Don't use default macos/arm64 deployment target in calculating the platform tag for fat binaries

src/wheel/macosx_libfile.py

@@ -33,6 +33,9 @@
 - All structures signatures are taken form macosx header files.
 - I think that binary format will be more stable than `otool` output.
   and if apple introduce some changes both implementation will need to be updated.
+- The system compile will set the deployment target no lower than
+  11.0 for arm64 builds. For "Universal 2" builds use the x86_64 deployment
+  target when the arm64 target is 11.0.
 """
 
 import ctypes
@@ -53,6 +56,7 @@
 LC_VERSION_MIN_MACOSX = 0x24
 LC_BUILD_VERSION = 0x32
 
+CPU_TYPE_ARM64 = 0x0100000c
 
 mach_header_fields = [
         ("magic", ctypes.c_uint32), ("cputype", ctypes.c_int),
@@ -271,6 +275,16 @@ class FatArch(BaseClass):
                 try:
                     version = read_mach_header(lib_file, el.offset)
                     if version is not None:
+                        if el.cputype == CPU_TYPE_ARM64 and len(fat_arch_list) != 1:
+                            # Xcode will not set the deployment target below 11.0.0
+                            # for the arm64 architecture. Ignore the arm64 deployment
+                            # in fat binaries when the target is 11.0.0, that way
+                            # the other architetures can select a lower deployment
+                            # target.
+                            # This is safe because there is no arm64 variant for
+                            # macOS 10.15 or earlier.
+                            if version == (11, 0, 0):
+                                continue
                         versions_list.append(version)
                 except ValueError:
                     pass

tests/test_macosx_libfile.py

@@ -23,6 +23,7 @@ def test_read_from_dylib():
         ("test_lib_multiple_fat.dylib", "10.14.0"),
         ("test_lib_10_10_10.dylib", "10.10.10"),
         ("test_lib_11.dylib", "11.0.0"),
+        ("test_lib_10_9_universal2.dylib", "10.9.0"),
     ]
     for file_name, ver in versions:
         extracted = extract_macosx_min_system_version(