ci-macos: the raw compiles use the SDK mcpp selects (xcode-27 image 20260912) (#665)

xcode-27-arm64 20260912 updated the Command Line Tools SDK, whose .tbd stubs
list arm64e.x1. ld64.lld 22.1.8 does not parse that architecture (support
landed upstream in llvm-project#222721, after 22.1.8), and the raw compiles in
this job took the SDK from the xlings LLVM package's clang++.cfg, which names
the Command Line Tools SDK. mcpp itself passes --no-default-config and uses
xcrun --show-sdk-path, and the mcpp e2e suite on the same image is green. The
raw compiles now state -isysroot from xcrun, so the job measures what mcpp
builds.

Co-authored-by: speak-agent <248744407+speak-agent@users.noreply.github.com>

Author: Sunrisepeak

.github/workflows/ci-macos.yml

@@ -81,6 +81,18 @@ jobs:
       - name: The floating macros mcpp states for an Apple target
         run: echo 'APPLE_FLOAT_MACROS=-DINFINITY=HUGE_VALF -DNAN=__builtin_nanf("0x7fc00000")' >> "$GITHUB_ENV"
 
+      # THE SDK mcpp SELECTS, NOT THE ONE THE PACKAGE'S clang++.cfg NAMES. The
+      # xlings LLVM package writes `--sysroot=/Library/Developer/CommandLineTools/
+      # SDKs/MacOSX.sdk` into its configuration; mcpp passes --no-default-config
+      # and uses `xcrun --show-sdk-path` (modules/platform/src/macos/macos.cppm).
+      # On xcode-27-arm64 20260912 the Command Line Tools SDK lists `arm64e.x1`
+      # in its .tbd stubs, which ld64.lld 22.1.8 does not parse (support landed
+      # upstream in llvm-project#222721, after 22.1.8), while the SDK xcrun
+      # selects links. These raw compiles state the SDK the way mcpp does, so the
+      # job measures what mcpp builds rather than the package's default.
+      - name: The SDK mcpp selects for an Apple target
+        run: echo "APPLE_SYSROOT=-isysroot $(xcrun --show-sdk-path)" >> "$GITHUB_ENV"
+
       - name: Test — non-module C++23 compilation
         run: |
           WORK=$(mktemp -d)
@@ -93,7 +105,7 @@ jobs:
               return 0;
           }
           EOF
-          "$CXX" -std=c++23 $APPLE_FLOAT_MACROS -o hello main.cpp
+          "$CXX" $APPLE_SYSROOT -std=c++23 $APPLE_FLOAT_MACROS -o hello main.cpp
           ./hello
 
       - name: Test — import std (two-stage module compilation)
@@ -112,11 +124,11 @@ jobs:
 
           echo "=== Step 1: Precompile std module ==="
           mkdir -p pcm.cache
-          "$CXX" -std=c++23 $APPLE_FLOAT_MACROS -Wno-reserved-module-identifier \
+          "$CXX" $APPLE_SYSROOT -std=c++23 $APPLE_FLOAT_MACROS -Wno-reserved-module-identifier \
             --precompile "$STD_CPPM" -o pcm.cache/std.pcm
 
           echo "=== Step 2: Compile std.pcm → std.o ==="
-          "$CXX" -std=c++23 $APPLE_FLOAT_MACROS -Wno-reserved-module-identifier \
+          "$CXX" $APPLE_SYSROOT -std=c++23 $APPLE_FLOAT_MACROS -Wno-reserved-module-identifier \
             pcm.cache/std.pcm -c -o std.o
 
           echo "=== Step 3: Compile main.cpp with import std ==="
@@ -127,10 +139,10 @@ jobs:
               return 0;
           }
           EOF
-          "$CXX" -std=c++23 $APPLE_FLOAT_MACROS -fmodule-file=std=pcm.cache/std.pcm -c main.cpp -o main.o
+          "$CXX" $APPLE_SYSROOT -std=c++23 $APPLE_FLOAT_MACROS -fmodule-file=std=pcm.cache/std.pcm -c main.cpp -o main.o
 
           echo "=== Step 4: Link ==="
-          "$CXX" main.o std.o -o hello_modules
+          "$CXX" $APPLE_SYSROOT main.o std.o -o hello_modules
           echo "=== Step 5: Run ==="
           ./hello_modules
 
