diff --git a/clang/lib/Driver/Tools.cpp b/clang/lib/Driver/Tools.cpp
@@ -386,9 +386,71 @@ void Clang::AddPreprocessingOptions(Compilation &C, const JobAction &JA,
   // wonky, but we include looking for .gch so we can support seamless
   // replacement into a build system already set up to be generating
   // .gch files.
+  int YcIndex = -1, YuIndex = -1;
+  {
+    int AI = -1;
+    const Arg *YcArg = Args.getLastArg(options::OPT__SLASH_Yc);
+    const Arg *YuArg = Args.getLastArg(options::OPT__SLASH_Yu);
+    for (const Arg *A : Args.filtered(options::OPT_clang_i_Group)) {
+      // Walk the whole i_Group and skip non "-include" flags so that the index
+      // here matches the index in the next loop below.
+      ++AI;
+      if (!A->getOption().matches(options::OPT_include))
+        continue;
+      if (YcArg && strcmp(A->getValue(), YcArg->getValue()) == 0)
+        YcIndex = AI;
+      if (YuArg && strcmp(A->getValue(), YuArg->getValue()) == 0)
+        YuIndex = AI;
+    }
+  }
+  if (isa<PrecompileJobAction>(JA) && YcIndex != -1) {
+    Driver::InputList Inputs;
+    D.BuildInputs(getToolChain(), C.getArgs(), Inputs);
+    assert(Inputs.size() == 1 && "Need one input when building pch");
+    CmdArgs.push_back(Args.MakeArgString(Twine("-find-pch-source=") +
+                                         Inputs[0].second->getValue()));
+  }
+
   bool RenderedImplicitInclude = false;
+  int AI = -1;
   for (const Arg *A : Args.filtered(options::OPT_clang_i_Group)) {
-    if (A->getOption().matches(options::OPT_include)) {
+    ++AI;
+
+    if (getToolChain().getDriver().IsCLMode()) {
+      // In clang-cl mode, /Ycfoo.h means that all code up to a foo.h 
+      // include is compiled into foo.h, and everything after goes into
+      // the .obj file. /Yufoo.h means that all includes prior to and including
+      // foo.h are completely skipped and replaced with a use of the pch file
+      // for foo.h.  (Each flag can have at most one value, multiple /Yc flags
+      // just mean that the last one wins.)  If /Yc and /Yu are both present
+      // and refer to the same file, /Yc wins.
+      // Note that OPT__SLASH_FI gets mapped to OPT_include.
+      // FIXME: The code here assumes that /Yc and /Yu refer to the same file.
+      // cl.exe seems to support both flags with different values, but that
+      // seems strange (which flag does /Fp now refer to?), so don't implement
+      // that until someone needs that.
+      int PchIndex = YcIndex != -1 ? YcIndex : YuIndex;
+      if (PchIndex != -1) {
+        if (isa<PrecompileJobAction>(JA)) {
+          // When building the pch, skip all includes after the pch.
+          assert(YcIndex != -1 && PchIndex == YcIndex);
+          if (AI >= YcIndex)
+            continue;
+        } else {
+          // When using the pch, skip all includes prior to the pch.
+          if (AI < PchIndex)
+            continue;
+          if (AI == PchIndex) {
+            A->claim();
+            CmdArgs.push_back("-include-pch");
+            CmdArgs.push_back(
+                Args.MakeArgString(D.GetClPchPath(C, A->getValue())));
+            continue;
+          }
+        }
+      }
+    } else if (A->getOption().matches(options::OPT_include)) {
+      // Handling of gcc-style gch precompiled headers.
       bool IsFirstImplicitInclude = !RenderedImplicitInclude;
       RenderedImplicitInclude = true;
 
@@ -5654,6 +5716,12 @@ void Clang::ConstructJob(Compilation &C, const JobAction &JA,
         getCLFallback()->GetCommand(C, JA, Output, Inputs, Args, LinkingOutput);
     C.addCommand(llvm::make_unique<FallbackCommand>(
         JA, *this, Exec, CmdArgs, Inputs, std::move(CLCommand)));
+  } else if (Args.hasArg(options::OPT__SLASH_fallback) &&
+             isa<PrecompileJobAction>(JA)) {
+    // In /fallback builds, run the main compilation even if the pch generation
+    // fails, so that the main compilation's fallback to cl.exe runs.
