From 35ad110f48ae8d57e659c19a4056026e002aeae1 Mon Sep 17 00:00:00 2001
From: Alexander Medvednikov <alexander@medvednikov.com>
Date: Wed, 15 Apr 2026 01:26:27 +0300
Subject: [PATCH] cgen: fix invalid memory access when smart casting an alias
 fntype (fixes #17799)

---
 vlib/v/gen/c/cgen.v                           | 11 ++++++++++-
 .../sumtype_alias_fntype_smartcast_test.v     | 19 +++++++++++++++++++
 2 files changed, 29 insertions(+), 1 deletion(-)
 create mode 100644 vlib/v/tests/sumtypes/sumtype_alias_fntype_smartcast_test.v

diff --git a/vlib/v/gen/c/cgen.v b/vlib/v/gen/c/cgen.v
index 038fd8a6f..bc58dd2d1 100644
--- a/vlib/v/gen/c/cgen.v
+++ b/vlib/v/gen/c/cgen.v
@@ -7835,6 +7835,9 @@ fn (mut g Gen) ident(node ast.Ident) {
 					for i, typ in smartcast_types {
 						is_option_unwrap := i == 0 && is_option
 							&& typ == resolved_var.orig_type.clear_flag(.option)
+						smartcast_final_sym := g.table.final_sym(g.unwrap_generic(typ))
+						sumtype_fn_value_smartcast := obj_sym.kind == .sum_type && !is_auto_heap
+							&& smartcast_final_sym.info is ast.FnType && !is_option_unwrap
 						g.write('(')
 						if i == 0 && resolved_var.is_unwrapped
 							&& resolved_var.ct_type_var == .smartcast {
@@ -7850,7 +7853,7 @@ fn (mut g Gen) ident(node ast.Ident) {
 									styp := g.base_type(resolved_var.typ)
 									g.write('*(${styp}*)')
 								}
-							} else if !g.arg_no_auto_deref && !typ.is_ptr() {
+							} else if !g.arg_no_auto_deref && !sumtype_fn_value_smartcast {
 								g.write('*')
 							}
 						} else if interface_var_needs_deref
@@ -7869,6 +7872,9 @@ fn (mut g Gen) ident(node ast.Ident) {
 					for i, typ in smartcast_types {
 						is_option_unwrap := is_option && typ == resolved_var.typ.clear_flag(.option)
 						cast_sym := g.table.sym(g.unwrap_generic(typ))
+						final_cast_sym := g.table.final_sym(g.unwrap_generic(typ))
+						sumtype_fn_value_smartcast := obj_sym.kind == .sum_type && !is_auto_heap
+							&& final_cast_sym.info is ast.FnType && !is_option_unwrap
 						if obj_sym.kind == .interface && cast_sym.kind == .interface {
 							if cast_sym.cname != obj_sym.cname {
 								ptr := '*'.repeat(resolved_var.typ.nr_muls())
@@ -7882,6 +7888,9 @@ fn (mut g Gen) ident(node ast.Ident) {
 								g.write('${ptr}${node.name}')
 							}
 						} else {
+							if sumtype_fn_value_smartcast {
+								g.write('(${g.styp(typ)})')
+							}
 							mut is_ptr := false
 							if i == 0 {
 								if resolved_var.is_inherited {
diff --git a/vlib/v/tests/sumtypes/sumtype_alias_fntype_smartcast_test.v b/vlib/v/tests/sumtypes/sumtype_alias_fntype_smartcast_test.v
new file mode 100644
index 000000000..679a8eb43
--- /dev/null
+++ b/vlib/v/tests/sumtypes/sumtype_alias_fntype_smartcast_test.v
@@ -0,0 +1,19 @@
+type FunString17799 = FnAlias17799 | string
+
+type FnAlias17799 = fn () string
+
+fn foo_bar_17799(foo FnAlias17799) string {
+	return foo()
+}
+
+fn test_sumtype_alias_fntype_smartcast() {
+	fs := FunString17799(FnAlias17799(fn () string {
+		return 'bar'
+	}))
+	assert foo_bar_17799(fs as FnAlias17799) == 'bar'
+	if fs is FnAlias17799 {
+		assert foo_bar_17799(fs) == 'bar'
+		return
+	}
+	assert false
+}
-- 
2.39.5