From 37adf10d133fb627282fc865ce88711f042ac7b6 Mon Sep 17 00:00:00 2001
From: Alexander Medvednikov <alexander@medvednikov.com>
Date: Tue, 14 Apr 2026 12:45:24 +0300
Subject: [PATCH] checker: Error handling, compile error (fixes #20980)

---
 vlib/v/checker/fn.v                           | 13 ++++++++++++
 .../tests/map_result_callback_fn_type_err.out | 20 +++++++++++++++++++
 .../tests/map_result_callback_fn_type_err.vv  | 14 +++++++++++++
 3 files changed, 47 insertions(+)
 create mode 100644 vlib/v/checker/tests/map_result_callback_fn_type_err.out
 create mode 100644 vlib/v/checker/tests/map_result_callback_fn_type_err.vv

diff --git a/vlib/v/checker/fn.v b/vlib/v/checker/fn.v
index 8d5f1b647..7eba0985b 100644
--- a/vlib/v/checker/fn.v
+++ b/vlib/v/checker/fn.v
@@ -4342,6 +4342,19 @@ fn (mut c Checker) array_builtin_method_call(mut node ast.CallExpr, left_type as
 				arg_type
 			}
 		}
+		normalized_ret_type := if c.table.sym(ret_type).kind == .alias {
+			unaliased_ret_type := c.table.unaliased_type(ret_type)
+			if unaliased_ret_type.has_option_or_result() {
+				unaliased_ret_type
+			} else {
+				ret_type
+			}
+		} else {
+			ret_type
+		}
+		if normalized_ret_type.has_flag(.result) {
+			c.error('cannot use Result type in `${node.name}`', arg0.expr.pos())
+		}
 		if c.pref.new_generic_solver {
 			node.return_type = c.table.find_or_register_array(ret_type)
 		} else {
diff --git a/vlib/v/checker/tests/map_result_callback_fn_type_err.out b/vlib/v/checker/tests/map_result_callback_fn_type_err.out
new file mode 100644
index 000000000..2b8d1858a
--- /dev/null
+++ b/vlib/v/checker/tests/map_result_callback_fn_type_err.out
@@ -0,0 +1,20 @@
+vlib/v/checker/tests/map_result_callback_fn_type_err.vv:9:20: error: cannot use Result type in `map`
+    7 |         return n
+    8 |     }
+    9 |     _ = [1, 2, 3].map(fn (n int) !int {
+      |                       ~~~~~~~~~~~~~~~~~
+   10 |         return n
+   11 |     })
+vlib/v/checker/tests/map_result_callback_fn_type_err.vv:12:20: error: cannot use Result type in `map`
+   10 |         return n
+   11 |     })
+   12 |     _ = [1, 2, 3].map(named_result_cb)
+      |                       ~~~~~~~~~~~~~~~
+   13 |     _ = [1, 2, 3].map(cb)
+   14 | }
+vlib/v/checker/tests/map_result_callback_fn_type_err.vv:13:20: error: cannot use Result type in `map`
+   11 |     })
+   12 |     _ = [1, 2, 3].map(named_result_cb)
+   13 |     _ = [1, 2, 3].map(cb)
+      |                       ~~
+   14 | }
diff --git a/vlib/v/checker/tests/map_result_callback_fn_type_err.vv b/vlib/v/checker/tests/map_result_callback_fn_type_err.vv
new file mode 100644
index 000000000..f609c98ef
--- /dev/null
+++ b/vlib/v/checker/tests/map_result_callback_fn_type_err.vv
@@ -0,0 +1,14 @@
+fn named_result_cb(n int) !int {
+	return n
+}
+
+fn main() {
+	cb := fn (n int) !int {
+		return n
+	}
+	_ = [1, 2, 3].map(fn (n int) !int {
+		return n
+	})
+	_ = [1, 2, 3].map(named_result_cb)
+	_ = [1, 2, 3].map(cb)
+}
-- 
2.39.5