ICE when reusing signal names in an if block that contains at least one template.

[DragonDev1906]

The following circuit results in an index out of bounds panic in circom:

pragma circom 2.2.3;

template Nop {
    signal output out;
}

template Demo {
    signal hash_bin;
    if (0 == 0) {
        signal foo <== 1;
        hash_bin <== Nop()();
    }
    signal foo <== 2;
}

component main = Demo();

Command: circom circuits/circom_bug.circom -o /tmp --O1

thread 'main' panicked at constraint_list/src/constraint_simplification.rs:82:45:
index out of bounds: the len is 4 but the index is 4
stack backtrace:
0: rust_begin_unwind
1: core::panicking::panic_fmt
2: core::panicking::panic_bounds_check
3: constraint_list::constraint_simplification::build_clusters
4: constraint_list::constraint_simplification::simplification
5: constraint_list::Simplifier::simplify_constraints
6: dag::map_to_constraint_list::map
7: dag::DAG::map_to_list
8: constraint_generation::build_circuit
9: circom::execution_user::execute_project
10: circom::main

Changing any of the following makes it compile:

• Rename either of the foo signals
• Remove the Nop template
• Remove the out signal (and the assignment to hash_bin).
• Make the condition false
• Disable optimizations (--O0)

[clararod9]

Hi, thank you for reporting this bug!

We solved this issue and others related to signal renaming in one of our last commits. If you use the latest version of circom (master branch) the compiler correctly returns an error indicating that signal names cannot be duplicated.

error[T3001]: The signal and component names must be unique after executing the template. Only variables can be redefined in different blocks
   ┌─ "bug_signal_if.circom":13:5
   │
13 │     signal foo <== 2;
   │     ^^^^^^^^^^^^^^^^ found here
   │
   = call trace:
     ->Demo

previous errors were found

We plan to publish a new stable version (circom 2.2.4) this week which will include this change along other improvements,

Thanks :)