From 78c5946200278130882d01ad7e35b0e23a6c7f9c Mon Sep 17 00:00:00 2001
From: Alexander Medvednikov <alexander@medvednikov.com>
Date: Sat, 31 Jan 2026 20:49:21 +0300
Subject: [PATCH] cgen: fix uninitialized interface bug (closes #17372)

---
 vlib/v/gen/c/auto_str_methods.v               |  1 +
 .../interface_uninitialized_field_str_test.v  | 32 +++++++++++++++++++
 2 files changed, 33 insertions(+)
 create mode 100644 vlib/v/tests/interfaces/interface_uninitialized_field_str_test.v

diff --git a/vlib/v/gen/c/auto_str_methods.v b/vlib/v/gen/c/auto_str_methods.v
index 6fd1585d5..166ed0008 100644
--- a/vlib/v/gen/c/auto_str_methods.v
+++ b/vlib/v/gen/c/auto_str_methods.v
@@ -400,6 +400,7 @@ fn (mut g Gen) gen_str_for_interface(info ast.Interface, styp string, typ_str st
 	mut fn_builder := strings.new_builder(512)
 	clean_interface_v_type_name := util.strip_main_name(typ_str)
 	fn_builder.writeln('${g.static_non_parallel}string indent_${str_fn_name}(${styp} x, ${ast.int_type_name} indent_count) { /* gen_str_for_interface */')
+	fn_builder.writeln('\tif (x._typ == 0 && x._object == NULL) return _S("nil");')
 	for typ in info.types {
 		sub_sym := g.table.sym(ast.mktyp(typ))
 		if g.pref.skip_unused && sub_sym.idx !in g.table.used_features.used_syms {
diff --git a/vlib/v/tests/interfaces/interface_uninitialized_field_str_test.v b/vlib/v/tests/interfaces/interface_uninitialized_field_str_test.v
new file mode 100644
index 000000000..73a98ecd0
--- /dev/null
+++ b/vlib/v/tests/interfaces/interface_uninitialized_field_str_test.v
@@ -0,0 +1,32 @@
+// Test for bug #17372: Uninitialized interface type field gives runtime error when accessing it
+// Expected behavior: print "nil" instead of causing a runtime crash
+
+interface Boo {}
+
+struct Foo {
+	boo Boo
+}
+
+@[noinit]
+struct Bar {
+	boo Boo
+}
+
+fn test_uninitialized_interface_field_str() {
+	// Printing uninitialized interface field should return "nil", not crash
+	s := '${Foo{}.boo}'
+	assert s == 'nil', 'Expected "nil" for uninitialized interface, got "${s}"'
+}
+
+fn test_uninitialized_interface_field_in_struct_str() {
+	// Struct containing uninitialized interface should also work
+	foo := Foo{}
+	s := '${foo}'
+	assert s.contains('boo: nil'), 'Expected struct str to contain "boo: nil", got "${s}"'
+}
+
+fn test_noinit_struct_with_interface_field() {
+	bar := Bar{}
+	s := '${bar.boo}'
+	assert s == 'nil', 'Expected "nil" for noinit struct interface field, got "${s}"'
+}
-- 
2.39.5