+    C.addCommand(llvm::make_unique<ForceSuccessCommand>(JA, *this, Exec,
+                                                        CmdArgs, Inputs));
   } else {
     C.addCommand(llvm::make_unique<Command>(JA, *this, Exec, CmdArgs, Inputs));
   }

diff --git a/clang/lib/Frontend/CompilerInstance.cpp b/clang/lib/Frontend/CompilerInstance.cpp
@@ -712,15 +712,18 @@ std::unique_ptr<llvm::raw_pwrite_stream> CompilerInstance::createOutputFile(
 // Initialization Utilities
 
 bool CompilerInstance::InitializeSourceManager(const FrontendInputFile &Input){
-  return InitializeSourceManager(Input, getDiagnostics(),
-                                 getFileManager(), getSourceManager(), 
-                                 getFrontendOpts());
+  return InitializeSourceManager(
+      Input, getDiagnostics(), getFileManager(), getSourceManager(),
+      hasPreprocessor() ? &getPreprocessor().getHeaderSearchInfo() : nullptr,
+      getFrontendOpts());
 }
 
+// static
 bool CompilerInstance::InitializeSourceManager(const FrontendInputFile &Input,
                                                DiagnosticsEngine &Diags,
                                                FileManager &FileMgr,
                                                SourceManager &SourceMgr,
+                                               HeaderSearch *HS,
                                                const FrontendOptions &Opts) {
   SrcMgr::CharacteristicKind
     Kind = Input.isSystem() ? SrcMgr::C_System : SrcMgr::C_User;
@@ -737,7 +740,32 @@ bool CompilerInstance::InitializeSourceManager(const FrontendInputFile &Input,
 
   // Figure out where to get and map in the main file.
   if (InputFile != "-") {
-    const FileEntry *File = FileMgr.getFile(InputFile, /*OpenFile=*/true);
+    const FileEntry *File;
+    if (Opts.FindPchSource.empty()) {
+      File = FileMgr.getFile(InputFile, /*OpenFile=*/true);
+    } else {
+      // When building a pch file in clang-cl mode, the .h file is built as if
+      // it was included by a cc file.  Since the driver doesn't know about
+      // all include search directories, the frontend must search the input
+      // file through HeaderSearch here, as if it had been included by the
+      // cc file at Opts.FindPchSource.
+      const FileEntry *FindFile = FileMgr.getFile(Opts.FindPchSource);
+      if (!FindFile) {
+        Diags.Report(diag::err_fe_error_reading) << Opts.FindPchSource;
+        return false;
+      }
+      const DirectoryLookup *UnusedCurDir;
+      SmallVector<std::pair<const FileEntry *, const DirectoryEntry *>, 16>
+          Includers;
+      Includers.push_back(std::make_pair(FindFile, FindFile->getDir()));
+      File = HS->LookupFile(InputFile, SourceLocation(), /*isAngled=*/false,
+                            /*FromDir=*/nullptr,
+                            /*CurDir=*/UnusedCurDir, Includers,
+                            /*SearchPath=*/nullptr,
+                            /*RelativePath=*/nullptr,
+                            /*RequestingModule=*/nullptr,
+                            /*SuggestedModule=*/nullptr, /*SkipCache=*/true);
+    }
     if (!File) {
       Diags.Report(diag::err_fe_error_reading) << InputFile;
       return false;

diff --git a/clang/lib/Frontend/CompilerInvocation.cpp b/clang/lib/Frontend/CompilerInvocation.cpp
@@ -1107,6 +1107,7 @@ static InputKind ParseFrontendArgs(FrontendOptions &Opts, ArgList &Args,
     = Args.getLastArgValue(OPT_foverride_record_layout_EQ);
   Opts.AuxTriple =
       llvm::Triple::normalize(Args.getLastArgValue(OPT_aux_triple));
+  Opts.FindPchSource = Args.getLastArgValue(OPT_find_pch_source_EQ);
 
   if (const Arg *A = Args.getLastArg(OPT_arcmt_check,
                                      OPT_arcmt_modify,

diff --git a/clang/test/Driver/Inputs/pchfile.cpp b/clang/test/Driver/Inputs/pchfile.cpp
diff --git a/clang/test/Driver/Inputs/pchfile.h b/clang/test/Driver/Inputs/pchfile.h
@@ -0,0 +1,3 @@
+#if defined(ERR_HEADER)
+#error nope1
+#endif
diff --git a/clang/test/Driver/cl-pch-errorhandling.cpp b/clang/test/Driver/cl-pch-errorhandling.cpp
@@ -0,0 +1,15 @@
+// Note: %s and %S must be preceded by --, otherwise it may be interpreted as a
+// command-line option, e.g. on Mac where %s is commonly under /Users.
