[Bug] Duplicate return value rejected

[evan-schott]

program inner.aleo {
    transition main(a: address) -> (address, address) {
        return (a, a);
    }
}

thread `main` panicked at compiler/passes/src/flattening/flatten_statement.rs:277:18:
internal error: entered unreachable code: SSA guarantees that the expression is always an identifier or unit expression.
stack backtrace: 
   0: backtrace::capture::Backtrace::create
   1: backtrace::capture::Backtrace::new
   2: leo::set_panic_hook::{{closure}}
   3: std::panicking::rust_panic_with_hook
   4: std::panicking::begin_panic_handler::{{closure}}
   5: std::sys_common::backtrace::__rust_end_short_backtrace
   6: _rust_begin_unwind
   7: core::panicking::panic_fmt
   8: leo_passes::flattening::flatten_statement::<impl leo_ast::passes::reconstructor::StatementReconstructor for leo_passes::flattening::flattener::Flattener>::reconstruct_return
   9: leo_ast::passes::reconstructor::StatementReconstructor::reconstruct_statement
  10: leo_passes::flattening::flatten_statement::<impl leo_ast::passes::reconstructor::StatementReconstructor for leo_passes::flattening::flattener::Flattener>::reconstruct_block
  11: leo_passes::flattening::flatten_program::<impl leo_ast::passes::reconstructor::ProgramReconstructor for leo_passes::flattening::flattener::Flattener>::reconstruct_function
  12: <core::iter::adapters::map::Map<I,F> as core::iter::traits::iterator::Iterator>::try_fold
  13: alloc::vec::in_place_collect::<impl alloc::vec::spec_from_iter::SpecFromIter<T,I> for alloc::vec::Vec<T>>::from_iter
  14: leo_ast::passes::reconstructor::ProgramReconstructor::reconstruct_program_scope
  15: <core::iter::adapters::map::Map<I,F> as core::iter::traits::iterator::Iterator>::fold
  16: <indexmap::map::IndexMap<K,V,S> as core::iter::traits::collect::FromIterator<(K,V)>>::from_iter
  17: leo_ast::passes::reconstructor::ProgramReconstructor::reconstruct_program
  18: leo_compiler::compiler::Compiler::flattening_pass
  19: leo_compiler::compiler::Compiler::compiler_stages
  20: leo_compiler::compiler::Compiler::compile
  21: <leo_lang::cli::commands::build::Build as leo_lang::cli::commands::Command>::apply
  22: leo_lang::cli::commands::Command::execute
  23: leo_lang::cli::cli::run_with_args
  24: scoped_tls::ScopedKey<T>::set
  25: leo::main
  26: std::sys_common::backtrace::__rust_begin_short_backtrace
  27: std::rt::lang_start::{{closure}}
  28: std::rt::lang_start_internal
  29: _main

error: internal compiler error: unexpected panic

note: the compiler unexpectedly panicked. this is a bug.

note: we would appreciate a bug report: https://github.com/AleoHQ/leo/issues/new?labels=bug,panic&template=bug.md&title=[Bug]

note: leo-lang 1.11.0 running on Darwin 23.2.0

note: compiler args: leo build

note: compiler flags: CLI { debug: false, quiet: false, command: Build { command: Build { options: BuildOptions { endpoint: "http://api.explorer.aleo.org/v1", non_recursive: false, offline: false, enable_symbol_table_spans: false, enable_initial_symbol_table_snapshot: false, enable_type_checked_symbol_table_snapshot: false, enable_unrolled_symbol_table_snapshot: false, enable_ast_spans: false, enable_dce: false, enable_all_ast_snapshots: false, enable_initial_ast_snapshot: false, enable_unrolled_ast_snapshot: false, enable_ssa_ast_snapshot: false, enable_flattened_ast_snapshot: false, enable_destructured_ast_snapshot: false, enable_inlined_ast_snapshot: false, enable_dce_ast_snapshot: false, conditional_block_max_depth: 10, disable_conditional_branch_type_checking: false } } }, path: None, home: None }