value_parser that should return Vec<u8> panics with Mismatch between definition and access of Payload. Could not downcast to u8, need to downcast to alloc::vec::Vec<u8>

[vslynko]

Please complete the following tasks

• I have searched the discussions
• I have searched the open and rejected issues

Rust Version

rustc 1.64.0 (a55dd71d5 2022-09-19)

Clap Version

clap = { version = "4.0.23", features = ["derive"] }

Minimal reproducible code

use clap::Parser;

fn parse_hex_values(s: &str) -> Result<Vec<u8>, hex::FromHexError> {
    hex::decode(s.trim_start_matches("0x"))
}

#[derive(Parser)]
#[command(author, version, about, long_about = None)]
pub struct Cli {
    #[arg(name = "Payload", value_parser(parse_hex_values))]
    payload: Vec<u8>,
}

fn main() {
    println!("Hello, world!");
    let _args = Cli::parse();
}

Steps to reproduce the bug with the above code

cargo run -- 0x11

cargo.Toml should also contain hex = "0.4.3" dependency

Actual Behaviour

Main thread panics with
thread 'main' panicked at 'Mismatch between definition and access of Payload. Could not downcast to u8, need to downcast to alloc::vec::Vec<u8>

~\source\clap_bug on master  ● λ cargo run -- 0x11
   Compiling adler v1.0.2
   Compiling gimli v0.26.2
   Compiling cfg-if v1.0.0
   Compiling rustc-demangle v0.1.21
   Compiling memchr v2.5.0
   Compiling libc v0.2.137
   Compiling miniz_oxide v0.5.4
   Compiling object v0.29.0                                                                                   
   Compiling addr2line v0.17.0
   Compiling backtrace v0.3.66
   Compiling clap v4.0.24
   Compiling clap_bug v0.1.0 (C:\Users\vadim\source\clap_bug)
    Finished dev [unoptimized + debuginfo] target(s) in 5.74s
     Running `target\debug\clap_bug.exe 0x11`
Hello, world!
[      clap::builder::command]  Command::_do_parse
[      clap::builder::command]  Command::_build: name="clap_bug"
[      clap::builder::command]  Command::_propagate:clap_bug
[      clap::builder::command]  Command::_check_help_and_version:clap_bug expand_help_tree=false
[      clap::builder::command]  Command::long_help_exists
[      clap::builder::command]  Command::_check_help_and_version: Building default --help
[      clap::builder::command]  Command::_check_help_and_version: Building default --version
[      clap::builder::command]  Command::_propagate_global_args:clap_bug
[clap::builder::debug_asserts]  Command::_debug_asserts
[clap::builder::debug_asserts]  Arg::_debug_asserts:Payload
[clap::builder::debug_asserts]  Arg::_debug_asserts:help
[clap::builder::debug_asserts]  Arg::_debug_asserts:version
[clap::builder::debug_asserts]  Command::_verify_positionals
[        clap::parser::parser]  Parser::get_matches_with
[        clap::parser::parser]  Parser::get_matches_with: Begin parsing 'RawOsStr("0x11")' ([48, 120, 49, 49])
[        clap::parser::parser]  Parser::possible_subcommand: arg=Ok("0x11")
[        clap::parser::parser]  Parser::get_matches_with: sc=None
[        clap::parser::parser]  Parser::get_matches_with: Positional counter...1
[        clap::parser::parser]  Parser::get_matches_with: Low index multiples...false
[        clap::parser::parser]  Parser::resolve_pending: id="Payload"
[        clap::parser::parser]  Parser::react action=Append, identifier=Some(Index), source=CommandLine       
[        clap::parser::parser]  Parser::remove_overrides: id="Payload"
[   clap::parser::arg_matcher]  ArgMatcher::start_custom_arg: id="Payload", source=CommandLine
[      clap::builder::command]  Command::groups_for_arg: id="Payload"
[   clap::parser::arg_matcher]  ArgMatcher::start_custom_arg: id="Cli", source=CommandLine
[        clap::parser::parser]  Parser::push_arg_values: ["0x11"]
[        clap::parser::parser]  Parser::add_single_val_to_arg: cur_idx:=1
[   clap::parser::arg_matcher]  ArgMatcher::needs_more_vals: o=Payload, pending=0