@@ -150,16 +162,16 @@ jobs:
 
           mkdir -p pcm.cache
           # Build std first
-          "$CXX" -std=c++23 $APPLE_FLOAT_MACROS -Wno-reserved-module-identifier \
+          "$CXX" $APPLE_SYSROOT -std=c++23 $APPLE_FLOAT_MACROS -Wno-reserved-module-identifier \
             --precompile "$STD_CPPM" -o pcm.cache/std.pcm
-          "$CXX" -std=c++23 $APPLE_FLOAT_MACROS -Wno-reserved-module-identifier \
+          "$CXX" $APPLE_SYSROOT -std=c++23 $APPLE_FLOAT_MACROS -Wno-reserved-module-identifier \
             pcm.cache/std.pcm -c -o std.o
 
           # Build std.compat (depends on std)
-          "$CXX" -std=c++23 $APPLE_FLOAT_MACROS -Wno-reserved-module-identifier \
+          "$CXX" $APPLE_SYSROOT -std=c++23 $APPLE_FLOAT_MACROS -Wno-reserved-module-identifier \
             -fmodule-file=std=pcm.cache/std.pcm \
             --precompile "$STD_COMPAT_CPPM" -o pcm.cache/std.compat.pcm
-          "$CXX" -std=c++23 $APPLE_FLOAT_MACROS -Wno-reserved-module-identifier \
+          "$CXX" $APPLE_SYSROOT -std=c++23 $APPLE_FLOAT_MACROS -Wno-reserved-module-identifier \
             -fmodule-file=std=pcm.cache/std.pcm \
             pcm.cache/std.compat.pcm -c -o std.compat.o
 
@@ -171,11 +183,11 @@ jobs:
               return 0;
           }
           EOF
-          "$CXX" -std=c++23 $APPLE_FLOAT_MACROS \
+          "$CXX" $APPLE_SYSROOT -std=c++23 $APPLE_FLOAT_MACROS \
             -fmodule-file=std=pcm.cache/std.pcm \
             -fmodule-file=std.compat=pcm.cache/std.compat.pcm \
             -c main.cpp -o main.o
-          "$CXX" main.o std.o std.compat.o -o compat_test
+          "$CXX" $APPLE_SYSROOT main.o std.o std.compat.o -o compat_test
           ./compat_test
 
       - name: Test — multi-module project
@@ -187,9 +199,9 @@ jobs:
           mkdir -p pcm.cache
 
           # Build std
-          "$CXX" -std=c++23 $APPLE_FLOAT_MACROS -Wno-reserved-module-identifier \
+          "$CXX" $APPLE_SYSROOT -std=c++23 $APPLE_FLOAT_MACROS -Wno-reserved-module-identifier \
             --precompile "$STD_CPPM" -o pcm.cache/std.pcm
-          "$CXX" -std=c++23 $APPLE_FLOAT_MACROS -Wno-reserved-module-identifier \
+          "$CXX" $APPLE_SYSROOT -std=c++23 $APPLE_FLOAT_MACROS -Wno-reserved-module-identifier \
             pcm.cache/std.pcm -c -o std.o
 
           # User module: greeter
@@ -202,9 +214,9 @@ jobs:
               }
           }
           EOF
-          "$CXX" -std=c++23 $APPLE_FLOAT_MACROS -fmodule-file=std=pcm.cache/std.pcm \
+          "$CXX" $APPLE_SYSROOT -std=c++23 $APPLE_FLOAT_MACROS -fmodule-file=std=pcm.cache/std.pcm \
             --precompile greeter.cppm -o pcm.cache/greeter.pcm
-          "$CXX" -std=c++23 $APPLE_FLOAT_MACROS -fmodule-file=std=pcm.cache/std.pcm \
+          "$CXX" $APPLE_SYSROOT -std=c++23 $APPLE_FLOAT_MACROS -fmodule-file=std=pcm.cache/std.pcm \
             pcm.cache/greeter.pcm -c -o greeter.o
 
           # Main
@@ -216,11 +228,11 @@ jobs:
               return 0;
           }
           EOF
-          "$CXX" -std=c++23 $APPLE_FLOAT_MACROS \
+          "$CXX" $APPLE_SYSROOT -std=c++23 $APPLE_FLOAT_MACROS \
             -fmodule-file=std=pcm.cache/std.pcm \
             -fmodule-file=greeter=pcm.cache/greeter.pcm \
             -c main.cpp -o main.o
-          "$CXX" main.o greeter.o std.o -o multimod
+          "$CXX" $APPLE_SYSROOT main.o greeter.o std.o -o multimod
           ./multimod
 
       - name: Validate mcpp probe logic expectations