From 17228e2121f47dd31e6d5845c98680234fc29c64 Mon Sep 17 00:00:00 2001
From: Alexander Medvednikov <alexander@medvednikov.com>
Date: Wed, 15 Apr 2026 01:44:22 +0300
Subject: [PATCH] checker: fix private C definition causing compile error if
 importing `os` earlier than others (fixes #15811)

---
 vlib/v/ast/table.v                            |  5 +++
 vlib/v/checker/checker.v                      | 34 ++++++++++++++++---
 vlib/v/checker/fn.v                           |  5 +++
 vlib/v/tests/project_issue_15811/a/a.c.v      |  6 ++++
 vlib/v/tests/project_issue_15811/b/b.c.v      | 18 ++++++++++
 vlib/v/tests/project_issue_15811/main.v       |  8 +++++
 vlib/v/tests/project_issue_15811/v.mod        |  3 ++
 vlib/v/tests/project_issue_15811_check_test.v |  9 +++++
 8 files changed, 84 insertions(+), 4 deletions(-)
 create mode 100644 vlib/v/tests/project_issue_15811/a/a.c.v
 create mode 100644 vlib/v/tests/project_issue_15811/b/b.c.v
 create mode 100644 vlib/v/tests/project_issue_15811/main.v
 create mode 100644 vlib/v/tests/project_issue_15811/v.mod
 create mode 100644 vlib/v/tests/project_issue_15811_check_test.v

diff --git a/vlib/v/ast/table.v b/vlib/v/ast/table.v
index c88422855..506bd8a12 100644
--- a/vlib/v/ast/table.v
+++ b/vlib/v/ast/table.v
@@ -1142,6 +1142,11 @@ fn (mut t Table) rewrite_already_registered_symbol(typ TypeSymbol, existing_idx
 		}
 		return existing_idx
 	}
+	// Keep concrete C type re-declarations so later modules can resolve their own
+	// symbol instead of inheriting the first module's private metadata.
+	if existing_symbol.language == .c && typ.language == .c {
+		return -2
+	}
 	// Override the already registered builtin types with the actual
 	// v struct declarations in the vlib/builtin module sources:
 	if (existing_idx >= string_type_idx && existing_idx <= map_type_idx)
diff --git a/vlib/v/checker/checker.v b/vlib/v/checker/checker.v
index e1f82d19a..d80b719b5 100644
--- a/vlib/v/checker/checker.v
+++ b/vlib/v/checker/checker.v
@@ -943,6 +943,7 @@ fn (mut c Checker) alias_type_decl(mut node ast.AliasTypeDecl) {
 	if c.file.mod.name != 'builtin' && !node.name.starts_with('C.') {
 		c.check_valid_pascal_case(node.name, 'type alias', node.pos)
 	}
+	node.parent_type = c.preferred_c_symbol_type(node.parent_type)
 	if c.pref.is_vls && c.pref.linfo.method == .definition {
 		if c.vls_is_the_node(node.type_pos) {
 			typ_str := c.table.type_to_str(node.parent_type)
@@ -1079,6 +1080,30 @@ fn (mut c Checker) alias_type_decl(mut node ast.AliasTypeDecl) {
 	}
 }
 
+fn (c &Checker) preferred_c_symbol_type(typ ast.Type) ast.Type {
+	sym := c.table.sym(typ)
+	if sym.language != .c || sym.name == '' {
+		return typ
+	}
+	mut public_typ := ast.invalid_type
+	for i := c.table.type_symbols.len - 1; i >= 1; i-- {
+		candidate := c.table.type_symbols[i]
+		if candidate.language != .c || candidate.name != sym.name || candidate.kind == .placeholder {
+			continue
+		}
+		if candidate.mod == c.mod {
+			return ast.new_type(i).derive(typ)
+		}
+		if candidate.is_pub && public_typ == ast.invalid_type {
+			public_typ = ast.new_type(i).derive(typ)
+		}
+	}
+	if public_typ != ast.invalid_type {
+		return public_typ
+	}
+	return typ
+}
+
 fn (mut c Checker) check_alias_vs_element_type_of_parent(node ast.AliasTypeDecl, element_type_of_parent ast.Type,
 	label string) {
 	if node.typ.idx() != element_type_of_parent.idx() {
@@ -8075,7 +8100,8 @@ fn (mut c Checker) ensure_generic_type_specify_type_names(typ ast.Type, pos toke
 }
 