[   clap::parser::arg_matcher]  ArgMatcher::needs_more_vals: expected=1..=18446744073709551615, actual=0      
[        clap::parser::parser]  Parser::react not enough values passed in, leaving it to the validator to complain
[        clap::parser::parser]  Parser::add_defaults
[        clap::parser::parser]  Parser::add_defaults:iter:Payload:
[        clap::parser::parser]  Parser::add_default_value: doesn't have conditional defaults
[        clap::parser::parser]  Parser::add_default_value:iter:Payload: doesn't have default vals
[        clap::parser::parser]  Parser::add_defaults:iter:help:
[        clap::parser::parser]  Parser::add_default_value: doesn't have conditional defaults
[        clap::parser::parser]  Parser::add_default_value:iter:help: doesn't have default vals
[        clap::parser::parser]  Parser::add_defaults:iter:version:
[        clap::parser::parser]  Parser::add_default_value: doesn't have conditional defaults
[        clap::parser::parser]  Parser::add_default_value:iter:version: doesn't have default vals
[     clap::parser::validator]  Validator::validate
[     clap::parser::validator]  Validator::validate_conflicts
[     clap::parser::validator]  Validator::validate_exclusive
[     clap::parser::validator]  Validator::validate_conflicts::iter: id="Payload"
[     clap::parser::validator]  Conflicts::gather_conflicts: arg="Payload"
[      clap::builder::command]  Command::groups_for_arg: id="Payload"
[     clap::parser::validator]  Conflicts::gather_direct_conflicts id="Payload", conflicts=[]
[     clap::parser::validator]  Conflicts::gather_direct_conflicts id="Cli", conflicts=[]
[     clap::parser::validator]  Conflicts::gather_conflicts: conflicts=[]
[     clap::parser::validator]  Validator::validate_required: required=ChildGraph([])
[     clap::parser::validator]  Validator::gather_requires
[     clap::parser::validator]  Validator::gather_requires:iter:"Payload"
[     clap::parser::validator]  Validator::gather_requires:iter:"Cli"
[     clap::parser::validator]  Validator::gather_requires:iter:"Cli":group
[     clap::parser::validator]  Validator::validate_required: is_exclusive_present=false
[   clap::parser::arg_matcher]  ArgMatcher::get_global_values: global_arg_vec=[]
thread 'main' panicked at 'Mismatch between definition and access of `Payload`. Could not downcast to u8, need to downcast to alloc::vec::Vec<u8>
', src\main.rs:11:14
stack backtrace:
   0: std::panicking::begin_panic_handler
             at /rustc/a55dd71d5fb0ec5a6a3a9e8c27b2127ba491ce52/library\std\src\panicking.rs:584
   1: core::panicking::panic_fmt
             at /rustc/a55dd71d5fb0ec5a6a3a9e8c27b2127ba491ce52/library\core\src\panicking.rs:142
   2: enum$<clap::parser::error::MatchesError, 1, 18446744073709551615, Downcast>::unwrap<enum$<core::option::Option<clap::parser::matches::arg_matches::Values<u8> >, 1, 18446744073709551615, Some> >
             at C:\Users\vadim\.cargo\registry\src\github.com-1ecc6299db9ec823\clap-4.0.24\src\parser\error.rs:30
   3: clap::parser::matches::arg_matches::ArgMatches::remove_many<u8>
             at C:\Users\vadim\.cargo\registry\src\github.com-1ecc6299db9ec823\clap-4.0.24\src\parser\matches\arg_matches.rs:338
   4: clap_bug::impl$2::from_arg_matches_mut
             at .\src\main.rs:7
   5: clap::derive::Parser::parse<clap_bug::Cli>
             at C:\Users\vadim\.cargo\registry\src\github.com-1ecc6299db9ec823\clap-4.0.24\src\derive.rs:83   
   6: clap_bug::main
             at .\src\main.rs:16
   7: core::ops::function::FnOnce::call_once<void (*)(),tuple$<> >
             at /rustc/a55dd71d5fb0ec5a6a3a9e8c27b2127ba491ce52\library\core\src\ops\function.rs:248
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.
error: process didn't exit successfully: `target\debug\clap_bug.exe 0x11` (exit code: 101)

Expected Behaviour

The result of parse_hex_values should be assigned to payload field.