+
+// /Yc but pch generation fails => main file not compiled
+// This is a separate file since executing this failure path requires
+// code generation, which makes this test require an x86 backend.
+// REQUIRES: x86-registered-target
+
+// RUN: not %clang_cl -internal-enable-pch -Werror /Yc%S/Inputs/pchfile.h /FI%S/Inputs/pchfile.h /c -DERR_HEADER -- %s 2>&1 \
+// RUN:   | FileCheck %s
+
+// CHECK: nope1
+// CHECK-NOT: nope2
+
+#error nope2
diff --git a/clang/test/Driver/cl-pch-search.cpp b/clang/test/Driver/cl-pch-search.cpp
@@ -0,0 +1,6 @@
+// Note: %s and %S must be preceded by --, otherwise it may be interpreted as a
+// command-line option, e.g. on Mac where %s is commonly under /Users.
+
+// REQUIRES: x86-registered-target
+// Check that pchfile.h next to to pchfile.cc is found correctly.
+// RUN: %clang_cl -internal-enable-pch -Werror /Ycpchfile.h /FIpchfile.h /c /Fo%t.obj /Fp%t.pch -- %S/Inputs/pchfile.cpp 
diff --git a/clang/test/Driver/cl-pch.c b/clang/test/Driver/cl-pch.c
@@ -0,0 +1,45 @@
+// Note: %s and %S must be preceded by --, otherwise it may be interpreted as a
+// command-line option, e.g. on Mac where %s is commonly under /Users.
+
+// The main test for clang-cl pch handling is cl-pch.cpp.  This file only checks
+// a few things for .c inputs.
+
+// /Yc with a .c file should build a c pch file.
+// RUN: %clang_cl -internal-enable-pch -Werror /Ycpchfile.h /FIpchfile.h /c -### -- %s 2>&1 \
+// RUN:   | FileCheck -check-prefix=CHECK-YC %s
+// CHECK-YC: cc1
+// CHECK-YC: -emit-pch
+// CHECK-YC: -o
+// CHECK-YC: pchfile.pch
+// CHECK-YC: -x
+// CHECK-YC: "c"
+
+// But not if /TP changes the input language to C++.
+// RUN: %clang_cl /TP -internal-enable-pch -Werror /Ycpchfile.h /FIpchfile.h /c -### -- %s 2>&1 \
+// RUN:   | FileCheck -check-prefix=CHECK-YCTP %s
+// CHECK-YCTP: cc1
+// CHECK-YCTP: -emit-pch
+// CHECK-YCTP: -o
+// CHECK-YCTP: pchfile.pch
+// CHECK-YCTP: -x
+// CHECK-YCTP: "c++"
+
+// Except if a later /TC changes it back.
+// RUN: %clang_cl -internal-enable-pch -Werror /Ycpchfile.h /FIpchfile.h /c -### -- %s 2>&1 \
+// RUN:   | FileCheck -check-prefix=CHECK-YCTPTC %s
+// CHECK-YCTPTC: cc1
+// CHECK-YCTPTC: -emit-pch
+// CHECK-YCTPTC: -o
+// CHECK-YCTPTC: pchfile.pch
+// CHECK-YCTPTC: -x
+// CHECK-YCTPTC: "c"
+
+// Also check lower-case /Tp flag.
