Issue #18080: When building a C extension module on OS X, if the compiler

is overriden with the CC environment variable, use the new compiler as
the default for linking if LDSHARED is not also overriden.  This restores
Distutils behavior introduced in 3.2.3 and inadvertently dropped in 3.3.0.

sysconfig.py

@@ -195,9 +195,15 @@ def customize_compiler(compiler):
             get_config_vars('CC', 'CXX', 'OPT', 'CFLAGS',
                             'CCSHARED', 'LDSHARED', 'SHLIB_SUFFIX', 'AR', 'ARFLAGS')
 
-        newcc = None
         if 'CC' in os.environ:
-            cc = os.environ['CC']
+            newcc = os.environ['CC']
+            if (sys.platform == 'darwin'
+                    and 'LDSHARED' not in os.environ
+                    and ldshared.startswith(cc)):
+                # On OS X, if CC is overridden, use that as the default
+                #       command for LDSHARED as well
+                ldshared = newcc + ldshared[len(cc):]
+            cc = newcc
         if 'CXX' in os.environ:
             cxx = os.environ['CXX']
         if 'LDSHARED' in os.environ:

tests/test_unixccompiler.py

 """Tests for distutils.unixccompiler."""
+import os
 import sys
 import unittest
-from test.support import run_unittest
+from test.support import EnvironmentVarGuard, run_unittest
 
 from distutils import sysconfig
 from distutils.unixccompiler import UnixCCompiler
@@ -94,7 +95,6 @@ class UnixCCompilerTestCase(unittest.TestCase):
         sysconfig.get_config_var = gcv
         self.assertEqual(self.cc.rpath_foo(), '-Wl,--enable-new-dtags,-R/foo')
 
-
         # non-GCC GNULD
         sys.platform = 'bar'
         def gcv(v):
@@ -115,6 +115,38 @@ class UnixCCompilerTestCase(unittest.TestCase):
         sysconfig.get_config_var = gcv
         self.assertEqual(self.cc.rpath_foo(), '-R/foo')
 
+    @unittest.skipUnless(sys.platform == 'darwin', 'test only relevant for OS X')
+    def test_osx_cc_overrides_ldshared(self):
+        # Issue #18080:
+        # ensure that setting CC env variable also changes default linker
+        def gcv(v):
+            if v == 'LDSHARED':
+                return 'gcc-4.2 -bundle -undefined dynamic_lookup '
+            return 'gcc-4.2'
+        sysconfig.get_config_var = gcv
+        with EnvironmentVarGuard() as env:
+            env['CC'] = 'my_cc'
+            del env['LDSHARED']
+            sysconfig.customize_compiler(self.cc)
+        self.assertEqual(self.cc.linker_so[0], 'my_cc')
+
+    @unittest.skipUnless(sys.platform == 'darwin', 'test only relevant for OS X')
+    def test_osx_explict_ldshared(self):
+        # Issue #18080:
+        # ensure that setting CC env variable does not change
+        #   explicit LDSHARED setting for linker
+        def gcv(v):
+            if v == 'LDSHARED':
+                return 'gcc-4.2 -bundle -undefined dynamic_lookup '
+            return 'gcc-4.2'
+        sysconfig.get_config_var = gcv
+        with EnvironmentVarGuard() as env:
+            env['CC'] = 'my_cc'
+            env['LDSHARED'] = 'my_ld -bundle -dynamic'
+            sysconfig.customize_compiler(self.cc)
+        self.assertEqual(self.cc.linker_so[0], 'my_ld')
+
+
 def test_suite():
     return unittest.makeSuite(UnixCCompilerTestCase)