Compilation fails if a single UDF is defined and used for two different tuple types

[nsiccha]

Not sure whether the title is clear. The following compiles:

functions {
int f3(tuple(int, int, int) x) {
    return x.3;
}
int f2(tuple(int, int) x) {
    return x.2;
}
}
transformed data {
    tuple(int, int, int) x3 = (1, 2, 3);
    tuple(int, int) x2 = (1, 2);
    int y3 = f3(x3);
    int y2 = f2(x2);
}

but the below does not, with a probably obscure-to-many g++ error message (reproduced at the end):

functions {
int f(tuple(int, int, int) x) {
    return x.3;
}
int f(tuple(int, int) x) {
    return x.2;
}
}
data {
}
transformed data {
    tuple(int, int, int) x3 = (1, 2, 3);
    tuple(int, int) x2 = (1, 2);
    int y3 = f(x3);
    int y2 = f(x2);
}

IMO, either, both models should compile, or stanc should already complain about the second model.

---

Output during attempted compilation of the second model (via the latest BridgeStan called from Julia):

┌ Error: Compilation failed!
│ Command: setenv(`make /home/niko/github/nsiccha/mwe/BridgeStan6/crash_model.so`; dir="/home/niko/.bridgestan/bridgestan-2.7.0")
│ stdout: 
│ --- Translating Stan model to C++ code ---
│ ./bin/stanc  --o=/home/niko/github/nsiccha/mwe/BridgeStan6/crash.hpp /home/niko/github/nsiccha/mwe/BridgeStan6/crash.stan
│ 
│ --- Compiling C++ code ---
│ g++ -std=c++17 -pthread -D_REENTRANT -Wno-sign-compare -Wno-ignored-attributes -Wno-class-memaccess      -I ./stan/lib/stan_math/lib/tbb_2020.3/include    -O3 -I ./stan/src -I ./stan/lib/rapidjson_1.1.0/ -I ./stan/lib/stan_math/ -I ./stan/lib/stan_math/lib/eigen_3.4.0 -I ./stan/lib/stan_math/lib/boost_1.87.0 -I ./stan/lib/stan_math/lib/sundials_6.1.1/include -I ./stan/lib/stan_math/lib/sundials_6.1.1/src/sundials -fPIC -fvisibility=hidden -fvisibility-inlines-hidden    -DBOOST_DISABLE_ASSERTS         -DBRIDGESTAN_EXPORT -DSTAN_OVERRIDE_EIGEN_ASSERT  -c  -x c++ -o /home/niko/github/nsiccha/mwe/BridgeStan6/crash.o /home/niko/github/nsiccha/mwe/BridgeStan6/crash.hpp
│ rm /home/niko/github/nsiccha/mwe/BridgeStan6/crash.hpp
│ 
│ stderr: /home/niko/github/nsiccha/mwe/BridgeStan6/crash.hpp: In constructor ‘crash_model_namespace::crash_model::crash_model(stan::io::var_context&, unsigned int, std::ostream*)’:
│ /home/niko/github/nsiccha/mwe/BridgeStan6/crash.hpp:129:13: error: call of overloaded ‘f(std::tuple<int, int, int>&, std::ostream*&)’ is ambiguous
│   129 |       y3 = f(x3, pstream__);
│       |            ~^~~~~~~~~~~~~~~
│ /home/niko/github/nsiccha/mwe/BridgeStan6/crash.hpp:42:5: note: candidate: ‘int crash_model_namespace::f(const T0__&, std::ostream*) [with T0__ = std::tuple<int, int, int>; stan::require_all_t<stan::math::is_tuple<T0__>, std::is_integral<typename std::decay<typename std::tuple_element<0, typename std::decay<_Tp>::type>::type>::type>, std::is_integral<typename std::decay<typename std::tuple_element<1, typename std::decay<_Tp>::type>::type>::type>, std::is_integral<typename std::decay<typename std::tuple_element<2, typename std::decay<_Tp>::type>::type>::type> >* <anonymous> = 0; std::ostream = std::basic_ostream<char>]’
│    42 | int f(const T0__& x, std::ostream* pstream__) {
│       |     ^