Additional Context

No response

Debug Output

~\source\clap_bug on master  ● λ cargo run -- 0x11
   Compiling adler v1.0.2
   Compiling gimli v0.26.2
   Compiling cfg-if v1.0.0
   Compiling rustc-demangle v0.1.21
   Compiling memchr v2.5.0
   Compiling libc v0.2.137
   Compiling miniz_oxide v0.5.4
   Compiling object v0.29.0                                                                                   
   Compiling addr2line v0.17.0
   Compiling backtrace v0.3.66
   Compiling clap v4.0.24
   Compiling clap_bug v0.1.0 (C:\Users\vadim\source\clap_bug)
    Finished dev [unoptimized + debuginfo] target(s) in 5.74s
     Running `target\debug\clap_bug.exe 0x11`
Hello, world!
[      clap::builder::command]  Command::_do_parse
[      clap::builder::command]  Command::_build: name="clap_bug"
[      clap::builder::command]  Command::_propagate:clap_bug
[      clap::builder::command]  Command::_check_help_and_version:clap_bug expand_help_tree=false
[      clap::builder::command]  Command::long_help_exists
[      clap::builder::command]  Command::_check_help_and_version: Building default --help
[      clap::builder::command]  Command::_check_help_and_version: Building default --version
[      clap::builder::command]  Command::_propagate_global_args:clap_bug
[clap::builder::debug_asserts]  Command::_debug_asserts
[clap::builder::debug_asserts]  Arg::_debug_asserts:Payload
[clap::builder::debug_asserts]  Arg::_debug_asserts:help
[clap::builder::debug_asserts]  Arg::_debug_asserts:version
[clap::builder::debug_asserts]  Command::_verify_positionals
[        clap::parser::parser]  Parser::get_matches_with
[        clap::parser::parser]  Parser::get_matches_with: Begin parsing 'RawOsStr("0x11")' ([48, 120, 49, 49])
[        clap::parser::parser]  Parser::possible_subcommand: arg=Ok("0x11")
[        clap::parser::parser]  Parser::get_matches_with: sc=None
[        clap::parser::parser]  Parser::get_matches_with: Positional counter...1
[        clap::parser::parser]  Parser::get_matches_with: Low index multiples...false
[        clap::parser::parser]  Parser::resolve_pending: id="Payload"
[        clap::parser::parser]  Parser::react action=Append, identifier=Some(Index), source=CommandLine       
[        clap::parser::parser]  Parser::remove_overrides: id="Payload"
[   clap::parser::arg_matcher]  ArgMatcher::start_custom_arg: id="Payload", source=CommandLine
[      clap::builder::command]  Command::groups_for_arg: id="Payload"
[   clap::parser::arg_matcher]  ArgMatcher::start_custom_arg: id="Cli", source=CommandLine
[        clap::parser::parser]  Parser::push_arg_values: ["0x11"]
[        clap::parser::parser]  Parser::add_single_val_to_arg: cur_idx:=1
[   clap::parser::arg_matcher]  ArgMatcher::needs_more_vals: o=Payload, pending=0
[   clap::parser::arg_matcher]  ArgMatcher::needs_more_vals: expected=1..=18446744073709551615, actual=0      
[        clap::parser::parser]  Parser::react not enough values passed in, leaving it to the validator to complain
[        clap::parser::parser]  Parser::add_defaults
[        clap::parser::parser]  Parser::add_defaults:iter:Payload:
[        clap::parser::parser]  Parser::add_default_value: doesn't have conditional defaults
[        clap::parser::parser]  Parser::add_default_value:iter:Payload: doesn't have default vals
[        clap::parser::parser]  Parser::add_defaults:iter:help:
[        clap::parser::parser]  Parser::add_default_value: doesn't have conditional defaults
[        clap::parser::parser]  Parser::add_default_value:iter:help: doesn't have default vals
[        clap::parser::parser]  Parser::add_defaults:iter:version:
[        clap::parser::parser]  Parser::add_default_value: doesn't have conditional defaults
[        clap::parser::parser]  Parser::add_default_value:iter:version: doesn't have default vals
[     clap::parser::validator]  Validator::validate
[     clap::parser::validator]  Validator::validate_conflicts
[     clap::parser::validator]  Validator::validate_exclusive
[     clap::parser::validator]  Validator::validate_conflicts::iter: id="Payload"
[     clap::parser::validator]  Conflicts::gather_conflicts: arg="Payload"
[      clap::builder::command]  Command::groups_for_arg: id="Payload"
[     clap::parser::validator]  Conflicts::gather_direct_conflicts id="Payload", conflicts=[]
[     clap::parser::validator]  Conflicts::gather_direct_conflicts id="Cli", conflicts=[]
[     clap::parser::validator]  Conflicts::gather_conflicts: conflicts=[]
[     clap::parser::validator]  Validator::validate_required: required=ChildGraph([])
[     clap::parser::validator]  Validator::gather_requires
[     clap::parser::validator]  Validator::gather_requires:iter:"Payload"
[     clap::parser::validator]  Validator::gather_requires:iter:"Cli"
[     clap::parser::validator]  Validator::gather_requires:iter:"Cli":group
[     clap::parser::validator]  Validator::validate_required: is_exclusive_present=false
[   clap::parser::arg_matcher]  ArgMatcher::get_global_values: global_arg_vec=[]
thread 'main' panicked at 'Mismatch between definition and access of `Payload`. Could not downcast to u8, need to downcast to alloc::vec::Vec<u8>
', src\main.rs:11:14
stack backtrace:
   0: std::panicking::begin_panic_handler
             at /rustc/a55dd71d5fb0ec5a6a3a9e8c27b2127ba491ce52/library\std\src\panicking.rs:584
   1: core::panicking::panic_fmt
             at /rustc/a55dd71d5fb0ec5a6a3a9e8c27b2127ba491ce52/library\core\src\panicking.rs:142
   2: enum$<clap::parser::error::MatchesError, 1, 18446744073709551615, Downcast>::unwrap<enum$<core::option::Option<clap::parser::matches::arg_matches::Values<u8> >, 1, 18446744073709551615, Some> >
             at C:\Users\vadim\.cargo\registry\src\github.com-1ecc6299db9ec823\clap-4.0.24\src\parser\error.rs:30
   3: clap::parser::matches::arg_matches::ArgMatches::remove_many<u8>
             at C:\Users\vadim\.cargo\registry\src\github.com-1ecc6299db9ec823\clap-4.0.24\src\parser\matches\arg_matches.rs:338
   4: clap_bug::impl$2::from_arg_matches_mut
             at .\src\main.rs:7
   5: clap::derive::Parser::parse<clap_bug::Cli>
             at C:\Users\vadim\.cargo\registry\src\github.com-1ecc6299db9ec823\clap-4.0.24\src\derive.rs:83   
   6: clap_bug::main
             at .\src\main.rs:16
   7: core::ops::function::FnOnce::call_once<void (*)(),tuple$<> >
             at /rustc/a55dd71d5fb0ec5a6a3a9e8c27b2127ba491ce52\library\core\src\ops\function.rs:248
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.
error: process didn't exit successfully: `target\debug\clap_bug.exe 0x11` (exit code: 101)

