diff --git a/llvm/test/tools/llvm-ar/flatten-thin-archive.test b/llvm/test/tools/llvm-ar/flatten-thin-archive.test
--- a/llvm/test/tools/llvm-ar/flatten-thin-archive.test
+++ b/llvm/test/tools/llvm-ar/flatten-thin-archive.test
@@ -11,8 +11,9 @@
 # RUN: llvm-ar rcs --thin %t-a-plus-b.a %t-a.txt %t-b.txt
 # RUN: llvm-ar rcs %t-d-plus-e.a %t-d.txt %t-e.txt
 # RUN: llvm-ar rcs --thin %t.a %t-a-plus-b.a %t-c.txt %t-d-plus-e.a
-# RUN: llvm-ar t %t.a | FileCheck %s
+# RUN: FileCheck --input-file=%t.a %s
 
+# CHECK:      !<thin>
 # CHECK:      a.txt
 # CHECK-NEXT: b.txt
 # CHECK-NEXT: c.txt
diff --git a/llvm/test/tools/llvm-ar/regular-to-thin-archive.test b/llvm/test/tools/llvm-ar/regular-to-thin-archive.test
--- a/llvm/test/tools/llvm-ar/regular-to-thin-archive.test
+++ b/llvm/test/tools/llvm-ar/regular-to-thin-archive.test
@@ -2,14 +2,28 @@
 ## Test regular archives do not convert to thin archives.
 
 # RUN: rm -rf %t && mkdir -p %t
-# RUN: llvm-ar qc %t/archive.a %s
+# RUN: llvm-ar qc %t/archive.a %S/Inputs/a.txt
 # RUN: not llvm-ar q --thin %t/archive.a %s 2>&1 | FileCheck %s
 # RUN: not llvm-ar r --thin %t/archive.a %s 2>&1 | FileCheck %s
 
 # CHECK: error: cannot convert a regular archive to a thin one
 
-## Test that you can add a regular archive's contents to a thin archive with 'L'
-# RUN: llvm-ar -qcL --thin %t/thin.a %t/archive.a
-# RUN: FileCheck --check-prefixes=THIN --input-file=%t/thin.a %s
+## Test that you can add a regular archive to a new thin archive with 'L'.
+# RUN: llvm-ar -qcL --thin %t/thin1.a %S/Inputs/b.txt %t/archive.a
+# RUN: FileCheck --check-prefixes=THIN --input-file=%t/thin1.a %s
 
-THIN: !<thin>
+## Test that you can add a regular archive to an existing thin archive with 'L'.
+# RUN: llvm-ar -qc --thin %t/thin2.a %S/Inputs/b.txt
+# RUN: llvm-ar -qL --thin %t/thin2.a %t/archive.a
+# RUN: FileCheck --check-prefixes=THIN --input-file=%t/thin2.a %s
+
+# THIN: !<thin>
+# THIN: archive.a
+
+## Test archives do not convert to thin archives with use of 'L' and '--thin'.
+# RUN: not llvm-ar -qL --thin %t/archive.a %t/thin2.a 2>&1 | FileCheck %s
+
+## Test that a regular archive can be read despite use of --thin 
+# RUN: llvm-ar -t --thin %t/archive.a
+# RUN: FileCheck --check-prefixes=REGULAR --input-file=%t/archive.a %s
+# REGULAR: !<arch>
diff --git a/llvm/test/tools/llvm-ar/thin-to-regular-archive.test b/llvm/test/tools/llvm-ar/thin-to-regular-archive.test
--- a/llvm/test/tools/llvm-ar/thin-to-regular-archive.test
+++ b/llvm/test/tools/llvm-ar/thin-to-regular-archive.test
@@ -1,7 +1,7 @@
 # XFAIL: system-aix
 ## Test thin archives do not siletly convert to regular archives on write.
 
-# RUN: rm -f %tthin.a %tregular.a 
+# RUN: rm -f %tthin.a %tregular1.a %tregular2.a
 
 # RUN: llvm-ar -rc --thin %tthin.a %S/Inputs/a.txt
 # RUN: FileCheck --check-prefixes=THIN --input-file=%tthin.a %s
@@ -20,8 +20,19 @@
 
 # THIN: !<thin>
 
-## Test that you can add a thin archive's contents to a regular archive with 'L'
-# RUN: llvm-ar -qcL %tregular.a %tthin.a
-# RUN: FileCheck --check-prefixes=REGULAR --input-file=%tregular.a %s
+## Test that you can add a thin archive's contents to a regular archive with 'L'.
+# RUN: llvm-ar -qcL %tregular1.a %tthin.a
+# RUN: FileCheck --check-prefixes=REGULAR --input-file=%tregular1.a --implicit-check-not=thin.a %s
 
-REGULAR: !<arch>
+# REGULAR: !<arch>
+# REGULAR: a.txt
+# REGULAR: b.txt
+
+## Test that you can add a thin archive's contents to an existing regular archive with 'L'.
+# RUN: llvm-ar -q %tregular2.a %S/Inputs/d.txt
+# RUN: llvm-ar -qL %tregular2.a %tthin.a
+# RUN: FileCheck --check-prefixes=REGULAR --input-file=%tregular2.a %s
+
+## Test that thin archives do not convert to regular archives with 'L' and no use of '--thin'.
+# RUN: llvm-ar -qL %tthin.a %tregular2.a
+# RUN: FileCheck --check-prefixes=THIN --input-file=%tthin.a %s
diff --git a/llvm/tools/llvm-ar/llvm-ar.cpp b/llvm/tools/llvm-ar/llvm-ar.cpp
--- a/llvm/tools/llvm-ar/llvm-ar.cpp
+++ b/llvm/tools/llvm-ar/llvm-ar.cpp
@@ -653,13 +653,6 @@
 static void addChildMember(std::vector<NewArchiveMember> &Members,
                            const object::Archive::Child &M,
                            bool FlattenArchive = false) {
-  if (Thin && !M.getParent()->isThin())
-    fail("cannot convert a regular archive to a thin one");
-
-  // Avoid converting an existing thin archive to a regular one.
-  if (!AddLibrary && M.getParent()->isThin())
-    Thin = true;
-
   Expected<NewArchiveMember> NMOrErr =
       NewArchiveMember::getOldMember(M, Deterministic);
   failIfError(NMOrErr.takeError());
@@ -927,6 +920,14 @@
                                   object::Archive *OldArchive,
                                   std::unique_ptr<MemoryBuffer> OldArchiveBuf,
                                   std::vector<NewArchiveMember> *NewMembersP) {
+  if (OldArchive) {
+    if (Thin && !OldArchive->isThin())
+      fail("cannot convert a regular archive to a thin one");
+
+    if (OldArchive->isThin())
+      Thin = true;
+  }
+
   std::vector<NewArchiveMember> NewMembers;
   if (!NewMembersP)
     NewMembers = computeNewArchiveMembers(Operation, OldArchive);