diff --git a/libcxx/test/libcxx/selftest/dsl/dsl.sh.py b/libcxx/test/libcxx/selftest/dsl/dsl.sh.py
--- a/libcxx/test/libcxx/selftest/dsl/dsl.sh.py
+++ b/libcxx/test/libcxx/selftest/dsl/dsl.sh.py
@@ -196,6 +196,7 @@
         #include <cstdio>
         int main(int, char**) {
             std::printf("MACRO=%u\\n", MACRO);
+            return 0;
         }
         """
         compileFlagsIndex = findIndex(self.config.substitutions, lambda x: x[0] == '%{compile_flags}')
@@ -209,6 +210,19 @@
         output2 = dsl.programOutput(self.config, source)
         self.assertEqual(output2, "MACRO=2\n")
 
+    def test_program_stderr_is_not_conflated_with_stdout(self):
+        # Run a program that produces stdout output and stderr output too, making
+        # sure the stderr output does not pollute the stdout output.
+        source = """
+        #include <cstdio>
+        int main(int, char**) {
+            std::fprintf(stdout, "STDOUT-OUTPUT");
+            std::fprintf(stderr, "STDERR-OUTPUT");
+            return 0;
+        }
+        """
+        self.assertEqual(dsl.programOutput(self.config, source), "STDOUT-OUTPUT")
+
 
 class TestHasLocale(SetupConfigs):
     """
diff --git a/libcxx/utils/libcxx/test/dsl.py b/libcxx/utils/libcxx/test/dsl.py
--- a/libcxx/utils/libcxx/test/dsl.py
+++ b/libcxx/utils/libcxx/test/dsl.py
@@ -64,9 +64,20 @@
     if not os.path.exists(d):
       os.makedirs(d)
   res = lit.TestRunner.executeScriptInternal(test, litConfig, tmpBase, parsedCommands, execDir)
-  if isinstance(res, lit.Test.Result):
-    res = ('', '', 127, None)
-  return res
+  if isinstance(res, lit.Test.Result): # Handle failure to parse the Lit test
+    res = ('', res.output, 127, None)
+  (out, err, exitCode, timeoutInfo) = res
+
+  # TODO: As a temporary workaround until https://reviews.llvm.org/D81892 lands, manually
+  #       split any stderr output that is included in stdout. It shouldn't be there, but
+  #       the Lit internal shell conflates stderr and stdout.
+  conflatedErrorOutput = re.search("(# command stderr:.+$)", out, flags=re.DOTALL)
+  if conflatedErrorOutput:
+    conflatedErrorOutput = conflatedErrorOutput.group(0)
+    out = out[:-len(conflatedErrorOutput)]
+    err += conflatedErrorOutput
+
+  return (out, err, exitCode, timeoutInfo)
 
 def _makeConfigTest(config, testPrefix=''):
   sourceRoot = os.path.join(config.test_exec_root, '__config_src__')
@@ -122,7 +133,7 @@
       if exitCode != 0:
         return None
 
-      actualOut = re.search("command output:\n(.+)\n$", out, flags=re.DOTALL)
+      actualOut = re.search("# command output:\n(.+)\n$", out, flags=re.DOTALL)
       actualOut = actualOut.group(1) if actualOut else ""
       return actualOut