+// RUN: %clang_cl -internal-enable-pch -Werror /Tp%s /Ycpchfile.h /FIpchfile.h /c -### 2>&1 \
+// RUN:   | FileCheck -check-prefix=CHECK-YCTp %s
+// CHECK-YCTp: cc1
+// CHECK-YCTp: -emit-pch
+// CHECK-YCTp: -o
+// CHECK-YCTp: pchfile.pch
+// CHECK-YCTp: -x
+// CHECK-YCTp: "c++"
diff --git a/clang/test/Driver/cl-pch.cpp b/clang/test/Driver/cl-pch.cpp
@@ -0,0 +1,309 @@
+// Note: %s and %S must be preceded by --, otherwise it may be interpreted as a
+// command-line option, e.g. on Mac where %s is commonly under /Users.
+
+// /Yc
+// RUN: %clang_cl -internal-enable-pch -Werror /Ycpchfile.h /FIpchfile.h /c -### -- %s 2>&1 \
+// RUN:   | FileCheck -check-prefix=CHECK-YC %s
+// 1. Build .pch file.
+// CHECK-YC: cc1
+// CHECK-YC: -emit-pch
+// CHECK-YC: -o
+// CHECK-YC: pchfile.pch
+// CHECK-YC: -x
+// CHECK-YC: "c++"
+// 2. Use .pch file.
+// CHECK-YC: cc1
+// CHECK-YC: -emit-obj
+// CHECK-YC: -include-pch
+// CHECK-YC: pchfile.pch
+
+// /Yc /Fo
+// /Fo overrides the .obj output filename, but not the .pch filename
+// RUN: %clang_cl -internal-enable-pch -Werror /Fomyobj.obj /Ycpchfile.h /FIpchfile.h /c -### -- %s 2>&1 \
+// RUN:   | FileCheck -check-prefix=CHECK-YCO %s
+// 1. Build .pch file.
+// CHECK-YCO: cc1
+// CHECK-YCO: -emit-pch
+// CHECK-YCO: -o
+// CHECK-YCO: pchfile.pch
+// 2. Use .pch file.
+// CHECK-YCO: cc1
+// CHECK-YCO: -emit-obj
+// CHECK-YCO: -include-pch
+// CHECK-YCO: pchfile.pch
+// CHECK-YCO: -o
+// CHECK-YCO: myobj.obj
+
+// /Yc /Y-
+// /Y- disables pch generation
+// RUN: %clang_cl -internal-enable-pch -Werror /Y- /Ycpchfile.h /FIpchfile.h /c -### -- %s 2>&1 \
+// RUN:   | FileCheck -check-prefix=CHECK-YC-Y_ %s
+// CHECK-YC-Y_-NOT: -emit-pch
+// CHECK-YC-Y_-NOT: -include-pch
+
+// /Yu
+// RUN: %clang_cl -internal-enable-pch -Werror /Yupchfile.h /FIpchfile.h /c -### -- %s 2>&1 \
+// RUN:   | FileCheck -check-prefix=CHECK-YU %s
+// Use .pch file, but don't build it.
+// CHECK-YU-NOT: -emit-pch
+// CHECK-YU: cc1
+// CHECK-YU: -emit-obj
+// CHECK-YU: -include-pch
+// CHECK-YU: pchfile.pch
+
+// /Yu /Y-
+// RUN: %clang_cl -internal-enable-pch -Werror /Y- /Yupchfile.h /FIpchfile.h /c -### -- %s 2>&1 \
+// RUN:   | FileCheck -check-prefix=CHECK-YU-Y_ %s
+// CHECK-YU-Y_-NOT: -emit-pch
+// CHECK-YU-Y_-NOT: -include-pch
+
+// /Yc /Yu -- /Yc overrides /Yc if they both refer to the same file
+// RUN: %clang_cl -internal-enable-pch -Werror /Ycpchfile.h /Yupchfile.h /FIpchfile.h /c -### -- %s 2>&1 \
+// RUN:   | FileCheck -check-prefix=CHECK-YC-YU %s
+// 1. Build .pch file.
