Index: libcxx/utils/libcxx/test/config.py
===================================================================
--- libcxx/utils/libcxx/test/config.py
+++ libcxx/utils/libcxx/test/config.py
@@ -272,6 +272,9 @@
             # Don't warn about using common but nonstandard unprefixed functions
             # like chdir, fileno.
             self.cxx.compile_flags += ['-D_CRT_NONSTDC_NO_WARNINGS']
+            # Build the tests in the same configuration as libcxx itself,
+            # to avoid mismatches if linked statically.
+            self.cxx.compile_flags += ['-D_CRT_STDIO_ISO_WIDE_SPECIFIERS']
             # Required so that tests using min/max don't fail on Windows,
             # and so that those tests don't have to be changed to tolerate
             # this insanity.