probe: i686-windows-msvc import std with llvm 23.1.3 and 22.1.8, and the MSVC STL coroutine guards

Author: Sunrisepeak

.github/probe/stl_coroutine_facts.sh

@@ -0,0 +1,23 @@
+#!/usr/bin/env bash
+# TEMPORARY PROBE (do not merge): how the machine's MSVC STL guards its
+# coroutine headers, and which headers of the std module depend on them.
+# Prints; asserts nothing.
+set -u
+vswhere="/c/Program Files (x86)/Microsoft Visual Studio/Installer/vswhere.exe"
+vs=$("$vswhere" -latest -products '*' -property installationPath | tr -d '\r')
+vs=$(cygpath -u "$vs")
+for tools in "$vs"/VC/Tools/MSVC/*; do
+  inc="$tools/include"; mods="$tools/modules"
+  echo "### toolset $(basename "$tools")"
+  echo "--- x86 libraries present:"; ls -d "$tools/lib/x86" 2>&1
+  echo "--- <coroutine> preprocessor lines:"
+  grep -nE '^\s*#\s*(if|ifdef|ifndef|elif|else|endif|error|define _COROUTINE_)|_EMIT_STL|__cpp_impl_coroutine' "$inc/coroutine" | head -40
+  echo "--- <generator> preprocessor lines:"
+  grep -nE '^\s*#\s*(if|ifdef|ifndef|elif|else|endif|error|define _GENERATOR_)|_EMIT_STL|__cpp_impl_coroutine|__cpp_lib_coroutine|include <coroutine>' "$inc/generator" | head -40
+  echo "--- yvals_core.h coroutine feature lines:"
+  grep -nE '__cpp_impl_coroutine|__cpp_lib_coroutine|__cpp_lib_generator' "$inc/yvals_core.h"
+  echo "--- std.ixx lines naming generator/coroutine (with the surrounding conditionals):"
+  grep -nE 'generator|coroutine|^\s*#\s*(if|elif|else|endif)' "$mods/std.ixx" | head -60
+  echo "--- STL headers that use coroutine_handle / suspend_always:"
+  grep -lE 'coroutine_handle|suspend_always' "$inc"/* 2>/dev/null | xargs -n1 basename
+done

.github/probe/x86_msvc_std_cases.sh

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+#!/usr/bin/env bash
+# TEMPORARY PROBE (do not merge): `import std` on i686-windows-msvc with
+# clang 23.1.3 and 22.1.8, mcpp built from the #781 head. Answers G1/G2 of
+# .agents/reviews/2026-10-08-llvm-2313-part3-review-fixes-and-x86-msvc-coroutines-design.md.
+# Prints; asserts nothing.
+set -u
+M="${MCPP_BUILT:?}"
+W="$RUNNER_TEMP/x86-msvc-std"; mkdir -p "$W"
+"$M" --version
+
+# case <name> <target> <standard> <toolchain-table> <source-kind>
+case_run() {
+  local name=$1 target=$2 std=$3 tc=$4 src=$5
+  local d="$W/$name"; rm -rf "$d"; mkdir -p "$d/src"; cd "$d"
+  printf '[package]\nname = "p"\nversion = "0.1.0"\nstandard = "%s"\n\n%s\n' "$std" "$tc" > mcpp.toml
+  case "$src" in
+    std)     printf 'import std;\nint main() { std::println("std ok {}", sizeof(void*)); }\n' > src/main.cpp ;;
+    compat)  printf 'import std.compat;\nint main() { std::printf("compat ok %%zu\\n", sizeof(void*)); }\n' > src/main.cpp ;;
+    std20)   printf 'import std;\nint main() { std::cout << "std ok " << sizeof(void*) << "\\n"; }\n' > src/main.cpp ;;
+    coro)    printf '#include <coroutine>\nstruct t { struct promise_type { t get_return_object() { return {}; } std::suspend_never initial_suspend() noexcept { return {}; } std::suspend_never final_suspend() noexcept { return {}; } void return_void() {} void unhandled_exception() {} }; };\nt f() { co_return; }\nint main() { f(); }\n' > src/main.cpp ;;
+  esac
+  echo; echo "################ $name  target=$target standard=$std src=$src"
+  echo "--- mcpp.toml"; cat mcpp.toml
+  echo "--- build"
+  "$M" build --verbose --target "$target" > build.log 2>&1; local rc=$?
+  cat build.log; echo "build rc=$rc"
+  echo "--- lines naming generator / coroutine / __cpp_impl_coroutine:"
+  grep -nE 'generator|coroutine|__cpp_impl_coroutine' build.log | head -30
+  if [ $rc -eq 0 ]; then
+    local exe; exe=$(find target -type f -name 'p.exe' | head -1)
+    echo "--- run $exe"; "$exe"; echo "run rc=$?"
+    command -v file >/dev/null && file "$exe"
+  fi
+}
+
+T23='[toolchain]
+windows = "llvm@23.1.3"'
+T22i='[toolchain]
+windows = "llvm@23.1.3"
+
+[target.i686-windows-msvc]
+toolchain = "llvm@22.1.8"'
+T22x='[toolchain]
+windows = "llvm@23.1.3"
+
+[target.x86-windows-msvc]
+toolchain = "llvm@22.1.8"'
+
+# G1: the reported failure, and what the compiler says where.
+case_run g1-23-cxx23-i686        i686-windows-msvc   c++23 "$T23"  std
+# G2: C++20 on the same compiler and target.
+case_run g2-23-cxx20-i686        i686-windows-msvc   c++20 "$T23"  std20
+case_run g2-23-cxx20-i686-compat i686-windows-msvc   c++20 "$T23"  compat
+# Using the coroutine library itself under C++20 (expected to be refused by the
+# STL or the compiler; recorded for the documentation).
+case_run g2-23-cxx20-i686-coro   i686-windows-msvc   c++20 "$T23"  coro
+# Option (b): llvm 22.1.8 for this target only, in the spelling the note uses
+# and in MSVC's x86 spelling.
+case_run g1b-22-cxx23-i686       i686-windows-msvc   c++23 "$T22i" std
+case_run g1b-22-cxx23-x86        x86-windows-msvc    c++23 "$T22x" std
+# G3: the 64-bit row is unaffected.
+case_run g3-23-cxx23-x64         x86_64-windows-msvc c++23 "$T23"  std