+// CHECK-YC-YU: cc1
+// CHECK-YC-YU: -emit-pch
+// CHECK-YC-YU: -o
+// CHECK-YC-YU: pchfile.pch
+// 2. Use .pch file.
+// CHECK-YC-YU: cc1
+// CHECK-YC-YU: -emit-obj
+// CHECK-YC-YU: -include-pch
+// CHECK-YC-YU: pchfile.pch
+
+// If /Yc /Yu refer to different files, semantics are pretty wonky.  Since this
+// doesn't seem like something that's important in practice, just punt for now.
+// RUN: %clang_cl -internal-enable-pch -Werror /Ycfoo1.h /Yufoo2.h /FIfoo1.h /FIfoo2.h /c -### -- %s 2>&1 \
+// RUN:   | FileCheck -check-prefix=CHECK-YC-YU-MISMATCH %s
+// CHECK-YC-YU-MISMATCH: error: support for '/Yc' and '/Yu' with different filenames not implemented yet; flags ignored
+
+// Similarly, punt on /Yc with more than one input file.
+// RUN: %clang_cl -internal-enable-pch -Werror /Ycfoo1.h /FIfoo1.h /c -### -- %s %s 2>&1 \
+// RUN:   | FileCheck -check-prefix=CHECK-YC-MULTIINPUT %s
+// CHECK-YC-MULTIINPUT: error: support for '/Yc' with more than one source file not implemented yet; flag ignored
+
+// /Yc /Yu /Y-
+// RUN: %clang_cl -internal-enable-pch -Werror /Ycpchfile.h /Yupchfile.h /FIpchfile.h /Y- /c -### -- %s 2>&1 \
+// RUN:   | FileCheck -check-prefix=CHECK-YC-YU-Y_ %s
+// CHECK-YC-YU-Y_-NOT: -emit-pch
+// CHECK-YC-YU-Y_-NOT: -include-pch
+
+// Test computation of pch filename in various cases.
+
+// /Yu /Fpout.pch => out.pch is filename
+// RUN: %clang_cl -internal-enable-pch -Werror /Yupchfile.h /FIpchfile.h /Fpout.pch /c -### -- %s 2>&1 \
+// RUN:   | FileCheck -check-prefix=CHECK-YUFP1 %s
+// Use .pch file, but don't build it.
+// CHECK-YUFP1: -include-pch
+// CHECK-YUFP1: out.pch
+
+// /Yu /Fpout => out.pch is filename (.pch gets added if no extension present)
+// RUN: %clang_cl -internal-enable-pch -Werror /Yupchfile.h /FIpchfile.h /Fpout.pch /c -### -- %s 2>&1 \
+// RUN:   | FileCheck -check-prefix=CHECK-YUFP2 %s
+// Use .pch file, but don't build it.
+// CHECK-YUFP2: -include-pch
+// CHECK-YUFP2: out.pch
+
+// /Yu /Fpout.bmp => out.bmp is filename (.pch not added when extension present)
+// RUN: %clang_cl -internal-enable-pch -Werror /Yupchfile.h /FIpchfile.h /Fpout.bmp /c -### -- %s 2>&1 \
+// RUN:   | FileCheck -check-prefix=CHECK-YUFP3 %s
+// Use .pch file, but don't build it.
+// CHECK-YUFP3: -include-pch
+// CHECK-YUFP3: out.bmp
+
+// /Yusub/dir.h => sub/dir.pch
+// RUN: %clang_cl -internal-enable-pch -Werror /Yusub/pchfile.h /FIsub/pchfile.h /c -### -- %s 2>&1 \
+// RUN:   | FileCheck -check-prefix=CHECK-YUFP4 %s
+// Use .pch file, but don't build it.
+// CHECK-YUFP4: -include-pch
+// CHECK-YUFP4: sub/pchfile.pch
+
+// /Yudir.h /Isub => dir.pch
+// RUN: %clang_cl -internal-enable-pch -Werror /Yupchfile.h /FIpchfile.h /Isub /c -### -- %s 2>&1 \
+// RUN:   | FileCheck -check-prefix=CHECK-YUFP5 %s
+// Use .pch file, but don't build it.
