From da1f22c128a004525a142124f1f29efd1236990d Mon Sep 17 00:00:00 2001
From: Alexander Medvednikov <alexander@medvednikov.com>
Date: Wed, 11 Mar 2026 11:40:55 +0300
Subject: [PATCH] checker: cannot return a function from a generic function
 (fixes #22011)

---
 vlib/v/ast/table.v                            | 34 ++++++++------
 ...eturn_closure_generic_struct_result_test.v | 44 +++++++++++++++++++
 2 files changed, 64 insertions(+), 14 deletions(-)
 create mode 100644 vlib/v/tests/generics/return_closure_generic_struct_result_test.v

diff --git a/vlib/v/ast/table.v b/vlib/v/ast/table.v
index 2677682de..bfb795116 100644
--- a/vlib/v/ast/table.v
+++ b/vlib/v/ast/table.v
@@ -1933,23 +1933,29 @@ pub fn (mut t Table) convert_generic_type(generic_type Type, generic_names []str
 		FnType {
 			mut func := sym.info.func
 			mut has_generic := false
-			if func.return_type.has_flag(.generic) {
-				if typ := t.convert_generic_type(func.return_type, generic_names, to_types) {
-					func.return_type = typ
-					if typ.has_flag(.generic) {
-						has_generic = true
-					}
-				}
+			orig_return_type := func.return_type
+			if typ := t.convert_generic_type(func.return_type, generic_names, to_types) {
+				func.return_type = typ
+			}
+			if t.sym(func.return_type).kind == .placeholder {
+				func.return_type = t.unwrap_generic_type_ex(orig_return_type, generic_names,
+					to_types, true)
+			}
+			if func.return_type.has_flag(.generic) || t.generic_type_names(func.return_type).len > 0 {
+				has_generic = true
 			}
 			func.params = func.params.clone()
 			for mut param in func.params {
-				if param.typ.has_flag(.generic) {
-					if typ := t.convert_generic_param_type(param, generic_names, to_types) {
-						param.typ = typ
-						if typ.has_flag(.generic) {
-							has_generic = true
-						}
-					}
+				orig_param_type := param.typ
+				if typ := t.convert_generic_param_type(param, generic_names, to_types) {
+					param.typ = typ
+				}
+				if t.sym(param.typ).kind == .placeholder {
+					param.typ = t.unwrap_generic_type_ex(orig_param_type, generic_names,
+						to_types, true)
+				}
+				if param.typ.has_flag(.generic) || t.generic_type_names(param.typ).len > 0 {
+					has_generic = true
 				}
 				if param.orig_typ.has_flag(.generic) {
 					if otyp := t.convert_generic_type(param.orig_typ, generic_names, to_types) {
diff --git a/vlib/v/tests/generics/return_closure_generic_struct_result_test.v b/vlib/v/tests/generics/return_closure_generic_struct_result_test.v
new file mode 100644
index 000000000..b5d213102
--- /dev/null
+++ b/vlib/v/tests/generics/return_closure_generic_struct_result_test.v
@@ -0,0 +1,44 @@
+struct ParserResult[O] {
+	i []u8
+	o O
+}
+
+type Parser[O] = fn ([]u8) !ParserResult[O]
+
+type Mapper[O] = fn ([]u8) O
+
+fn take_part(count int) Parser[[]u8] {
+	return fn [count] (i []u8) !ParserResult[[]u8] {
+		if i.len < count {
+			return error('invalid input length')
+		}
+		return ParserResult[[]u8]{
+			i: i[count..]
+			o: i[..count]
+		}
+	}
+}
+
+fn map_parser[O](p Parser[[]u8], f Mapper[O]) Parser[O] {
+	return fn [p, f] [O](i []u8) !ParserResult[O] {
+		r := p(i)!
+		return ParserResult[O]{
+			i: r.i
+			o: f[O](r.o)
+		}
+	}
+}
+
+fn first_byte_value(input []u8) int {
+	return int(input[0])
+}
+
+fn test_return_closure_generic_struct_result() {
+	parser := map_parser[int](take_part(2), first_byte_value)
+	result := parser('ab_rest'.bytes()) or {
+		assert false
+		return
+	}
+	assert result.o == int(`a`)
+	assert result.i == '_rest'.bytes()
+}
-- 
2.39.5