@@ -1,42 +1,39 @@
 #!/usr/bin/env bash
-# macOS 27: why libc++ 22's std.cppm loses INFINITY/NAN. One READING per fact.
+# macOS 27: libc++ 22 std.cppm loses INFINITY/NAN. Round 2: mechanism and candidate fixes.
 set +e
 r() { echo "READING $*"; }
-r "os $(sw_vers -productVersion) $(sw_vers -buildVersion) xcode=$(xcodebuild -version | tr '\n' ' ')"
-r "xcrun sdk=$(xcrun --show-sdk-path) ver=$(xcrun --show-sdk-version)"
-CLT=/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk
-r "clt sdk link=$(readlink $CLT) exists=$([ -d $CLT ] && echo y || echo n) settings=$(plutil -extract Version raw $CLT/SDKSettings.plist 2>/dev/null)"
-ls -la /Library/Developer/CommandLineTools/SDKs/ 2>&1 | sed 's/^/READING clt-sdks /'
-XSDK=$(xcrun --show-sdk-path)
-for s in $CLT $XSDK; do
-  r "math.h in $s: INFINITY defs: $(grep -rn 'define[[:space:]]*INFINITY' $s/usr/include/math.h $s/usr/include/_math.h $s/usr/include/math 2>/dev/null | head -3 | tr '\n' ' ')"
-  r "math.h head in $s: $(grep -n 'include\|__MATH' $s/usr/include/math.h 2>/dev/null | head -12 | tr '\n' ' ')"
-done
-# xlings llvm
-curl -fsSL https://raw.githubusercontent.com/openxlings/xlings/main/tools/other/quick_install.sh | bash -s v2026.9.16.1 >/dev/null 2>&1
-export PATH="$HOME/.xlings/subos/current/bin:$PATH"
-xlings install llvm@22.1.8 -y >/dev/null 2>&1
-L=$(ls -d $HOME/.xlings/data/xpkgs/xim-x-llvm/22.1.8)
-r "llvm cfg: $(cat $L/bin/clang++.cfg | tr '\n' ' ')"
-S=$L/share/libc++/v1/std.cppm
+L="$LLVM_ROOT"; SDK=$(xcrun --show-sdk-path); S=$L/share/libc++/v1/std.cppm
+r "llvm=$L cfg=$(tr '\n' ' ' < $L/bin/clang++.cfg)"
+r "math.h 55-95:"; sed -n 55,95p $SDK/usr/include/math.h | sed 's/^/READING   math.h| /'
+FH=$($L/bin/clang -print-resource-dir)/include/float.h
+r "clang float.h INFINITY region:"; grep -n -B4 -A3 'define INFINITY' $FH | sed 's/^/READING   float.h| /'
+AF=$(/usr/bin/clang -print-resource-dir)/include/float.h
+r "apple float.h INFINITY region:"; grep -n -B4 -A3 'define INFINITY' $AF | sed 's/^/READING   apple-float.h| /'
 W=$(mktemp -d); cd $W
-try() { name=$1; shift; out=$("$@" 2>&1); rc=$?; r "$name rc=$rc $(printf '%s' "$out" | grep -m2 'error' | tr '\n' ' ')"; }
-try cfg-clt      $L/bin/clang++ -std=c++23 -Wno-reserved-module-identifier --precompile $S -o a.pcm
-try isysroot-xc  $L/bin/clang++ -std=c++23 -Wno-reserved-module-identifier --precompile $S -o b.pcm --sysroot=$XSDK
-try no-cfg-xc    $L/bin/clang++ --no-default-config -std=c++23 -nostdinc++ -isystem $L/include/c++/v1 -isysroot $XSDK -Wno-reserved-module-identifier --precompile $S -o c.pcm
-printf '#include <cmath>\n#ifndef INFINITY\n#error no INFINITY after cmath\n#endif\n#include <complex>\nint main(){}\n' > t.cpp
-try hdr-clt      $L/bin/clang++ -std=c++23 -fsyntax-only t.cpp
-try hdr-xc       $L/bin/clang++ -std=c++23 -fsyntax-only t.cpp --sysroot=$XSDK
-printf '#include <math.h>\n#ifndef INFINITY\n#error no INFINITY after math.h\n#endif\nint main(){}\n' > m.c
-try c-math-clt   $L/bin/clang -fsyntax-only m.c
-try c-math-apple /usr/bin/clang -fsyntax-only m.c
-try cxx-apple    /usr/bin/clang++ -std=c++23 -fsyntax-only t.cpp
-$L/bin/clang++ -std=c++23 -E -dD t.cpp 2>/dev/null | grep -n 'INFINITY\|# 1 ".*math' | head -10 | sed 's/^/READING pp-clt /'
-$L/bin/clang++ -std=c++23 -H -fsyntax-only t.cpp 2>&1 | grep -i 'math' | head -12 | sed 's/^/READING includes-clt /'
-# the released mcpp
-xlings install mcpp@2026.9.16.2 -y >/dev/null 2>&1
-M=$(ls $HOME/.xlings/data/xpkgs/xim-x-mcpp/2026.9.16.2/bin/mcpp)
-cd $(mktemp -d); "$M" new h >/dev/null 2>&1; cd h
-out=$("$M" run 2>&1); rc=$?