+// CHECK-YUFP5: -include-pch
+// CHECK-YUFP5: pchfile.pch
+
+// FIXME: /Fpdir: use dir/VCx0.pch when dir is directory, where x is major MSVS
+// version in use.
+
+// Spot-check one use of /Fp with /Yc too, else trust the /Yu test cases above
+// also all assume to /Yc.
+// RUN: %clang_cl -internal-enable-pch -Werror /Ycpchfile.h /FIpchfile.h /Fpsub/file.pch /c -### -- %s 2>&1 \
+// RUN:   | FileCheck -check-prefix=CHECK-YCFP %s
+// 1. Build .pch file.
+// CHECK-YCFP: cc1
+// CHECK-YCFP: -emit-pch
+// CHECK-YCFP: -o
+// CHECK-YCFP: sub/file.pch
+// 2. Use .pch file.
+// CHECK-YCFP: cc1
+// CHECK-YCFP: -emit-obj
+// CHECK-YCFP: -include-pch
+// CHECK-YCFP: sub/file.pch
+
+// /Ycfoo2.h /FIfoo1.h /FIfoo2.h /FIfoo3.h
+// => foo1 and foo2 go into pch, foo3 into main compilation
+// /Yc
+// RUN: %clang_cl -internal-enable-pch -Werror /Ycfoo2.h /FIfoo1.h /FIfoo2.h /FIfoo3.h /c -### -- %s 2>&1 \
+// RUN:   | FileCheck -check-prefix=CHECK-YCFIFIFI %s
+// 1. Build .pch file: Includes foo1.h (but NOT foo3.h) and compiles foo2.h
+// CHECK-YCFIFIFI: cc1
+// CHECK-YCFIFIFI: -emit-pch
+// CHECK-YCFIFIFI: -include
+// CHECK-YCFIFIFI: foo1.h
+// CHECK-YCFIFIFI-NOT: foo2.h
+// CHECK-YCFIFIFI-NOT: foo3.h
+// CHECK-YCFIFIFI: -o
+// CHECK-YCFIFIFI: foo2.pch
+// CHECK-YCFIFIFI: -x
+// CHECK-YCFIFIFI: "c++"
+// CHECK-YCFIFIFI: foo2.h
+// 2. Use .pch file: Inlucdes foo2.pch and foo3.h
+// CHECK-YCFIFIFI: cc1
+// CHECK-YCFIFIFI: -emit-obj
+// CHECK-YCFIFIFI-NOT: foo1.h
+// CHECK-YCFIFIFI-NOT: foo2.h
+// CHECK-YCFIFIFI: -include-pch
+// CHECK-YCFIFIFI: foo2.pch
+// CHECK-YCFIFIFI: -include
+// CHECK-YCFIFIFI: foo3.h
+
+// /Yucfoo2.h /FIfoo1.h /FIfoo2.h /FIfoo3.h
+// => foo1 foo2 filtered out, foo3 into main compilation
+// RUN: %clang_cl -internal-enable-pch -Werror /Yufoo2.h /FIfoo1.h /FIfoo2.h /FIfoo3.h /c -### -- %s 2>&1 \
+// RUN:   | FileCheck -check-prefix=CHECK-YUFIFIFI %s
+// Use .pch file, but don't build it.