[epage]

clap determines what to do based on the wrapping type, in this case Vec. There isn't a way to opt-out and there isn't a guaranteed way for us to take the value parsers output type into account.

To work around this, you can fully qualify the type, like std::vec::Vec<u8>.

[fritzrehde]

I am having this issue as well. I get the following rust panic:

thread 'main' panicked at 'Mismatch between definition and access of `keybindings`. Could not downcast to TypeId { t: 8482930911198856167 }, need to downcast to TypeId { t: 12752778543207947612 }
', /home/fritz/.cargo/registry/src/.../clap-4.0.32/src/parser/error.rs:30:9

I have the following code snippet from my derived struct:

	/// Comma-seperated list of keybindings in the format KEY:OP[+OP]*[,KEY:OP[+OP]*]*
	#[arg(short = 'b', long = "bind", value_name = "KEYBINDINGS", value_delimiter = ',', value_parser = keybindings::parse_str)]
	keybindings: Option<Vec<(String, Vec<String>)>>,

This previously worked when Option<Vec<(String, Vec<String>)>> was just Vec<(String, Vec<String>)>. I assumed adding an option would just work out of the box. The behaviour I expected: if I don't specify the ---bind option keybindings will be none, otherwise it will be some of a vector delimited by value_delimiter and parsed by value_parser. Am I wrong in my assumption that this works?

[fritzrehde]

Here's some extra debug output running in the project instead of the binary:

thread 'main' panicked at 'Mismatch between definition and access of `keybindings`. Could not downcast to alloc::vec::Vec<(alloc::string::String, alloc::vec::Vec<alloc::string::String>)>, need to downcast to (alloc::string::String, alloc::vec::Vec<alloc::string::String>)
', src/config/mod.rs:195:18
stack backtrace:
   0: rust_begin_unwind
             at /rustc/69f9c33d71c871fc16ac445211281c6e7a340943/library/std/src/panicking.rs:575:5
   1: core::panicking::panic_fmt
             at /rustc/69f9c33d71c871fc16ac445211281c6e7a340943/library/core/src/panicking.rs:65:14
   2: clap::parser::error::MatchesError::unwrap
             at /home/fritz/.cargo/registry/src/github.com-1ecc6299db9ec823/clap-4.1.4/src/parser/error.rs:30:9
   3: clap::parser::matches::arg_matches::ArgMatches::remove_one
             at /home/fritz/.cargo/registry/src/github.com-1ecc6299db9ec823/clap-4.1.4/src/parser/matches/arg_matches.rs:390:9
   4: <watchbind::config::ClapConfig as clap::derive::FromArgMatches>::from_arg_matches_mut
             at ./src/config/mod.rs:148:10
   5: clap::derive::Parser::parse
             at /home/fritz/.cargo/registry/src/github.com-1ecc6299db9ec823/clap-4.1.4/src/derive.rs:83:19
   6: watchbind::config::Config::parse
             at ./src/config/mod.rs:24:13
   7: watchbind::main
             at ./src/main.rs:9:12
   8: core::ops::function::FnOnce::call_once
             at /rustc/69f9c33d71c871fc16ac445211281c6e7a340943/library/core/src/ops/function.rs:251:5
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

[epage]

btw I lost track of this being the issue for improving customizing the type, so we now have #4626 which has more information in that direction, so closing in favor of that issue.

@fritzrehde as your case is more of a support issue (helping debug), could you start a discussion and, at minimum, include the signature for keybindings::parse_str. If you could include a full reproduction case, that'd be even better.