cgen: fix generating invalid Option decls for generic interfaces (#10817)

vlib/v/gen/c/cgen.v

@@ -1375,10 +1375,13 @@ fn (mut g Gen) stmt(node ast.Stmt) {
 		ast.Import {}
 		ast.InterfaceDecl {
 			// definitions are sorted and added in write_types
-			for method in node.methods {
+			ts := g.table.get_type_symbol(node.typ)
-				if method.return_type.has_flag(.optional) {
+			if !(ts.info as ast.Interface).is_generic {
-					// Register an optional if it's not registered yet
+				for method in node.methods {
-					g.register_optional(method.return_type)
+					if method.return_type.has_flag(.optional) {
+						// Register an optional if it's not registered yet
+						g.register_optional(method.return_type)
+					}
 				}
 			}
 		}

vlib/v/tests/generic_interface_test.v

@@ -63,3 +63,36 @@ fn test_extract_basic() {
 	cc := extract_basic(c)
 	assert '$aa | $bb | $cc' == '123 | 456 | 789'
 }
+
+//////
+interface Iterator<T> {
+	next() ?T
+}
+
+struct NumberIterator<T> {
+	limit T
+mut:
+	val T
+}
+
+fn (mut i NumberIterator<T>) next<T>() ?T {
+	if i.val >= i.limit {
+		return none
+	}
+	i.val++
+	return i.val
+}
+
+fn test_iterator_implementation() {
+	mut i := Iterator<int>(NumberIterator<int>{
+		limit: 10
+	})
+	for {
+		if val := i.next() {
+			println(val)
+		} else {
+			println('iterator is done')
+			break
+		}
+	}
+}