From a7039193c3425333ef1f754a2780111733af8167 Mon Sep 17 00:00:00 2001
From: Alexander Medvednikov <alexander@medvednikov.com>
Date: Tue, 21 Apr 2026 15:30:02 +0300
Subject: [PATCH] checker: fix returning private struct from public function
 works without noinit (fixes #18427)

---
 vlib/v/checker/checker.v                      | 22 +++++++++++++++++++
 vlib/v/checker/fn.v                           |  7 ++++++
 vlib/v/checker/str.v                          |  2 ++
 .../private_struct_return_autostr_err.out     |  6 +++++
 .../private_struct_return_autostr_err.vv      |  6 +++++
 5 files changed, 43 insertions(+)
 create mode 100644 vlib/v/checker/tests/private_struct_return_autostr_err.out
 create mode 100644 vlib/v/checker/tests/private_struct_return_autostr_err.vv

diff --git a/vlib/v/checker/checker.v b/vlib/v/checker/checker.v
index 0579622bf..1aaa6a389 100644
--- a/vlib/v/checker/checker.v
+++ b/vlib/v/checker/checker.v
@@ -4908,6 +4908,9 @@ pub fn (mut c Checker) expr(mut node ast.Expr) ast.Type {
 					node.expr.pos())
 				return ast.void_type
 			}
+			if c.fail_if_private_implicit_str(node.expr_type, node.expr.pos(), 'dump') {
+				return ast.void_type
+			}
 
 			unwrapped_expr_type := c.unwrap_generic(node.expr_type)
 			tsym := c.table.sym(unwrapped_expr_type)
@@ -8643,6 +8646,25 @@ fn (mut c Checker) fail_if_unreadable(expr ast.Expr, typ ast.Type, what string)
 	return false
 }
 
+fn (mut c Checker) fail_if_private_implicit_str(typ ast.Type, pos token.Pos, action string) bool {
+	base_typ := c.unwrap_generic(typ.clear_option_and_result().clear_flags(.variadic, .shared_f,
+		.atomic_f).clear_ref())
+	if base_typ == 0 {
+		return false
+	}
+	final_sym := c.table.final_sym(base_typ)
+	if final_sym.language != .v || final_sym.kind != .struct || final_sym.is_pub
+		|| final_sym.mod == c.mod || final_sym.mod in ['builtin', 'main'] {
+		return false
+	}
+	if final_sym.has_method_with_generic_parent('str') {
+		return false
+	}
+	c.error('cannot ${action} private type `${final_sym.name}` outside module `${final_sym.mod}` without an explicit `str()` method',
+		pos)
+	return true
+}
+
 fn (mut c Checker) interface_embeds_interface(interface_type ast.Type, embedded_interface_type ast.Type) bool {
 	mut visited := map[int]bool{}
 	return c.interface_embeds_interface_recursive(interface_type, embedded_interface_type, mut
diff --git a/vlib/v/checker/fn.v b/vlib/v/checker/fn.v
index 5425fb172..bf5e31bd1 100644
--- a/vlib/v/checker/fn.v
+++ b/vlib/v/checker/fn.v
@@ -1468,6 +1468,8 @@ fn (mut c Checker) builtin_args(mut node ast.CallExpr, fn_name string, func &ast
 			node.pos)
 	} else if arg.expr is ast.ArrayDecompose {
 		c.error('`${fn_name}` cannot print variadic values', node.pos)
+	} else if c.fail_if_private_implicit_str(arg.typ, node.pos, 'print') {
+		return
 	}
 	c.fail_if_unreadable(arg.expr, arg.typ, 'argument to print')
 	c.inside_interface_deref = false
@@ -3107,6 +3109,11 @@ fn (mut c Checker) method_call(mut node ast.CallExpr, mut continue_check &bool)
 				c.error('interface `${iname}` does not have a .str() method. Use typeof() instead',
 					node.pos)
 			}
+			if c.fail_if_private_implicit_str(left_type, node.pos,
+				'call auto-generated `.str()` on')
+			{
+				return ast.string_type
+			}
 			node.receiver_type = left_type.clear_ref()
 			node.return_type = ast.string_type
 			if node.args.len > 0 {
diff --git a/vlib/v/checker/str.v b/vlib/v/checker/str.v
index 8eb7aeb84..b5644cdba 100644
--- a/vlib/v/checker/str.v
+++ b/vlib/v/checker/str.v
@@ -90,6 +90,8 @@ fn (mut c Checker) string_inter_lit(mut node ast.StringInterLiteral) ast.Type {
 		} else if ftyp == ast.char_type && ftyp.nr_muls() == 0 {
 			c.error('expression returning type `char` cannot be used in string interpolation directly, print its address or cast it to an integer instead',
 				expr.pos())
+		} else if c.fail_if_private_implicit_str(ftyp, expr.pos(), 'interpolate') {
+			return ast.string_type
 		}
 		if ftyp == 0 {
 			return ast.void_type
diff --git a/vlib/v/checker/tests/private_struct_return_autostr_err.out b/vlib/v/checker/tests/private_struct_return_autostr_err.out
new file mode 100644
index 000000000..fe54c3fc2
--- /dev/null
+++ b/vlib/v/checker/tests/private_struct_return_autostr_err.out
@@ -0,0 +1,6 @@
+vlib/v/checker/tests/private_struct_return_autostr_err.vv:5:7: error: cannot dump private type `v.checker.tests.amod.PrivateFoo` outside module `v.checker.tests.amod` without an explicit `str()` method
+    3 | fn main() {
+    4 |     a := amod.new_private_foo(1)
+    5 |     dump(a)
+      |          ^
+    6 | }
diff --git a/vlib/v/checker/tests/private_struct_return_autostr_err.vv b/vlib/v/checker/tests/private_struct_return_autostr_err.vv
new file mode 100644
index 000000000..3ba341eee
--- /dev/null
+++ b/vlib/v/checker/tests/private_struct_return_autostr_err.vv
@@ -0,0 +1,6 @@
+import v.checker.tests.amod
+
+fn main() {
+	a := amod.new_private_foo(1)
+	dump(a)
+}
-- 
2.39.5