GCC 16 build fails due to -Wsfinae-incomplete in start_detached test

Reproduce the failure with GCC 16 and start in test/exec/test_start_detached.cpp around custom_scheduler::sender::get_env(), then inspect the referenced __get_env_member_result_t in include/stdexec/__detail/__env.hpp. Check the test and its CMake target, and consider the issue complete when the start_detached test builds successfully with the reported warning-as-error settings.

Build fails with g++ 16 due to -Wsfinae-incomplete

Error Description:

[1/3] Building CXX object test/exec/CMakeFiles/test.exec.dir/test_start_detached.cpp.o
FAILED: [code=1] test/exec/CMakeFiles/test.exec.dir/test_start_detached.cpp.o
/usr/bin/c++ -DSTDEXEC_NAMESPACE=std::execution -I/home/dane/src/stdexec/build/_deps/catch2-src/src/catch2/.. -I/home/dane/src/stdexec/build/_deps/catch2-build/generated-includes -I/home/dane/src/stdexec/test -isystem /home/dane/src/stdexec/include -isystem /home/dane/src/stdexec/build/include -O3 -DNDEBUG -Wall -Wextra -Wpedantic -Werror -Wno-c2y-extensions -fcoroutines -fconcepts-diagnostics-depth=10 -Werror=unused-parameter -Wno-non-template-friend -Wno-maybe-uninitialized -MD -MT test/exec/CMakeFiles/test.exec.dir/test_start_detached.cpp.o -MF test/exec/CMakeFiles/test.exec.dir/test_start_detached.cpp.o.d -o test/exec/CMakeFiles/test.exec.dir/test_start_detached.cpp.o -c /home/dane/src/stdexec/test/exec/test_start_detached.cpp
/home/dane/src/stdexec/test/exec/test_start_detached.cpp: In member function ‘auto {anonymous}::custom_scheduler::sender::get_env() const’:
/home/dane/src/stdexec/test/exec/test_start_detached.cpp:148:7: error: defining ‘auto {anonymous}::custom_scheduler::sender::get_env() const’, which previously failed to be deduced in a SFINAE context [-Werror=sfinae-incomplete=]
148 | }
| ^
In file included from /home/dane/src/stdexec/include/exec/start_detached.hpp:20,
from /home/dane/src/stdexec/test/exec/test_start_detached.cpp:17:
/home/dane/src/stdexec/include/stdexec/__detail/__env.hpp:277:81: note: here. Use ‘-Wsfinae-incomplete=2’ for a diagnostic at that point
277 | using __get_env_member_result_t = decltype(__declval<_EnvProvider>().get_env());
| ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~
At global scope:
cc1plus: note: unrecognized command-line option ‘-Wno-c2y-extensions’ may have been intended to silence earlier diagnostics
cc1plus: all warnings being treated as errors
ninja: build stopped: subcommand failed.

Environment:
g++ -v:
Using built-in specs.
COLLECT_GCC=g++
COLLECT_LTO_WRAPPER=/usr/libexec/gcc/x86_64-redhat-linux/16/lto-wrapper
OFFLOAD_TARGET_NAMES=nvptx-none:amdgcn-amdhsa
OFFLOAD_TARGET_DEFAULT=1
Target: x86_64-redhat-linux
Configured with: ../configure --enable-bootstrap --enable-languages=c,c++,fortran,objc,obj-c++,ada,go,d,m2,cobol,algol68,lto --prefix=/usr --mandir=/usr/share/man --infodir=/usr/share/info --with-bugurl=https://bugzilla.redhat.com/ --enable-shared --enable-threads=posix --enable-checking=release --enable-multilib --with-system-zlib --enable-__cxa_atexit --disable-libunwind-exceptions --enable-gnu-unique-object --enable-linker-build-id --with-gcc-major-version-only --enable-libstdcxx-backtrace --with-libstdcxx-zoneinfo=/usr/share/zoneinfo --with-linker-hash-style=gnu --enable-plugin --enable-initfini-array --with-isl=/builddir/build/BUILD/gcc-16.2.1-build/gcc-16.2.1-20260819/obj-x86_64-redhat-linux/isl-install --enable-offload-targets=nvptx-none,amdgcn-amdhsa --enable-offload-defaulted --without-cuda-driver --enable-gnu-indirect-function --enable-cet --with-tune=generic --with-tls=gnu2 --with-arch_32=i686 --build=x86_64-redhat-linux --with-build-config=bootstrap-lto --enable-link-serialization=1 --disable-libssp
Thread model: posix
Supported LTO compression algorithms: zlib zstd
gcc version 16.2.1 20260819 (Red Hat 16.2.1-2) (GCC)

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