v.parser: allow anonymous function to return a function (#10592)

vlib/v/parser/fn.v

@@ -648,7 +648,8 @@ fn (mut p Parser) anon_fn() ast.AnonFn {
 	mut return_type_pos := p.tok.position()
 	// lpar: multiple return types
 	if same_line {
-		if p.tok.kind.is_start_of_type() {
+		if (p.tok.kind.is_start_of_type() && (same_line || p.tok.kind != .lsbr))
+			|| (same_line && p.tok.kind == .key_fn) {
 			return_type = p.parse_type()
 			return_type_pos = return_type_pos.extend(p.tok.position())
 		} else if p.tok.kind != .lcbr {

vlib/v/tests/fn_return_fn_test.v

@@ -0,0 +1,20 @@
+fn make_squarer() fn (int) int {
+	return fn (n int) int {
+		return n * n
+	}
+}
+
+fn test_fn_return_fn() {
+	squarer := make_squarer()
+	assert squarer(10) == 100
+}
+
+fn test_anon_fn_return_fn() {
+	make_doubler := fn () fn (int) int {
+		return fn (n int) int {
+			return n + n
+		}
+	}
+	doubler := make_doubler()
+	assert doubler(10) == 20
+}