cgen: fix generic option/result reference return (#21922)

vlib/v/gen/c/cgen.v

@@ -1161,7 +1161,7 @@ fn (mut g Gen) option_type_name(t ast.Type) (string, string) {
 	} else {
 		styp = '${c.option_name}_${base}'
 	}
-	if t.is_ptr() {
+	if t.is_ptr() || t.has_flag(.generic) {
 		styp = styp.replace('*', '_ptr')
 	}
 	return styp, base
@@ -1183,7 +1183,7 @@ fn (mut g Gen) result_type_name(t ast.Type) (string, string) {
 	} else {
 		styp = '${c.result_name}_${base}'
 	}
-	if t.is_ptr() {
+	if t.is_ptr() || t.has_flag(.generic) {
 		styp = styp.replace('*', '_ptr')
 	}
 	return styp, base

vlib/v/tests/option_ret_ptr_generic_test.v

@@ -0,0 +1,30 @@
+struct Stack[T] {
+mut:
+	elements []T
+}
+
+pub fn (stack Stack[T]) is_empty() bool {
+	return stack.elements.len == 0
+}
+
+pub fn (stack Stack[T]) peek() ?T {
+	return if !stack.is_empty() { stack.elements.last() } else { none }
+}
+
+pub fn (stack Stack[T]) peek2() !T {
+	return if !stack.is_empty() { stack.elements.last() } else { error('Stack is empty') }
+}
+
+@[heap]
+struct Element {
+mut:
+	name  string
+	value string
+}
+
+fn test_main() {
+	mut parent := &Element{
+		name: 'parent element'
+	}
+	mut stack := Stack[&Element]{}
+}