From a41a13ac62cef64c319e42b075ad7305ed567cbc Mon Sep 17 00:00:00 2001
From: Alexander Medvednikov <alexander@medvednikov.com>
Date: Fri, 19 Jun 2026 23:19:15 +0300
Subject: [PATCH] fmt: don't prefix current-module types inside `fn` typed
 struct fields (#27475) (#27506)

---
 vlib/v/ast/types.v                                 | 14 ++++++++++++++
 vlib/v/fmt/fmt.v                                   |  5 +++++
 .../struct_fn_field_current_module_type_keep.vv    | 14 ++++++++++++++
 3 files changed, 33 insertions(+)
 create mode 100644 vlib/v/fmt/tests/struct_fn_field_current_module_type_keep.vv

diff --git a/vlib/v/ast/types.v b/vlib/v/ast/types.v
index 62d5ad5a6..156ef7050 100644
--- a/vlib/v/ast/types.v
+++ b/vlib/v/ast/types.v
@@ -1642,6 +1642,20 @@ pub fn (t &Table) delete_cached_type_to_str(typ Type, import_aliases_len int) {
 	}
 }
 
+// invalidate_type_to_str_cache clears every cached `type_to_str` result.
+// vfmt calls this whenever it enters a new module (see Fmt.set_current_module_name),
+// because strings that were memoized before the current module was known (e.g. while
+// parsing, when `cmod_prefix` was still empty) keep stale `mod.` prefixes on types
+// that actually belong to the current module. Those stale entries are otherwise reused
+// when the signature of a `fn` typed struct field is rebuilt, producing output like
+// `fn (s main.Struct) bool` instead of `fn (s Struct) bool` (issue #27475).
+pub fn (t &Table) invalidate_type_to_str_cache() {
+	mut mt := unsafe { &Table(t) }
+	lock mt.cached_type_to_str {
+		mt.cached_type_to_str.clear()
+	}
+}
+
 // import_aliases is a map of imported symbol aliases 'module.Type' => 'Type'
 pub fn (t &Table) type_to_str_using_aliases(typ Type, import_aliases map[string]string) string {
 	cache_key := (u64(import_aliases.len) << 32) | u64(typ)
diff --git a/vlib/v/fmt/fmt.v b/vlib/v/fmt/fmt.v
index a347dd617..e68bd614d 100644
--- a/vlib/v/fmt/fmt.v
+++ b/vlib/v/fmt/fmt.v
@@ -268,6 +268,11 @@ fn (mut f Fmt) write_generic_types(gtypes []ast.Type) {
 pub fn (mut f Fmt) set_current_module_name(cmodname string) {
 	f.cur_mod = cmodname
 	f.table.cmod_prefix = cmodname + '.'
+	// Drop type_to_str strings that were memoized before the module context was
+	// known (during parsing `cmod_prefix` is still empty), so that types of the
+	// current module are not left with a stale `mod.` prefix when a `fn` typed
+	// field signature is rebuilt later on (issue #27475).
+	f.table.invalidate_type_to_str_cache()
 }
 
 fn (f &Fmt) get_modname_prefix(mname string) (string, string) {
diff --git a/vlib/v/fmt/tests/struct_fn_field_current_module_type_keep.vv b/vlib/v/fmt/tests/struct_fn_field_current_module_type_keep.vv
new file mode 100644
index 000000000..b801f37d8
--- /dev/null
+++ b/vlib/v/fmt/tests/struct_fn_field_current_module_type_keep.vv
@@ -0,0 +1,14 @@
+struct Struct1 {}
+
+struct Struct2 {}
+
+// Regression test for https://github.com/vlang/v/issues/27475 :
+// types of the current module must not gain a `main.` prefix inside the
+// signature of a `fn` typed struct field (the bug only showed up when the
+// file had no imports, i.e. an empty alias map).
+struct Holder {
+	field1 fn (s1 Struct1, s2 Struct2) bool
+	field2 Struct1
+	field3 fn (s Struct1) Struct2
+	field4 fn (int, Struct1) (Struct2, bool)
+}
-- 
2.39.5