Index: llvm/test/tools/llvm-ar/regular-to-thin-archive-55527.test
===================================================================
--- /dev/null
+++ llvm/test/tools/llvm-ar/regular-to-thin-archive-55527.test
@@ -0,0 +1,15 @@
+## Test regular archives do not convert to thin archives
+## in the special case of replacing a single file in an
+## archive with itself from the current directory.
+
+# RUN: rm -rf %t && mkdir -p %t && cd %t
+# RUN: echo "a" > a.txt
+# RUN: llvm-ar --format=gnu cr foo.a a.txt 2>&1
+
+## Check that an error is issued.
+# RUN: not llvm-ar --format=gnu cr --thin foo.a a.txt 2>&1 | FileCheck %s
+# CHECK: error: cannot convert a regular archive to a thin one
+
+## Check that the archive is still the correct flavor.
+# RUN: FileCheck --input-file=foo.a %s --check-prefix=REGULAR
+# REGULAR: !<arch>