 fn (mut c Checker) ensure_type_exists(typ ast.Type, pos token.Pos) bool {
-	if typ == 0 {
+	mut checked_typ := c.preferred_c_symbol_type(typ)
+	if checked_typ == 0 {
 		c.error('unknown type', pos)
 		return c.pref.is_vls
 	}
@@ -8088,9 +8114,9 @@ fn (mut c Checker) ensure_type_exists(typ ast.Type, pos token.Pos) bool {
 			pos)
 		return c.pref.is_vls
 	}
-	sym := c.table.sym(typ)
+	sym := c.table.sym(checked_typ)
 	if !c.is_builtin_mod && !sym.is_pub && sym.mod != c.mod && sym.mod != 'main'
-		&& typ !in c.table.cur_concrete_types {
+		&& checked_typ !in c.table.cur_concrete_types {
 		if sym.kind == .function {
 			fn_info := sym.info as ast.FnType
 			// hack: recover fn mod from func name
@@ -8199,7 +8225,7 @@ fn (mut c Checker) ensure_type_exists(typ ast.Type, pos token.Pos) bool {
 		else {}
 	}
 	if sym.kind == .map {
-		if !c.ensure_supported_map_key_types(typ, pos) {
+		if !c.ensure_supported_map_key_types(checked_typ, pos) {
 			return c.pref.is_vls
 		}
 	}
diff --git a/vlib/v/checker/fn.v b/vlib/v/checker/fn.v
index 8217e9334..cbceadb63 100644
--- a/vlib/v/checker/fn.v
+++ b/vlib/v/checker/fn.v
@@ -569,6 +569,10 @@ fn (mut c Checker) fn_decl(mut node ast.FnDecl) {
 	if node.language == .v {
 		// Make sure all types are valid
 		for mut param in node.params {
+			param.typ = c.preferred_c_symbol_type(param.typ)
+			if mut scoped_param := node.scope.find_var(param.name) {
+				scoped_param.typ = param.typ
+			}
 			// handle vls go to definition for parameter types
 			if c.pref.is_vls && c.pref.linfo.method == .definition {
 				if c.vls_is_the_node(param.type_pos) {
@@ -703,6 +707,7 @@ fn (mut c Checker) fn_decl(mut node ast.FnDecl) {
 		}
 	}
 	if node.return_type != ast.no_type {
+		node.return_type = c.preferred_c_symbol_type(node.return_type)
 		if !c.ensure_type_exists(node.return_type, node.return_type_pos) {
 			return
 		}
diff --git a/vlib/v/tests/project_issue_15811/a/a.c.v b/vlib/v/tests/project_issue_15811/a/a.c.v
new file mode 100644
index 000000000..ddea660b2
--- /dev/null
+++ b/vlib/v/tests/project_issue_15811/a/a.c.v
@@ -0,0 +1,6 @@
+module a
+
+@[typedef]
+struct C.dupe {
+	old int
+}
diff --git a/vlib/v/tests/project_issue_15811/b/b.c.v b/vlib/v/tests/project_issue_15811/b/b.c.v
new file mode 100644
index 000000000..2c051a5b4
--- /dev/null
+++ b/vlib/v/tests/project_issue_15811/b/b.c.v
@@ -0,0 +1,18 @@
+module b
+
+pub fn take(x &C.dupe) int {
+	return x.new
+}
+
+@[typedef]
+pub struct C.dupe {
+	new int
+}
+
+pub type Dupe = C.dupe
+
+pub fn make() &C.dupe {
+	return &C.dupe{
+		new: 1
+	}
+}
diff --git a/vlib/v/tests/project_issue_15811/main.v b/vlib/v/tests/project_issue_15811/main.v
new file mode 100644
index 000000000..f5ed44533
--- /dev/null
+++ b/vlib/v/tests/project_issue_15811/main.v
@@ -0,0 +1,8 @@
+module main
+
+import a as _
+import b
+
+fn main() {
+	assert b.take(b.make()) == 1
+}
diff --git a/vlib/v/tests/project_issue_15811/v.mod b/vlib/v/tests/project_issue_15811/v.mod
new file mode 100644
index 000000000..e44dd0c1e
--- /dev/null
+++ b/vlib/v/tests/project_issue_15811/v.mod
@@ -0,0 +1,3 @@
+Module {
+	name: 'project_issue_15811'
+}
diff --git a/vlib/v/tests/project_issue_15811_check_test.v b/vlib/v/tests/project_issue_15811_check_test.v
new file mode 100644
index 000000000..0306d9801
--- /dev/null
+++ b/vlib/v/tests/project_issue_15811_check_test.v
@@ -0,0 +1,9 @@
+import os
+
+const vexe = os.quoted_path(@VEXE)
+const issue_15811_project = os.join_path(os.dir(@FILE), 'project_issue_15811')
+
+fn test_private_c_redeclaration_order_checks_cleanly() {
+	res := os.execute('${vexe} -check ${os.quoted_path(issue_15811_project)}')
+	assert res.exit_code == 0, res.output
+}
-- 
2.39.5