parser: check interface name using single letter capital (#14878)

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)

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

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)
+}