diff --git a/clang/lib/Driver/ToolChains/CommonArgs.cpp b/clang/lib/Driver/ToolChains/CommonArgs.cpp
--- a/clang/lib/Driver/ToolChains/CommonArgs.cpp
+++ b/clang/lib/Driver/ToolChains/CommonArgs.cpp
@@ -1602,9 +1602,10 @@
     if (TC.getTriple().isKnownWindowsMSVCEnvironment()) {
       // Issue error diagnostic if libgcc is explicitly specified
       // through command line as --rtlib option argument.
-      if (Args.hasArg(options::OPT_rtlib_EQ)) {
+      Arg *A = Args.getLastArg(options::OPT_rtlib_EQ);
+      if (A && A->getValue() != StringRef("platform")) {
         TC.getDriver().Diag(diag::err_drv_unsupported_rtlib_for_platform)
-            << Args.getLastArg(options::OPT_rtlib_EQ)->getValue() << "MSVC";
+            << A->getValue() << "MSVC";
       }
     } else
       AddLibgcc(TC, D, CmdArgs, Args);
diff --git a/clang/lib/Driver/ToolChains/Darwin.cpp b/clang/lib/Driver/ToolChains/Darwin.cpp
--- a/clang/lib/Driver/ToolChains/Darwin.cpp
+++ b/clang/lib/Driver/ToolChains/Darwin.cpp
@@ -1385,7 +1385,7 @@
     const ArgList &Args) const {
   if (Arg* A = Args.getLastArg(options::OPT_rtlib_EQ)) {
     StringRef Value = A->getValue();
-    if (Value != "compiler-rt")
+    if (Value != "compiler-rt" && Value != "platform")
       getDriver().Diag(clang::diag::err_drv_unsupported_rtlib_for_platform)
           << Value << "darwin";
   }
diff --git a/clang/test/Driver/msvc-compiler-rt.c b/clang/test/Driver/msvc-compiler-rt.c
--- a/clang/test/Driver/msvc-compiler-rt.c
+++ b/clang/test/Driver/msvc-compiler-rt.c
@@ -1,5 +1,7 @@
 // RUN: %clang -target x86_64-pc-windows-msvc --rtlib=compiler-rt -### %s 2>&1 | FileCheck %s -check-prefix MSVC-COMPILER-RT
+// RUN: %clang -target x86_64-pc-windows-msvc --rtlib=compiler-rt --rtlib=platform -### %s 2>&1 | FileCheck %s -check-prefix MSVC-DEFAULT
 // RUN: not %clang %s -target x86_64-pc-windows-msvc --rtlib=libgcc 2>&1 | FileCheck %s -check-prefix CHECK-ERROR
 
 // MSVC-COMPILER-RT: "{{.*}}clang_rt.builtins{{.*}}"
+// MSVC-DEFAULT-NOT: "{{.*}}clang_rt.builtins{{.*}}"
 // CHECK-ERROR: unsupported runtime library 'libgcc' for platform 'MSVC'
diff --git a/clang/test/Driver/rtlib-darwin.c b/clang/test/Driver/rtlib-darwin.c
new file mode 100644
--- /dev/null
+++ b/clang/test/Driver/rtlib-darwin.c
@@ -0,0 +1,6 @@
+// RUN: %clang -target x86_64-apple-darwin -resource-dir=%S/Inputs/resource_dir --rtlib=compiler-rt -### %s 2>&1 | FileCheck %s -check-prefix DARWIN-COMPILER-RT
+// RUN: %clang -target x86_64-apple-darwin -resource-dir=%S/Inputs/resource_dir --rtlib=platform -### %s 2>&1 | FileCheck %s -check-prefix DARWIN-COMPILER-RT
+// RUN: not %clang %s -target x86_64-apple-darwin --rtlib=libgcc 2>&1 | FileCheck %s -check-prefix CHECK-ERROR
+
+// DARWIN-COMPILER-RT: "{{.*}}clang_rt.osx{{.*}}"
+// CHECK-ERROR: unsupported runtime library 'libgcc' for platform 'darwin'