Index: llvm/test/tools/llvm-ar/flatten-thin-archive.test
===================================================================
--- llvm/test/tools/llvm-ar/flatten-thin-archive.test
+++ llvm/test/tools/llvm-ar/flatten-thin-archive.test
@@ -1,19 +1,20 @@
 # XFAIL: system-aix
-# This test creates a thin archive that contains a thin archive, a regular
-# archive, and a file.
-#
-# The inner thin archive should be flattened, but the regular archive should
-# not. The order of members in the archive should match the input order, with
-# flattened members appearing together.
+## This test creates a thin archive that contains a thin archive, a regular
+## archive, and a file.
+##
+## The inner thin archive should be flattened, but the regular archive should
+## not. The order of members in the archive should match the input order, with
+## flattened members appearing together.
 
-RUN: touch %t-a.txt %t-b.txt %t-c.txt %t-d.txt %t-e.txt
-RUN: rm -f %t-a-plus-b.a %t-d-plus-e.a %t.a
-RUN: llvm-ar rcsT %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 rcsT %t.a %t-a-plus-b.a %t-c.txt %t-d-plus-e.a
-RUN: llvm-ar t %t.a | FileCheck %s
+# RUN: touch %t-a.txt %t-b.txt %t-c.txt %t-d.txt %t-e.txt
+# RUN: rm -f %t-a-plus-b.a %t-d-plus-e.a %t.a
+# 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: FileCheck --input-file=%t.a %s
 
-CHECK:      a.txt
-CHECK-NEXT: b.txt
-CHECK-NEXT: c.txt
-CHECK-NEXT: -d-plus-e.a
+# CHECK:      !<thin>
+# CHECK:      a.txt
+# CHECK-NEXT: b.txt
+# CHECK-NEXT: c.txt
+# CHECK-NEXT: -d-plus-e.a
Index: llvm/test/tools/llvm-ar/full-to-thin-archive.test
===================================================================
--- llvm/test/tools/llvm-ar/full-to-thin-archive.test
+++ /dev/null
@@ -1,15 +0,0 @@
-# XFAIL: system-aix
-## Test archives do not convert to thin archives.
-
-# RUN: rm -rf %t && mkdir -p %t
-# RUN: llvm-ar qc %t/archive.a %s
-# RUN: not llvm-ar qT %t/archive.a %s 2>&1 | FileCheck %s
-# RUN: not llvm-ar rT %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 full archive's contents to a thin archive with 'L'
-# RUN: llvm-ar -TqcL %t/thin.a %t/archive.a
-# RUN: FileCheck --check-prefixes=THIN --input-file=%t/thin.a %s
-
-THIN: !<thin>
Index: llvm/test/tools/llvm-ar/regular-to-thin-archive.test
===================================================================
--- /dev/null
+++ llvm/test/tools/llvm-ar/regular-to-thin-archive.test
@@ -0,0 +1,27 @@
+# XFAIL: system-aix
+## Test archives do not convert to thin archives.
+
+# RUN: rm -rf %t && mkdir -p %t
+# 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 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
+
+## Test that you can add a regular archive to an existing thin archive with 'L'.
+# RUN: llvm-ar -q --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
+
+# RUN: FileCheck --check-prefixes=REGULAR --input-file=%t/archive.a %s
+# REGULAR: !<arch>
Index: llvm/test/tools/llvm-ar/thin-to-full-archive.test
===================================================================
--- llvm/test/tools/llvm-ar/thin-to-full-archive.test
+++ /dev/null
@@ -1,27 +0,0 @@
-# XFAIL: system-aix
-## Test thin archives do not siletly convert to full archives on write.
-
-# RUN: rm -f %tthin.a %tfull.a 
-
-# RUN: llvm-ar -Trc %tthin.a %S/Inputs/a.txt
-# RUN: FileCheck --check-prefixes=THIN --input-file=%tthin.a %s
-
-# RUN: llvm-ar -q %tthin.a %S/Inputs/b.txt
-# RUN: FileCheck --check-prefixes=THIN --input-file=%tthin.a %s
-
-# RUN: llvm-ar -r %tthin.a %S/Inputs/c.txt
-# RUN: FileCheck --check-prefixes=THIN --input-file=%tthin.a %s
-
-# RUN: llvm-ar -am %S/Inputs/a.txt %tthin.a %S/Inputs/c.txt
-# RUN: FileCheck --check-prefixes=THIN --input-file=%tthin.a %s
-
-# RUN: llvm-ar -d %tthin.a %S/Inputs/c.txt
-# RUN: FileCheck --check-prefixes=THIN --input-file=%tthin.a %s
-
-THIN: !<thin>
-
-## Test that you can add a thin archive's contents to a full archive with 'L'
-# RUN: llvm-ar -qcL %tfull.a %tthin.a
-# RUN: FileCheck --check-prefixes=FULL --input-file=%tfull.a %s
-
-FULL: !<arch>
Index: llvm/test/tools/llvm-ar/thin-to-regular-archive.test
===================================================================
--- /dev/null
+++ llvm/test/tools/llvm-ar/thin-to-regular-archive.test
@@ -0,0 +1,38 @@
+# XFAIL: system-aix
+## Test thin archives do not siletly convert to full archives on write.
+
+# RUN: rm -f %tthin.a %tfull1.a %tfull2.a 
+
+# RUN: llvm-ar -rc --thin %tthin.a %S/Inputs/a.txt
+# RUN: FileCheck --check-prefixes=THIN --input-file=%tthin.a %s
+
+# RUN: llvm-ar -q %tthin.a %S/Inputs/b.txt
+# RUN: FileCheck --check-prefixes=THIN --input-file=%tthin.a %s
+
+# RUN: llvm-ar -r %tthin.a %S/Inputs/c.txt
+# RUN: FileCheck --check-prefixes=THIN --input-file=%tthin.a %s
+
+# RUN: llvm-ar -am %S/Inputs/a.txt %tthin.a %S/Inputs/c.txt
+# RUN: FileCheck --check-prefixes=THIN --input-file=%tthin.a %s
+
+# RUN: llvm-ar -d %tthin.a %S/Inputs/c.txt
+# RUN: FileCheck --check-prefixes=THIN --input-file=%tthin.a %s
+
+# THIN: !<thin>
+
+## Test that you can add a thin archive's contents to a regular archive with 'L'.
+# RUN: llvm-ar -qcL %tfull1.a %tthin.a
+# RUN: FileCheck --check-prefixes=REGULAR --input-file=%tfull1.a --implicit-check-not=thin.a %s
+
+# 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 %tfull2.a %S/Inputs/d.txt
+# RUN: llvm-ar -qL %tfull2.a %tthin.a
+# RUN: FileCheck --check-prefixes=REGULAR --input-file=%tfull2.a %s
+
+## Test that thin archives do not convert to a regular archive with 'L' and no use of '--thin'.
+# RUN: llvm-ar -qL %tthin.a %tfull2.a
+# RUN: FileCheck --check-prefixes=THIN --input-file=%tthin.a %s
Index: llvm/tools/llvm-ar/llvm-ar.cpp
===================================================================
--- llvm/tools/llvm-ar/llvm-ar.cpp
+++ 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);