│ /home/niko/github/nsiccha/mwe/BridgeStan6/crash.hpp:67:5: note: candidate: ‘int crash_model_namespace::f(const T0__&, std::ostream*) [with T0__ = std::tuple<int, int, int>; stan::require_all_t<stan::math::is_tuple<T0__>, std::is_integral<typename std::decay<typename std::tuple_element<0, typename std::decay<_Tp>::type>::type>::type>, std::is_integral<typename std::decay<typename std::tuple_element<1, typename std::decay<_Tp>::type>::type>::type> >* <anonymous> = 0; std::ostream = std::basic_ostream<char>]’
│    67 | int f(const T0__& x, std::ostream* pstream__) {
│       |     ^
│ In file included from /usr/include/c++/11/functional:54,
│                  from ./stan/lib/stan_math/lib/eigen_3.4.0/Eigen/Core:85,
│                  from ./stan/lib/stan_math/lib/eigen_3.4.0/Eigen/Dense:1,
│                  from ./stan/lib/stan_math/stan/math/prim/fun/Eigen.hpp:32,
│                  from ./stan/lib/stan_math/stan/math/prim/meta/append_return_type.hpp:4,
│                  from ./stan/lib/stan_math/stan/math/prim/meta.hpp:72,
│                  from ./stan/lib/stan_math/stan/math/rev/core/accumulate_adjoints.hpp:4,
│                  from ./stan/lib/stan_math/stan/math/rev/core.hpp:4,
│                  from ./stan/src/stan/model/model_base.hpp:8,
│                  from ./stan/src/stan/model/model_header.hpp:4,
│                  from /home/niko/github/nsiccha/mwe/BridgeStan6/crash.hpp:2:
│ /usr/include/c++/11/tuple: In instantiation of ‘struct std::tuple_element<0, std::tuple<> >’:
│ /usr/include/c++/11/tuple:1353:12:   recursively required from ‘struct std::tuple_element<1, std::tuple<int> >’
│ /usr/include/c++/11/tuple:1353:12:   required from ‘struct std::tuple_element<2, std::tuple<int, int> >’
│ /home/niko/github/nsiccha/mwe/BridgeStan6/crash.hpp:24:89:   required by substitution of ‘template<class T0__, stan::require_all_t<stan::math::is_tuple<T0__>, std::is_integral<typename std::decay<typename std::tuple_element<0, typename std::decay<_Tp>::type>::type>::type>, std::is_integral<typename std::decay<typename std::tuple_element<1, typename std::decay<_Tp>::type>::type>::type>, std::is_integral<typename std::decay<typename std::tuple_element<2, typename std::decay<_Tp>::type>::type>::type> >* <anonymous> > int crash_model_namespace::f(const T0__&, std::ostream*) [with T0__ = std::tuple<int, int>; stan::require_all_t<stan::math::is_tuple<T0__>, std::is_integral<typename std::decay<typename std::tuple_element<0, typename std::decay<_Tp>::type>::type>::type>, std::is_integral<typename std::decay<typename std::tuple_element<1, typename std::decay<_Tp>::type>::type>::type>, std::is_integral<typename std::decay<typename std::tuple_element<2, typename std::decay<_Tp>::type>::type>::type> >* <anonymous> = <missing>]’
│ /home/niko/github/nsiccha/mwe/BridgeStan6/crash.hpp:133:13:   required from here
│ /usr/include/c++/11/tuple:1371:25: error: static assertion failed: tuple index must be in range
│  1371 |       static_assert(__i < tuple_size<tuple<>>::value,
│       |                     ~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~
│ /usr/include/c++/11/tuple:1371:25: note: ‘(0 < ((long unsigned int)std::integral_constant<long unsigned int, 0>::value))’ evaluates to false
│ make: *** [Makefile:77: /home/niko/github/nsiccha/mwe/BridgeStan6/crash.o] Error 1
└ @ Main ~/github/nsiccha/mwe/BridgeStan6/crash.jl:9

[WardBrian]

Thanks for reporting @nsiccha. This is an unfortunate behavior of std::tuple_element, but I think it should be easy enough to work around by writing our own version which returns void as the type whenever the requested number is larger than the tuple size.