -0,0 +1,35 @@
+interface Iter<T> {
+	next() ?T
+}
+
+fn (mut it Iter<T>) collect<T>() []T {
+	mut data := []T{}
+	for {
+		val := it.next() or { break }
+		data << val
+	}
+	return data
+}
+
+struct ArrayIter<T> {
+	data []T
+mut:
+	index int
+}
+
+fn (mut it ArrayIter<T>) next<T>() ?T {
+	if it.index >= it.data.len {
+		return none
+	}
+	defer {
+		it.index++
+	}
+	return it.data[it.index]
+}
+
+fn test_generics_interface_method() {
+	mut iter := Iter<int>(ArrayIter<int>{
+		data: [1, 2, 3]
+	})
+	assert iter.collect() == [1, 2, 3]
+}