-r "mcpp-2026.9.16.2 new+run rc=$rc $(printf '%s' "$out" | grep -m3 -E 'error|Hello|sysroot' | tr '\n' ' ')"
+BASE=(--no-default-config -nostdinc++ -isystem $L/include/c++/v1 -isysroot $SDK -Wno-reserved-module-identifier)
+try() { name=$1; shift; out=$("$@" 2>&1); rc=$?; r "$name rc=$rc $(printf '%s' "$out" | grep -m1 'error' | cut -c1-200)"; }
+printf 'int main(){ return __has_feature(modules); }\n' > hf.cpp
+try hasfeature-modules-in-precompile-of-plain  $L/bin/clang++ "${BASE[@]}" -std=c++23 -fsyntax-only hf.cpp
+printf 'export module m;\n#if __has_feature(modules)\n#error MODULES_ON\n#endif\n' > hf.cppm
+try hasfeature-modules-in-module-interface $L/bin/clang++ "${BASE[@]}" -std=c++23 --precompile hf.cppm -o hf.pcm
+try c++23            $L/bin/clang++ "${BASE[@]}" -std=c++23   --precompile $S -o a.pcm
+try gnu++23          $L/bin/clang++ "${BASE[@]}" -std=gnu++23 --precompile $S -o b.pcm
+try c++23-U-strict   $L/bin/clang++ "${BASE[@]}" -std=c++23 -U__STRICT_ANSI__ --precompile $S -o c.pcm
+try c++23-inc-math   $L/bin/clang++ "${BASE[@]}" -std=c++23 -include math.h --precompile $S -o d.pcm
+try c++23-fbuiltin-inf $L/bin/clang++ "${BASE[@]}" -std=c++23 "-DINFINITY=__builtin_inff()" "-DNAN=__builtin_nanf(\"\")" --precompile $S -o e.pcm
+try c++26            $L/bin/clang++ "${BASE[@]}" -std=c++26   --precompile $S -o f.pcm
+SDK26=/Library/Developer/CommandLineTools/SDKs/MacOSX26.5.sdk
+BASE26=(--no-default-config -nostdinc++ -isystem $L/include/c++/v1 -isysroot $SDK26 -Wno-reserved-module-identifier)
+try c++23-sdk26.5    $L/bin/clang++ "${BASE26[@]}" -std=c++23 --precompile $S -o g.pcm
+# a program importing std, end to end, with the variant that works
+for std in c++23 gnu++23; do
+  rm -rf p; mkdir p; cd p
+  $L/bin/clang++ "${BASE[@]}" -std=$std --precompile $S -o std.pcm >/dev/null 2>&1
+  $L/bin/clang++ "${BASE[@]}" -std=$std -c std.pcm -o std.o >/dev/null 2>&1
+  printf 'import std;\nint main(){ std::println("inf={} nan={}", std::numeric_limits<double>::infinity(), std::isnan(std::nan(""))); }\n' > m.cpp
+  $L/bin/clang++ "${BASE[@]}" -std=$std -fmodule-file=std=std.pcm -c m.cpp -o m.o >/dev/null 2>&1
+  $L/bin/clang++ --no-default-config -isysroot $SDK -fuse-ld=lld -nostdlib++ m.o std.o $L/lib/libc++.a $L/lib/libc++abi.a -o m >/dev/null 2>&1
+  r "program $std: $(./m 2>&1)"
+  cd ..
+done
 exit 0
@@ -8,4 +8,5 @@ jobs:
     timeout-minutes: 40
     steps:
       - uses: actions/checkout@v4
+      - uses: ./.github/actions/setup-macos-llvm
       - run: bash .github/probe/m27.sh