Index: llvm/test/tools/llvm-ar/non-windows-name-case.test
===================================================================
--- /dev/null
+++ llvm/test/tools/llvm-ar/non-windows-name-case.test
@@ -0,0 +1,27 @@
+## Test that on windows, members are case insensitive.
+# UNSUPPORTED: system-windows
+
+# RUN: mkdir -p %t
+# RUN: rm -rf %t/lowerCase %t/UPPERCASE && mkdir %t/lowerCase %t/UPPERCASE
+# RUN: echo lowerCase > %t/lowerCase/file.txt
+# RUN: echo UPPERCASE > %t/UPPERCASE/FILE.TXT
+
+# RUN: rm -f %t/archive.a
+# RUN: llvm-ar rc %t/archive.a %t/lowerCase/file.txt
+# RUN: llvm-ar rc %t/archive.a %t/UPPERCASE/FILE.TXT
+# RUN: FileCheck %s -input-file=%t/archive.a --check-prefix=ARCHIVE
+
+# ARCHIVE: file.txt
+# ARCHIVE: FILE.TXT
+
+# RUN: rm -f %t/thin-archive.a
+# RUN: llvm-ar Trc %t/thin-archive.a %t/lowerCase/file.txt %t/UPPERCASE/FILE.TXT
+# RUN: llvm-ar dTP %t/thin-archive.a %t/uppercase/file.txt
+
+# RUN: FileCheck %s -input-file=%t/thin-archive.a --check-prefix=THIN-ARCHIVE -DPATH=%/t
+
+# RUN: llvm-ar dTP %t/thin-archive.a %t/LOWERCASE/FILE.TXT
+# RUN: FileCheck %s -input-file=%t/thin-archive.a --check-prefix=THIN-ARCHIVE -DPATH=%/t
+
+# THIN-ARCHIVE: [[PATH]]/lowerCase/file.txt
+# THIN-ARCHIVE: [[PATH]]/UPPERCASE/FILE.TXT
Index: llvm/test/tools/llvm-ar/windows-name-case.test
===================================================================
--- /dev/null
+++ llvm/test/tools/llvm-ar/windows-name-case.test
@@ -0,0 +1,29 @@
+## Test that on windows, members are case insensitive.
+# REQUIRES: system-windows
+
+# RUN: mkdir -p %t
+# RUN: rm -rf %t/lowerCase %t/UPPERCASE && mkdir %t/lowerCase %t/UPPERCASE
+# RUN: echo lowerCase > %t/lowerCase/file.txt
+# RUN: echo UPPERCASE > %t/UPPERCASE/FILE.TXT
+
+# RUN: rm -f %t/archive.a
+# RUN: llvm-ar rc %t/archive.a %t/lowerCase/file.txt
+# RUN: FileCheck %s -input-file=%t/archive.a --check-prefix=LOWER --implicit-check-not=FILE.TXT
+# RUN: llvm-ar rc %t/archive.a %t/UPPERCASE/FILE.TXT
+# RUN: FileCheck %s -input-file=%t/archive.a --check-prefix=UPPER --implicit-check-not=file.txt
+
+# LOWER: file.txt
+# UPPER: FILE.TXT
+
+# RUN: rm -f %t/thin-archive.a
+# RUN: llvm-ar Trc %t/thin-archive.a %t/lowerCase/file.txt %t/UPPERCASE/FILE.txt
+# RUN: llvm-ar dTP %t/thin-archive.a %t/uppercase/file.txt
+
+# RUN: FileCheck %s -input-file=%t/thin-archive.a --check-prefix=THIN-ARCHIVE -DPATH=%/t
+# THIN-ARCHIVE-NOT: [[PATH]]/UPPERCASE/FILE.TXT
+# THIN-ARCHIVE: [[PATH]]/lowerCase/file.txt
+
+# RUN: llvm-ar dTP %t/thin-archive.a %t/LOWERCASE/FILE.TXT
+# RUN: FileCheck %s -input-file=%t/thin-archive.a --check-prefix=EMPTY --implicit-check-not {{.}}
+
+EMPTY: !<thin>
Index: llvm/tools/llvm-ar/llvm-ar.cpp
===================================================================
--- llvm/tools/llvm-ar/llvm-ar.cpp
+++ llvm/tools/llvm-ar/llvm-ar.cpp
@@ -484,8 +484,14 @@
 }
 
 static std::string normalizePath(StringRef Path) {
-  return CompareFullPath ? sys::path::convert_to_slash(Path)
-                         : std::string(sys::path::filename(Path));
+  std::string NormalizedPath = CompareFullPath
+                                   ? sys::path::convert_to_slash(Path)
+                                   : std::string(sys::path::filename(Path));
+
+  if (Triple(sys::getProcessTriple()).isOSWindows())
+    llvm::transform(NormalizedPath, NormalizedPath.begin(), ::toupper);
+
+  return NormalizedPath;
 }
 
 // Implement the 'x' operation. This function extracts files back to the file
@@ -561,7 +567,7 @@
 
       if (Filter) {
         auto I = find_if(Members, [Name](StringRef Path) {
-          return Name == normalizePath(Path);
+          return normalizePath(Name) == normalizePath(Path);
         });
         if (I == Members.end())
           continue;
@@ -690,8 +696,9 @@
                                         StringMap<int> &MemberCount) {
   if (Operation == QuickAppend || Members.empty())
     return IA_AddOldMember;
-  auto MI = find_if(
-      Members, [Name](StringRef Path) { return Name == normalizePath(Path); });
+  auto MI = find_if(Members, [Name](StringRef Path) {
+    return normalizePath(Name) == normalizePath(Path);
+  });
 
   if (MI == Members.end())
     return IA_AddOldMember;
@@ -749,7 +756,7 @@
       int Pos = Ret.size();
       Expected<StringRef> NameOrErr = Child.getName();
       failIfError(NameOrErr.takeError());
-      StringRef Name = NameOrErr.get();
+      std::string Name = normalizePath(NameOrErr.get());
       if (Name == PosName) {
         assert(AddAfter || AddBefore);
         if (AddBefore)
@@ -1019,8 +1026,9 @@
       break;
     case MRICommand::Delete: {
       std::string Name = normalizePath(Rest);
-      llvm::erase_if(NewMembers,
-                     [=](NewArchiveMember &M) { return M.MemberName == Name; });
+      llvm::erase_if(NewMembers, [=](NewArchiveMember &M) {
+        return normalizePath(M.MemberName) == Name;
+      });
       break;
     }
     case MRICommand::Save: