From 7870c08b9033f6f67bea4097730ef71877530956 Mon Sep 17 00:00:00 2001
From: Alexander Medvednikov <alexander@medvednikov.com>
Date: Wed, 25 Mar 2026 16:42:15 +0300
Subject: [PATCH] builtin: fix strings builder bug with -cg (fixes #13613)

---
 vlib/builtin/array.v                          |  3 +-
 vlib/builtin/array_test.v                     |  8 ++++
 .../strings_builder_issue_13613_cg_test.v     | 41 +++++++++++++++++++
 3 files changed, 51 insertions(+), 1 deletion(-)
 create mode 100644 vlib/v/tests/builtin_strings_and_interpolation/strings_builder_issue_13613_cg_test.v

diff --git a/vlib/builtin/array.v b/vlib/builtin/array.v
index 2245f4c99..71c2c7934 100644
--- a/vlib/builtin/array.v
+++ b/vlib/builtin/array.v
@@ -789,10 +789,11 @@ pub fn (mut a array) push_many(val voidptr, size int) {
 		// string interpolation also uses <<; avoid it, use a fixed string for the panic
 		panic('array.push_many: new len exceeds max_int')
 	}
+	is_self_append := a.data == val && a.data != 0
 	if new_len >= a.cap {
 		a.ensure_cap(int(new_len))
 	}
-	if a.data == val && a.data != 0 {
+	if is_self_append {
 		// handle `arr << arr`
 		copy := a.clone()
 		unsafe {
diff --git a/vlib/builtin/array_test.v b/vlib/builtin/array_test.v
index 47fbbafa8..5267b1db2 100644
--- a/vlib/builtin/array_test.v
+++ b/vlib/builtin/array_test.v
@@ -353,6 +353,14 @@ fn test_push_many() {
 	assert a[5] == 6
 }
 
+fn test_push_many_self_append_with_growth() {
+	mut a := [u8(`a`), `b`, `c`]
+	a << a
+	assert a.bytestr() == 'abcabc'
+	a << a
+	assert a.bytestr() == 'abcabcabcabc'
+}
+
 fn test_reverse() {
 	a := [1, 2, 3, 4]
 	b := ['test', 'array', 'reverse']
diff --git a/vlib/v/tests/builtin_strings_and_interpolation/strings_builder_issue_13613_cg_test.v b/vlib/v/tests/builtin_strings_and_interpolation/strings_builder_issue_13613_cg_test.v
new file mode 100644
index 000000000..893a9cf3a
--- /dev/null
+++ b/vlib/v/tests/builtin_strings_and_interpolation/strings_builder_issue_13613_cg_test.v
@@ -0,0 +1,41 @@
+// vtest vflags: -cg
+module main
+
+import strings
+
+struct Issue13613Writer {
+mut:
+	out strings.Builder = strings.new_builder(200)
+}
+
+fn (mut w Issue13613Writer) build() string {
+	w.out.writeln('module main')
+	w.out.writeln('')
+	w.out.writeln('fn main() {')
+	w.out.writeln('\tprint("You\'re")')
+	w.out.writeln('\tmatch 27 {')
+	w.out.writeln('\t\t18 {')
+	w.out.writeln('\t\t\tmut age := 27')
+	w.out.writeln("\t\t\tprintln(' of age')")
+	w.out.writeln('\t\t}')
+	w.out.writeln('\t\t10 {')
+	w.out.writeln('\t\t\tmut age := 27')
+	w.out.writeln("\t\t\tprintln(' a child')")
+	w.out.writeln('\t\t}')
+	w.out.writeln('\t\telse {')
+	w.out.writeln('\t\t\tmut age := 27')
+	w.out.writeln('\t\t\tprintln(".. I don\'t know my program is pretty dumb :/")')
+	w.out.writeln('\t\t}')
+	w.out.writeln('\t}')
+	w.out.write_string('}')
+	return w.out.str()
+}
+
+fn test_strings_builder_growth_with_cg_regression() {
+	expected := 'module main\n\nfn main() {\n\tprint("You\'re")\n\tmatch 27 {\n\t\t18 {\n\t\t\tmut age := 27\n\t\t\tprintln(\' of age\')\n\t\t}\n\t\t10 {\n\t\t\tmut age := 27\n\t\t\tprintln(\' a child\')\n\t\t}\n\t\telse {\n\t\t\tmut age := 27\n\t\t\tprintln(".. I don\'t know my program is pretty dumb :/")\n\t\t}\n\t}\n}'
+	mut w := Issue13613Writer{}
+	got := w.build()
+	assert got.len == expected.len
+	assert got[0] == `m`
+	assert got == expected
+}
-- 
2.39.5