From 9e8f69efbae14402ca57b93f97c8737ed12279c1 Mon Sep 17 00:00:00 2001
From: Alexander Medvednikov <alexander@medvednikov.com>
Date: Wed, 25 Mar 2026 16:42:16 +0300
Subject: [PATCH] cgen: fix not finding structure defined in a separate file
 (fixes #15724)

---
 vlib/v/gen/c/cgen.v                           | 22 +++++++++++++++++++
 vlib/v/tests/project_issue_15724/main.v       | 10 +++++++++
 .../project_compiles_test.v                   |  5 +++++
 vlib/v/tests/project_issue_15724/v.mod        |  0
 vlib/v/tests/project_issue_15724/z_decl.v     |  5 +++++
 5 files changed, 42 insertions(+)
 create mode 100644 vlib/v/tests/project_issue_15724/main.v
 create mode 100644 vlib/v/tests/project_issue_15724/project_compiles_test.v
 create mode 100644 vlib/v/tests/project_issue_15724/v.mod
 create mode 100644 vlib/v/tests/project_issue_15724/z_decl.v

diff --git a/vlib/v/gen/c/cgen.v b/vlib/v/gen/c/cgen.v
index 8007f9752..25a6f029e 100644
--- a/vlib/v/gen/c/cgen.v
+++ b/vlib/v/gen/c/cgen.v
@@ -8776,12 +8776,30 @@ fn (mut g Gen) ensure_fixed_array_option_definition(elem_type ast.Type) bool {
 	return true
 }
 
+fn (g &Gen) should_emit_private_c_struct(sym &ast.TypeSymbol, info ast.Struct) bool {
+	if sym.language != .c || sym.kind != .struct || info.is_anon || info.is_typedef {
+		return false
+	}
+	if !sym.name.all_after('C.').starts_with('_') {
+		return false
+	}
+	if info.name_pos.file_idx < 0 || int(info.name_pos.file_idx) >= g.table.filelist.len {
+		return false
+	}
+	decl_path := g.table.filelist[info.name_pos.file_idx]
+	return !decl_path.ends_with('.c.v')
+}
+
 fn (mut g Gen) write_types(symbols []&ast.TypeSymbol) {
 	mut struct_names := map[string]bool{}
 	for sym in symbols {
 		if sym.name.starts_with('C.') {
 			if sym.info is ast.Struct && sym.info.is_anon {
 				// For `C___VAnonStruct`, we need to create a new struct to make auto_str work.
+			} else if sym.info is ast.Struct && g.should_emit_private_c_struct(sym, sym.info) {
+				// Private C tags like `C._gpgme_key` are often only forward-declared in headers.
+				// When they are defined in a plain `.v` file, cgen needs to emit the backing
+				// struct body instead of assuming the C headers will provide it.
 			} else {
 				continue
 			}
@@ -8793,6 +8811,10 @@ fn (mut g Gen) write_types(symbols []&ast.TypeSymbol) {
 			g.typedefs.writeln('typedef struct none none;')
 		}
 		mut name := sym.scoped_cname()
+		if sym.name.starts_with('C.') && sym.info is ast.Struct
+			&& g.should_emit_private_c_struct(sym, sym.info) {
+			name = sym.name.all_after('C.')
+		}
 		if g.pref.skip_unused && g.table.used_features.used_maps == 0 {
 			if name in ['map', 'mapnode', 'SortedMap', 'MapMode', 'DenseArray'] {
 				continue
diff --git a/vlib/v/tests/project_issue_15724/main.v b/vlib/v/tests/project_issue_15724/main.v
new file mode 100644
index 000000000..e0be177fa
--- /dev/null
+++ b/vlib/v/tests/project_issue_15724/main.v
@@ -0,0 +1,10 @@
+module main
+
+fn get_key_refcount() u32 {
+	key := &C._gpgme_key(unsafe { nil })
+	return key._refs
+}
+
+fn main() {
+	assert get_key_refcount() == 0
+}
diff --git a/vlib/v/tests/project_issue_15724/project_compiles_test.v b/vlib/v/tests/project_issue_15724/project_compiles_test.v
new file mode 100644
index 000000000..edb1e414d
--- /dev/null
+++ b/vlib/v/tests/project_issue_15724/project_compiles_test.v
@@ -0,0 +1,5 @@
+module main
+
+fn test_compiles() {
+	assert true
+}
diff --git a/vlib/v/tests/project_issue_15724/v.mod b/vlib/v/tests/project_issue_15724/v.mod
new file mode 100644
index 000000000..e69de29bb
diff --git a/vlib/v/tests/project_issue_15724/z_decl.v b/vlib/v/tests/project_issue_15724/z_decl.v
new file mode 100644
index 000000000..7823d348b
--- /dev/null
+++ b/vlib/v/tests/project_issue_15724/z_decl.v
@@ -0,0 +1,5 @@
+module main
+
+struct C._gpgme_key {
+	_refs u32
+}
-- 
2.39.5