From 476486007164a36917687b6c6d195faba990c93f Mon Sep 17 00:00:00 2001
From: Alexander Medvednikov <alexander@medvednikov.com>
Date: Mon, 11 May 2026 04:26:40 +0300
Subject: [PATCH] ast: detect C aliases by name prefix in
 rewrite_already_registered_symbol, instead of marking every C alias
 TypeSymbol with language=.c

---
 vlib/v/ast/table.v                                    |  6 ++++--
 .../gen/c/testdata/c_alias_redeclaration.c.must_have  |  2 ++
 vlib/v/gen/c/testdata/c_alias_redeclaration.vv        | 11 +++++++++++
 vlib/v/parser/parser.v                                |  1 -
 4 files changed, 17 insertions(+), 3 deletions(-)
 create mode 100644 vlib/v/gen/c/testdata/c_alias_redeclaration.c.must_have
 create mode 100644 vlib/v/gen/c/testdata/c_alias_redeclaration.vv

diff --git a/vlib/v/ast/table.v b/vlib/v/ast/table.v
index cbdb72aea..ccebd507b 100644
--- a/vlib/v/ast/table.v
+++ b/vlib/v/ast/table.v
@@ -1158,8 +1158,10 @@ fn (mut t Table) rewrite_already_registered_symbol(typ TypeSymbol, existing_idx
 		return existing_idx
 	}
 	// Allow C type aliases to override existing C types (e.g. `type C.WCHAR = u16`
-	// on Windows where WCHAR is already registered from system headers):
-	if typ.kind == .alias && typ.language == .c && existing_symbol.language == .c {
+	// on Windows where WCHAR is already registered from system headers).
+	// Detect C aliases by the `C.` name prefix, since alias TypeSymbols
+	// themselves don't carry .language == .c (only Alias.info.language does):
+	if typ.kind == .alias && typ.name.starts_with('C.') && existing_symbol.name.starts_with('C.') {
 		t.type_symbols[existing_idx] = &TypeSymbol{
 			...typ
 			idx:        existing_idx
diff --git a/vlib/v/gen/c/testdata/c_alias_redeclaration.c.must_have b/vlib/v/gen/c/testdata/c_alias_redeclaration.c.must_have
new file mode 100644
index 000000000..6b6743bb6
--- /dev/null
+++ b/vlib/v/gen/c/testdata/c_alias_redeclaration.c.must_have
@@ -0,0 +1,2 @@
+#define C__RedeclareLong RedeclareLong
+VV_LOC C__RedeclareLong main__use_redeclare_long(C__RedeclareLong x);
diff --git a/vlib/v/gen/c/testdata/c_alias_redeclaration.vv b/vlib/v/gen/c/testdata/c_alias_redeclaration.vv
new file mode 100644
index 000000000..4a0143071
--- /dev/null
+++ b/vlib/v/gen/c/testdata/c_alias_redeclaration.vv
@@ -0,0 +1,11 @@
+// Regression: redeclaring a C alias must not set the alias TypeSymbol's
+// language to .c. If it does, the C backend emits the raw C name instead of
+// the `C__Name` cname for any later use of the alias.
+
+type C.RedeclareLong = int
+type C.RedeclareLong = int
+
+@[markused]
+fn use_redeclare_long(x C.RedeclareLong) C.RedeclareLong {
+	return x
+}
diff --git a/vlib/v/parser/parser.v b/vlib/v/parser/parser.v
index 41bd55a8d..c1a6c23bf 100644
--- a/vlib/v/parser/parser.v
+++ b/vlib/v/parser/parser.v
@@ -3278,7 +3278,6 @@ fn (mut p Parser) type_decl() ast.TypeDecl {
 		cname:      util.no_dots(prepend_mod_name)
 		mod:        p.mod
 		parent_idx: pidx
-		language:   language
 		info:       ast.Alias{
 			parent_type: parent_type
 			language:    parent_language
-- 
2.39.5