From 09630dd0bc33bbc059e014c63f624bcc713441c4 Mon Sep 17 00:00:00 2001
From: yuyi <yuyi98@163.com>
Date: Tue, 28 Jun 2022 13:29:23 +0800
Subject: [PATCH] parser: check interface name using single letter capital
 (#14878)

---
 vlib/v/parser/struct.v                     |  5 +++++
 vlib/v/parser/tests/interface_name_err.out |  5 +++++
 vlib/v/parser/tests/interface_name_err.vv  | 19 +++++++++++++++++++
 3 files changed, 29 insertions(+)
 create mode 100644 vlib/v/parser/tests/interface_name_err.out
 create mode 100644 vlib/v/parser/tests/interface_name_err.vv

diff --git a/vlib/v/parser/struct.v b/vlib/v/parser/struct.v
index c12a4437f1..affd50a570 100644
--- a/vlib/v/parser/struct.v
+++ b/vlib/v/parser/struct.v
@@ -493,6 +493,11 @@ fn (mut p Parser) interface_decl() ast.InterfaceDecl {
 		return ast.InterfaceDecl{}
 	}
 	modless_name := p.check_name()
+	if modless_name.len == 1 && modless_name[0].is_capital() {
+		p.error_with_pos('single letter capital names are reserved for generic template types.',
+			name_pos)
+		return ast.InterfaceDecl{}
+	}
 	if modless_name == 'IError' && p.mod != 'builtin' {
 		p.error_with_pos('cannot register interface `IError`, it is builtin interface type',
 			name_pos)
diff --git a/vlib/v/parser/tests/interface_name_err.out b/vlib/v/parser/tests/interface_name_err.out
new file mode 100644
index 0000000000..34d5e71d9a
--- /dev/null
+++ b/vlib/v/parser/tests/interface_name_err.out
@@ -0,0 +1,5 @@
+vlib/v/parser/tests/interface_name_err.vv:1:11: error: single letter capital names are reserved for generic template types.
+    1 | interface A {
+      |           ^
+    2 |     a int
+    3 |     b() int
diff --git a/vlib/v/parser/tests/interface_name_err.vv b/vlib/v/parser/tests/interface_name_err.vv
new file mode 100644
index 0000000000..f67cfdac85
--- /dev/null
+++ b/vlib/v/parser/tests/interface_name_err.vv
@@ -0,0 +1,19 @@
+interface A {
+	a int
+	b() int
+}
+
+struct Ba {
+	a int
+}
+
+fn (b &Ba) b() int {
+	return b.a
+}
+
+fn main() {
+	a := &Ba{
+		a: 1
+	}
+	b := A(a)
+}