Index: llvm/test/tools/llvm-ar/full-to-thin-archive.test
===================================================================
--- llvm/test/tools/llvm-ar/full-to-thin-archive.test
+++ llvm/test/tools/llvm-ar/full-to-thin-archive.test
@@ -6,3 +6,9 @@
 # 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
+# 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/thin-to-full-archive.test
===================================================================
--- /dev/null
+++ llvm/test/tools/llvm-ar/thin-to-full-archive.test
@@ -0,0 +1,26 @@
+## 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
+# RUN: llvm-ar -qcL %tfull.a %tthin.a
+# RUN: FileCheck -input-file=%tfull.a %s
+
+CHECK-NOT: !<thin>
Index: llvm/tools/llvm-ar/llvm-ar.cpp
===================================================================
--- llvm/tools/llvm-ar/llvm-ar.cpp
+++ llvm/tools/llvm-ar/llvm-ar.cpp
@@ -651,6 +651,11 @@
                            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());