[veljjanoski]

Fixes #1116.

What

The tests build with -Wall -Wextra -Wpedantic -Werror, but CMake adds the headers as SYSTEM, so the tests never see warnings inside them. Users who include the headers with a plain -I still get those warnings, for example through meson's include_directories('include') with warning_level=3. On main, building the tests with the headers through -I gives 23 warning sites with GCC 12 and 9 with Clang 21. This PR fixes them:

• 19 extra ; after member function definitions, in 16 files (GCC -Wpedantic).
• -Wmissing-field-initializers (GCC and Clang): __continues_on's __state_base::__data_ and __finally's __opstate_base::__result_ get a {} default member initializer. Both structs are only built by aggregate initialization that leaves this member out, so it was already initialized from {}. Behaviour does not change.
• -Wunused-local-typedef(s): an unused shape_t alias in thread_pool_base.hpp.
• Clang -Wmissing-braces in __when_all.hpp: ignored for the file after __prologue.hpp, as __bulk.hpp, __just.hpp and __tuple.hpp already do.
• -Wunused-value in __receiver_ref.hpp: ignored for the file the same way, as exec/repeat_n.hpp does. The left operand of the comma in __indirect_receiver::set_next has no effect, but it stays, because it is what makes the call depend on _Item.

In thread_pool_base.hpp, _bulk_sender::connect ends with STDEXEC_AUTO_RETURN(...). Without the ;, clang-format treats the next member as part of connect, so connect and the next member are now inside // clang-format off/on, as in basic_sequence.hpp.

Not changed: GCC -Wsubobject-linkage at 4 places in __any.hpp. GCC reports it when the tests instantiate those templates with types from an anonymous namespace.

Verification

Docker (Debian 12), GCC 12.2 and Clang 21.1 with libstdc++ 12, Debug, the tests' own flags, with the headers through -I instead of -isystem. The GCC build also had -Wno-error=subobject-linkage.

Warning sites in the headers	main (9d292a4)	this PR
GCC 12	23, plus 4 subobject-linkage	0, plus the same 4
Clang 21	9	0

With this PR, test.stdexec and test.exec build with -Werror and pass with both compilers. That is 641 and 454 test cases with Clang. GCC runs one fewer in test.stdexec, because one test is skipped on GCC.

Each header in include/stdexec and include/exec was also compiled on its own with both compilers (-fsyntax-only -Wall -Wextra -Wpedantic, through -I). This skipped the nvexec, asio, tbb, taskflow, libdispatch and Windows headers. Only the deprecation #warnings in exec/repeat_effect_until.hpp and exec/system_context.hpp remain. clang-format 21 is clean.

Not tested: MSVC, nvc++, Release builds, and the other CI configurations.

These warnings came back because the tests cannot see them. If you want, I can follow up with a change that builds the tests with the headers as non-system includes, so CI catches new ones. GCC would need -Wno-subobject-linkage for that.

AI-usage disclosure: Claude (Anthropic) was used as assistant.

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[ericniebler]

/ok to test a3eee61