+// CHECK-YUFIFIFI-NOT: -emit-pch
+// CHECK-YUFIFIFI: cc1
+// CHECK-YUFIFIFI: -emit-obj
+// CHECK-YUFIFIFI-NOT: foo1.h
+// CHECK-YUFIFIFI-NOT: foo2.h
+// CHECK-YUFIFIFI: -include-pch
+// CHECK-YUFIFIFI: foo2.pch
+// CHECK-YUFIFIFI: -include
+// CHECK-YUFIFIFI: foo3.h
+
+// FIXME: Implement support for /Ycfoo.h / /Yufoo.h without /FIfoo.h
+// RUN: %clang_cl -internal-enable-pch -Werror /Ycfoo.h /c -### -- %s 2>&1 \
+// RUN:   | FileCheck -check-prefix=CHECK-YC-NOFI %s
+// CHECK-YC-NOFI: error: support for '/Yc' without a corresponding /FI flag not implemented yet; flag ignored
+// RUN: %clang_cl -internal-enable-pch -Werror /Yufoo.h /c -### -- %s 2>&1 \
+// RUN:   | FileCheck -check-prefix=CHECK-YU-NOFI %s
+// CHECK-YU-NOFI: error: support for '/Yu' without a corresponding /FI flag not implemented yet; flag ignored
+
+// /Yc and /FI relative to /I paths...
+// The rules are:
+// Yu/Yc and FI parameter must match exactly, else it's not found
+// Must match literally exactly: /FI./foo.h /Ycfoo.h does _not_ work.
+// However, the path can be relative to /I paths.
+// FIXME: Update the error messages below once /FI is no longer required, but
+// these test cases all should stay failures as they fail with cl.exe.
+
+// Check that ./ isn't canonicalized away.
+// RUN: %clang_cl -internal-enable-pch -Werror /Ycpchfile.h /FI./pchfile.h /c -### -- %s 2>&1 \
+// RUN:   | FileCheck -check-prefix=CHECK-YC-I1 %s
+// CHECK-YC-I1: support for '/Yc' without a corresponding /FI flag not implemented yet; flag ignored
+
+// Check that ./ isn't canonicalized away.
+// RUN: %clang_cl -internal-enable-pch -Werror /Yc./pchfile.h /FIpchfile.h /c -### -- %s 2>&1 \
+// RUN:   | FileCheck -check-prefix=CHECK-YC-I2 %s
+// CHECK-YC-I2: support for '/Yc' without a corresponding /FI flag not implemented yet; flag ignored
+
+// With an actual /I argument.
+// RUN: %clang_cl -internal-enable-pch -Werror /Ifoo /Ycpchfile.h /FIpchfile.h /c -### -- %s 2>&1 \
+// RUN:   | FileCheck -check-prefix=CHECK-YC-I3 %s
+// 1. This writes pchfile.pch into the root dir, even if this will pick up
+//    foo/pchfile.h
+// CHECK-YC-I3: cc1
+// CHECK-YC-I3: -emit-pch
+// CHECK-YC-I3: -o
+// CHECK-YC-I3: pchfile.pch
+// 2. Use .pch file.
+// CHECK-YC-I3: cc1
+// CHECK-YC-I3: -emit-obj
+// CHECK-YC-I3: -include-pch
+// CHECK-YC-I3: pchfile.pch
+
+// Check that ./ isn't canonicalized away for /Yu either.
+// RUN: %clang_cl -internal-enable-pch -Werror /Yupchfile.h /FI./pchfile.h /c -### -- %s 2>&1 \
+// RUN:   | FileCheck -check-prefix=CHECK-YU-I1 %s
+// CHECK-YU-I1: support for '/Yu' without a corresponding /FI flag not implemented yet; flag ignored
+
+// But /FIfoo/bar.h /Ycfoo\bar.h does work, as does /FIfOo.h /Ycfoo.H
+// FIXME: This part isn't implemented yet. The following two tests should not
+// show an error but do regular /Yu handling.
+// RUN: %clang_cl -internal-enable-pch -Werror /YupchFILE.h /FI./pchfile.h /c -### -- %s 2>&1 \
+// RUN:   | FileCheck -check-prefix=CHECK-YU-CASE %s
+// CHECK-YU-CASE: support for '/Yu' without a corresponding /FI flag not implemented yet; flag ignored
+// RUN: %clang_cl -internal-enable-pch -Werror /Yu./pchfile.h /FI.\pchfile.h /c -### -- %s 2>&1 \
+// RUN:   | FileCheck -check-prefix=CHECK-YU-SLASH %s
+// CHECK-YU-SLASH: support for '/Yu' without a corresponding /FI flag not implemented yet; flag ignored
+
+// cl.exe warns on multiple /Yc, /Yu, /Fp arguments, but clang-cl silently just
+// uses the last one.  This is true for e.g. /Fo too, so not warning on this
+// is self-consistent with clang-cl's flag handling.
