From 9989453673ee8cb6a3324abb16f551669d9c4768 Mon Sep 17 00:00:00 2001
From: Alexander Medvednikov <alexander@medvednikov.com>
Date: Wed, 11 Mar 2026 12:41:13 +0300
Subject: [PATCH] checker: C error: cannot convert 'struct _option_string' to
 'unsigned char *' (fixes #26347)

---
 vlib/v/checker/checker.v                    | 40 ++++++++++++++++++++-
 vlib/v/checker/tests/map_option_key_err.out | 20 +++++++++++
 vlib/v/checker/tests/map_option_key_err.vv  | 10 ++++++
 3 files changed, 69 insertions(+), 1 deletion(-)
 create mode 100644 vlib/v/checker/tests/map_option_key_err.out
 create mode 100644 vlib/v/checker/tests/map_option_key_err.vv

diff --git a/vlib/v/checker/checker.v b/vlib/v/checker/checker.v
index b4cfb4af8..f04cca20e 100644
--- a/vlib/v/checker/checker.v
+++ b/vlib/v/checker/checker.v
@@ -1795,6 +1795,36 @@ fn (mut c Checker) check_expr_option_or_result_call(expr ast.Expr, ret_type ast.
 	return ret_type
 }
 
+fn (mut c Checker) expr_unhandled_option_type(expr ast.Expr) ast.Type {
+	match expr {
+		ast.Ident {
+			if expr.or_expr.kind == .absent && expr.info is ast.IdentVar {
+				return c.type_resolver.get_type_or_default(expr, expr.info.typ)
+			}
+		}
+		ast.CallExpr {
+			if expr.or_block.kind == .absent {
+				return expr.return_type
+			}
+		}
+		ast.SelectorExpr {
+			if expr.or_block.kind == .absent {
+				return expr.typ
+			}
+		}
+		ast.IndexExpr {
+			if expr.or_expr.kind == .absent {
+				return expr.typ
+			}
+		}
+		ast.ParExpr {
+			return c.expr_unhandled_option_type(expr.expr)
+		}
+		else {}
+	}
+	return ast.void_type
+}
+
 fn (mut c Checker) check_or_expr(node ast.OrExpr, ret_type ast.Type, expr_return_type ast.Type, expr ast.Expr) {
 	if node.kind == .propagate_option {
 		if c.table.cur_fn != unsafe { nil } && !c.table.cur_fn.return_type.has_flag(.option)
@@ -6034,10 +6064,18 @@ fn (mut c Checker) index_expr(mut node ast.IndexExpr) ast.Type {
 	} else { // [1]
 		if typ_sym.kind == .map {
 			info := typ_sym.info as ast.Map
+			old_expected_type := c.expected_type
 			c.expected_type = info.key_type
 			index_type := c.expr(mut node.index)
+			c.expected_type = old_expected_type
 			key_type := c.unwrap_generic(info.key_type)
-			if !c.check_types(index_type, key_type) {
+			actual_index_type := c.expr_unhandled_option_type(node.index)
+			if actual_index_type.has_flag(.option) && !key_type.has_flag(.option) {
+				got_typ_str, expected_typ_str := c.get_string_names_of(actual_index_type,
+					key_type)
+				c.error('invalid key: cannot use `${got_typ_str}` as `${expected_typ_str}`, it must be unwrapped first',
+					node.index.pos())
+			} else if !c.check_types(index_type, key_type) {
 				err := c.expected_msg(index_type, key_type)
 				c.error('invalid key: ${err}', node.pos)
 			}
diff --git a/vlib/v/checker/tests/map_option_key_err.out b/vlib/v/checker/tests/map_option_key_err.out
new file mode 100644
index 000000000..44c671346
--- /dev/null
+++ b/vlib/v/checker/tests/map_option_key_err.out
@@ -0,0 +1,20 @@
+vlib/v/checker/tests/map_option_key_err.vv:7:10: error: invalid key: cannot use `?string` as `string`, it must be unwrapped first
+    5 |     mut c := cuid2.new()
+    6 |     key := c.next()
+    7 |     _ = m[c.next()]
+      |             ~~~~~~
+    8 |     m[c.next()] = 1
+    9 |     m[key]++
+vlib/v/checker/tests/map_option_key_err.vv:8:6: error: invalid key: cannot use `?string` as `string`, it must be unwrapped first
+    6 |     key := c.next()
+    7 |     _ = m[c.next()]
+    8 |     m[c.next()] = 1
+      |         ~~~~~~
+    9 |     m[key]++
+   10 | }
+vlib/v/checker/tests/map_option_key_err.vv:9:4: error: invalid key: cannot use `?string` as `string`, it must be unwrapped first
+    7 |     _ = m[c.next()]
+    8 |     m[c.next()] = 1
+    9 |     m[key]++
+      |       ~~~
+   10 | }
diff --git a/vlib/v/checker/tests/map_option_key_err.vv b/vlib/v/checker/tests/map_option_key_err.vv
new file mode 100644
index 000000000..981e1462c
--- /dev/null
+++ b/vlib/v/checker/tests/map_option_key_err.vv
@@ -0,0 +1,10 @@
+import rand.cuid2
+
+fn main() {
+	mut m := map[string]int{}
+	mut c := cuid2.new()
+	key := c.next()
+	_ = m[c.next()]
+	m[c.next()] = 1
+	m[key]++
+}
-- 
2.39.5