cgen: fix uninitialized interface bug (closes #17372)

Author: medvednikov

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 {

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}"'
+}