+
+// Interaction with /fallback
+
+// /Yc /fallback => /Yc not passed on (but /FI is)
+// RUN: %clang_cl -internal-enable-pch -Werror /Ycpchfile.h /FIpchfile.h /Fpfoo.pch /fallback /c -### -- %s 2>&1 \
+// RUN:   | FileCheck -check-prefix=CHECK-YC-FALLBACK %s
+// Note that in /fallback builds, if creation of the pch fails the main compile
+// does still run so that /fallback can have an effect (this part is not tested)
+// CHECK-YC-FALLBACK: cc1
+// CHECK-YC-FALLBACK: -emit-obj
+// CHECK-YC-FALLBACK: -include-pch
+// CHECK-YC-FALLBACK: foo.pch
+// CHECK-YC-FALLBACK: ||
+// CHECK-YC-FALLBACK: cl.exe
+// CHECK-YC-FALLBACK-NOT: -include-pch
+// CHECK-YC-FALLBACK-NOT: /Ycpchfile.h
+// CHECK-YC-FALLBACK: /FIpchfile.h
+// CHECK-YC-FALLBACK-NOT: /Fpfoo.pch
+
+// /Yu /fallback => /Yu not passed on (but /FI is)
+// RUN: %clang_cl -internal-enable-pch -Werror /Yupchfile.h /FIpchfile.h /Fpfoo.pch /fallback /c -### -- %s 2>&1 \
+// RUN:   | FileCheck -check-prefix=CHECK-YU-FALLBACK %s
+// CHECK-YU-FALLBACK-NOT: -emit-pch
+// CHECK-YU-FALLBACK: cc1
+// CHECK-YU-FALLBACK: -emit-obj
+// CHECK-YU-FALLBACK: -include-pch
+// CHECK-YU-FALLBACK: foo.pch
+// CHECK-YU-FALLBACK: ||
+// CHECK-YU-FALLBACK: cl.exe
+// CHECK-YU-FALLBACK-NOT: -include-pch
+// CHECK-YU-FALLBACK-NOT: /Yupchfile.h
+// CHECK-YU-FALLBACK: /FIpchfile.h
+// CHECK-YU-FALLBACK-NOT: /Fpfoo.pch
+
+// /FI without /Yu => pch file not used, even if it exists (different from
+// -include, which picks up .gch files if they exist).
+// RUN: touch %t.pch
+// RUN: %clang_cl -internal-enable-pch -Werror /FI%t.pch /Fp%t.pch /c -### -- %s 2>&1 \
+// RUN:   | FileCheck -check-prefix=CHECK-FI %s
+// CHECK-FI-NOT: -include-pch
+// CHECK-FI: -include
+
+// Test interaction of /Yc with language mode flags.
+
+// If /TC changes the input language to C, a c pch file should be produced.
+// RUN: %clang_cl /TC -internal-enable-pch -Werror /Ycpchfile.h /FIpchfile.h /c -### -- %s 2>&1 \
+// RUN:   | FileCheck -check-prefix=CHECK-YCTC %s
+// CHECK-YCTC: cc1
+// CHECK-YCTC: -emit-pch
+// CHECK-YCTC: -o
+// CHECK-YCTC: pchfile.pch
+// CHECK-YCTC: -x
+// CHECK-YCTP: "c"
+
+// Also check lower-case /Tc variant.
+// RUN: %clang_cl -internal-enable-pch -Werror /Ycpchfile.h /FIpchfile.h /c -### /Tc%s 2>&1 \
+// RUN:   | FileCheck -check-prefix=CHECK-YCTc %s
+// CHECK-YCTc: cc1
+// CHECK-YCTc: -emit-pch
+// CHECK-YCTc: -o
+// CHECK-YCTc: pchfile.pch
+// CHECK-YCTc: -x
+// CHECK-YCTc: "c"