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checker: detect recursive .str() method calls #27926

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6 changes: 6 additions & 0 deletions vlib/v/checker/fn.v
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Original file line number Diff line number Diff line change
Expand Up @@ -3590,6 +3590,12 @@ fn (mut c Checker) method_call(mut node ast.CallExpr, mut continue_check &bool)
&& c.table.register_fn_concrete_types(method.fkey(), concrete_types) {
c.need_recheck_generic_fns = true
}
if method_name == 'str' && c.table.cur_fn != unsafe { nil } && c.table.cur_fn.is_method
&& c.table.cur_fn.name == 'str' && node.left is ast.Ident
&& (node.left as ast.Ident).name == c.table.cur_fn.receiver.name
Comment on lines +3594 to +3595

@chatgpt-codex-connector chatgpt-codex-connector Bot Jul 24, 2026

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P2 Badge Handle parenthesized receivers in recursive str check

In a str method, (n).str() is semantically the same recursive call as n.str(), but node.left remains an ast.ParExpr, so this node.left is ast.Ident guard skips the new diagnostic and still allows the runtime recursion/stack overflow that this check is meant to prevent. method_call already computes an unwrapped left_expr := node.left.remove_par() earlier, so using that here would catch both forms.

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&& left_type.idx() == c.table.cur_fn.receiver.typ.idx() {
c.error('cannot call `str()` method recursively', node.pos)
Comment on lines +3593 to +3597

@chatgpt-codex-connector chatgpt-codex-connector Bot Jul 24, 2026

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P1 Badge Preserve existing alias-array str implementations

This condition also fires while checking alias receivers that implement an interface by providing str() and delegate to the underlying array formatting, for example the existing type List = []Value; fn (x List) str() string { return x.str() } fixtures in vlib/v/tests/aliases/alias_array_returned_as_interface_test.v and vlib/v/tests/aliases/modules/value/value.v. In those files x still has the alias receiver type, so left_type.idx() equals the receiver idx and the checker now rejects existing vlib tests before they can run; either the check needs to distinguish this alias-delegation case or those fixtures need to be updated with an explicit cast.

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}
node.is_noreturn = method.is_noreturn
node.is_expand_simple_interpolation = method.is_expand_simple_interpolation
node.is_ctor_new = method.is_ctor_new
Expand Down
7 changes: 7 additions & 0 deletions vlib/v/checker/tests/sum_type_recursive_str_err.out
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@@ -0,0 +1,7 @@
vlib/v/checker/tests/sum_type_recursive_str_err.vv:6:12: error: cannot call `str()` method recursively
4 | return match n {
5 | u8 { n.str() }
6 | else { n.str() }
| ~~~~~
7 | }
8 | }
12 changes: 12 additions & 0 deletions vlib/v/checker/tests/sum_type_recursive_str_err.vv
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@@ -0,0 +1,12 @@
type Num = u8 | u16

fn (n Num) str() string {
return match n {
u8 { n.str() }
else { n.str() }
}
}

fn main() {
println(Num(u16(0)))
}
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