parser: fix @@ escape before complex at-expressions in templates (#27701)

Author: EvgenyGuguev

vlib/v/parser/tmpl.v

@@ -205,7 +205,19 @@ fn rewrite_complex_template_at_expressions(line string) string {
 			continue
 		}
 		next := line[i + 1]
-		if next == `@` || next == `{` {
+		// `@@` is the documented escape for a literal `@` (see vlib/v/TEMPLATES.md).
+		// Write BOTH characters and consume them fully (i += 2): otherwise the
+		// second `@` is re-parsed below as the start of a complex at-expression,
+		// turning `@@get('/x')` into `@{get('/x')}`. The final `@@` -> `@` collapse
+		// is performed later in insert_template_code.
+		if next == `@` {
+			b.write_string('@@')
+			i += 2
+			continue
+		}
+		// `@{expr}` is an interpolation; leave it untouched, insert_template_code
+		// turns `@{` into `${`.
+		if next == `{` {
 			b.write_u8(`@`)
 			i++
 			continue

vlib/v/tests/tmpl/at_before_complex.txt

@@ -0,0 +1 @@
+@@get('/x') @@post('/save')

vlib/v/tests/tmpl_escape_test.v

@@ -16,3 +16,13 @@ fn test_keeps_at_in_html_urls() {
 	assert res.contains('https://unpkg.com/hyperscript.org@0.8.1')
 	assert res.contains('<div>template ok</div>')
 }
+
+fn test_escape_double_at_before_complex_at_expression() {
+	res := $tmpl('tmpl/at_before_complex.txt')
+	// @@ before ident(...) must yield the literal `@ident(...)`, not `@{ident(...)}`.
+	// Regression for `@@get('/x')` being rewritten to `@{get('/x')}`.
+	assert res.contains("@get('/x')")
+	assert res.contains("@post('/save')")
+	assert !res.contains('@{get(')
+	assert !res.contains('@{post(')
+}