pkg/apple-sdk: support Xcode 27 SDK headers (#13419)

Xcode 27's math.h uses the __need_infinity_nan protocol provided by
matching Clang resource headers. Zig 0.16's bundled float.h predates
that protocol, causing the bundled libc++ compilation to fail.

Overlay the SDK math.h through the Apple SDK libc include path and
provide the missing infinity and NaN definitions. The compatibility
header can be removed once Zig's bundled Clang headers support the
protocol.

.github/workflows/test.yml

@@ -102,6 +102,7 @@ jobs:
       - build-nix
       # - build-nix-macos
       - build-macos
+      - build-macos-xcode-27
       - build-macos-freetype
       - build-snap
       - test
@@ -1109,6 +1110,46 @@ jobs:
             COMPILATION_CACHE_CAS_PATH=/Users/runner/Library/Developer/Xcode/DerivedData/CompilationCache.noindex \
             COMPILATION_CACHE_KEEP_CAS_DIRECTORY=YES
 
+  build-macos-xcode-27:
+    runs-on: namespace-profile-ghostty-macos-tahoe
+    needs: test
+    env:
+      ZIG_LOCAL_CACHE_DIR: /Users/runner/zig/local-cache
+      ZIG_GLOBAL_CACHE_DIR: /Users/runner/zig/global-cache
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
+
+      - name: Setup Cache
+        uses: namespacelabs/nscloud-cache-action@c5f8dab7560444c4bf8dbc64f1b203431873c547 # v1.6.1
+        with:
+          cache: |
+            xcode
+          path: |
+            /Users/runner/zig
+
+      # TODO(tahoe): https://github.com/NixOS/nix/issues/13342
+      - uses: DeterminateSystems/nix-installer-action@main
+        with:
+          determinate: true
+      - uses: cachix/cachix-action@5f2d7c5294214f71b873db4b969586b980625e71 # v17
+        with:
+          name: ghostty
+          authToken: "${{ secrets.CACHIX_AUTH_TOKEN }}"
+
+      - name: Xcode Select
+        run: sudo xcode-select -s /Applications/Xcode_27.app
+
+      - name: Xcode Version
+        run: xcodebuild -version
+
+      - name: get the Zig deps
+        id: deps
+        run: nix build -L .#deps && echo "deps=$(readlink ./result)" >> $GITHUB_OUTPUT
+
+      - name: Build with Xcode 27
+        run: nix develop -c zig build --system ${{ steps.deps.outputs.deps }} -Demit-macos-app=false
+
   build-macos-freetype:
     runs-on: namespace-profile-ghostty-macos-tahoe
     needs: test

pkg/apple-sdk/build.zig

@@ -49,7 +49,7 @@ pub fn pathsForTarget(b: *std.Build, target: std.Target) !Cache.Value {
             // Detect our SDK using the "findNative" Zig stdlib function.
             // This is really important because it forces using `xcrun` to
             // find the SDK path.
-            const libc = std.zig.LibCInstallation.findNative(
+            var libc = std.zig.LibCInstallation.findNative(
                 b.allocator,
                 b.graph.io,
                 .{
@@ -59,6 +59,20 @@ pub fn pathsForTarget(b: *std.Build, target: std.Target) !Cache.Value {
                 },
             ) catch break :darwin;
 
+            // Xcode 27's math.h requests infinity and NaN definitions from
+            // Clang's float.h using the __need_infinity_nan protocol. Zig
+            // 0.16's bundled Clang resource headers predate that protocol, so
+            // compiling Zig's bundled libc++ against the new SDK fails.
+            //
+            // Put our compatibility include directory between Zig's resource
+            // headers and the selected SDK headers. Its math.h forwards to the
+            // SDK with #include_next, then supplies the definitions missing
+            // from Zig's float.h. This can be removed once Zig's bundled Clang
+            // headers implement __need_infinity_nan.
+            libc.include_dir = b.dependency("apple_sdk", .{})
+                .path("include")
+                .getPath(b);
+
             // Render the file compatible with the `--libc` Zig flag.
             var stream: std.Io.Writer.Allocating = .init(b.allocator);
             defer stream.deinit();

pkg/apple-sdk/include/math.h

@@ -0,0 +1,21 @@
+#ifndef GHOSTTY_APPLE_SDK_MATH_H
+#define GHOSTTY_APPLE_SDK_MATH_H
+
+// Resume the header search after this compatibility directory so we include
+// the selected Xcode SDK's math.h. A regular #include <math.h> would find this
+// wrapper again and loop forever. This is a Clang extension but Zig uses
+// Clang so we good.
+#include_next <math.h>
+
+// Xcode 27's math.h expects Clang's float.h to define these when requested
+// with __need_infinity_nan. Zig 0.16's bundled float.h predates that protocol;
+// remove these fallbacks once Zig's bundled Clang headers implement it.
+#ifndef INFINITY
+#define INFINITY (__builtin_inff())
+#endif
+
+#ifndef NAN
+#define NAN (__builtin_nanf(""))
+#endif
+
+#endif