diff --git a/clang/include/clang/Driver/Types.def b/clang/include/clang/Driver/Types.def
--- a/clang/include/clang/Driver/Types.def
+++ b/clang/include/clang/Driver/Types.def
@@ -64,7 +64,11 @@
 TYPE("objective-c-header",       ObjCHeader,   PP_ObjCHeader,   "h",      phases::Preprocess, phases::Precompile)
 TYPE("c++-header-cpp-output",    PP_CXXHeader, INVALID,         "ii",     phases::Precompile)
 TYPE("c++-header",               CXXHeader,    PP_CXXHeader,    "hh",     phases::Preprocess, phases::Precompile)
-TYPE("objective-c++-header-cpp-output", PP_ObjCXXHeader, INVALID, "mii",  phases::Precompile)
+TYPE("c++-header-unit-cpp-output", PP_CXXHeaderUnit,INVALID,    "iih",    phases::Precompile)
+TYPE("c++-header-unit-header",   CXXHUHeader,  PP_CXXHeaderUnit,"hh",     phases::Preprocess, phases::Precompile)
+TYPE("c++-system-header",        CXXSHeader,   PP_CXXHeaderUnit,"hh",     phases::Preprocess, phases::Precompile)
+TYPE("c++-user-header",          CXXUHeader,   PP_CXXHeaderUnit,"hh",     phases::Preprocess, phases::Precompile)
+TYPE("objective-c++-header-cpp-output", PP_ObjCXXHeader, INVALID,"mii",   phases::Precompile)
 TYPE("objective-c++-header",     ObjCXXHeader, PP_ObjCXXHeader, "h",      phases::Preprocess, phases::Precompile)
 TYPE("c++-module",               CXXModule,    PP_CXXModule,    "cppm",   phases::Preprocess, phases::Precompile, phases::Compile, phases::Backend, phases::Assemble, phases::Link)
 TYPE("c++-module-cpp-output",    PP_CXXModule, INVALID,         "iim",    phases::Precompile, phases::Compile, phases::Backend, phases::Assemble, phases::Link)
@@ -89,6 +93,7 @@
 TYPE("ifs",                      IFS,          INVALID,         "ifs",    phases::IfsMerge)
 TYPE("ifs-cpp",                  IFS_CPP,      INVALID,         "ifs",    phases::Compile, phases::IfsMerge)
 TYPE("pcm",                      ModuleFile,   INVALID,         "pcm",    phases::Compile, phases::Backend, phases::Assemble, phases::Link)
+TYPE("header-unit",              HeaderUnit,   INVALID,         "pcm",    phases::Compile, phases::Backend, phases::Assemble, phases::Link)
 TYPE("plist",                    Plist,        INVALID,         "plist",  phases::Compile, phases::Backend, phases::Assemble, phases::Link)
 TYPE("rewritten-objc",           RewrittenObjC,INVALID,         "cpp",    phases::Compile, phases::Backend, phases::Assemble, phases::Link)
 TYPE("rewritten-legacy-objc",    RewrittenLegacyObjC,INVALID,   "cpp",    phases::Compile, phases::Backend, phases::Assemble, phases::Link)
diff --git a/clang/lib/Driver/Driver.cpp b/clang/lib/Driver/Driver.cpp
--- a/clang/lib/Driver/Driver.cpp
+++ b/clang/lib/Driver/Driver.cpp
@@ -2219,6 +2219,11 @@
   if (Value == "-")
     return true;
 
+  // If it's a header to be found in the system or user search path, then defer
+  // complaints about its absence until those searches can be done.
+  if (Ty == types::TY_CXXSHeader || Ty == types::TY_CXXUHeader)
+    return true;
+
   if (getVFS().exists(Value))
     return true;
 
diff --git a/clang/lib/Driver/ToolChains/Clang.cpp b/clang/lib/Driver/ToolChains/Clang.cpp
--- a/clang/lib/Driver/ToolChains/Clang.cpp
+++ b/clang/lib/Driver/ToolChains/Clang.cpp
@@ -4609,6 +4609,8 @@
       CmdArgs.push_back(IsHeaderModulePrecompile
                             ? "-emit-header-module"
                             : "-emit-module-interface");
+    else if (JA.getType() == types::TY_HeaderUnit)
+      CmdArgs.push_back("-emit-header-unit");
     else
       CmdArgs.push_back("-emit-pch");
   } else if (isa<VerifyPCHJobAction>(JA)) {
diff --git a/clang/lib/Driver/Types.cpp b/clang/lib/Driver/Types.cpp
--- a/clang/lib/Driver/Types.cpp
+++ b/clang/lib/Driver/Types.cpp
@@ -65,9 +65,16 @@
   return Id == TY_CXXModule || Id == TY_PP_CXXModule;
 }
 
+static bool isPreprocessedHeaderUnitType(ID Id) {
+  return Id == TY_CXXSHeader || Id == TY_CXXUHeader || Id == TY_CXXHUHeader ||
+         Id == TY_PP_CXXHeaderUnit;
+}
+
 types::ID types::getPrecompiledType(ID Id) {
   if (isPreprocessedModuleType(Id))
     return TY_ModuleFile;
+  if (isPreprocessedHeaderUnitType(Id))
+    return TY_HeaderUnit;
   if (onlyPrecompileType(Id))
     return TY_PCH;
   return TY_INVALID;
@@ -139,6 +146,10 @@
   case TY_CLHeader:
   case TY_ObjCHeader: case TY_PP_ObjCHeader:
   case TY_CXXHeader: case TY_PP_CXXHeader:
+  case TY_CXXSHeader:
+  case TY_CXXUHeader:
+  case TY_CXXHUHeader:
+  case TY_PP_CXXHeaderUnit:
   case TY_ObjCXXHeader: case TY_PP_ObjCXXHeader:
   case TY_CXXModule: case TY_PP_CXXModule:
   case TY_AST: case TY_ModuleFile: case TY_PCH:
@@ -210,6 +221,10 @@
   case TY_CXX: case TY_PP_CXX:
   case TY_ObjCXX: case TY_PP_ObjCXX: case TY_PP_ObjCXX_Alias:
   case TY_CXXHeader: case TY_PP_CXXHeader:
+  case TY_CXXSHeader:
+  case TY_CXXUHeader:
+  case TY_CXXHUHeader:
+  case TY_PP_CXXHeaderUnit:
   case TY_ObjCXXHeader: case TY_PP_ObjCXXHeader:
   case TY_CXXModule: case TY_PP_CXXModule:
   case TY_CUDA: case TY_PP_CUDA: case TY_CUDA_DEVICE:
@@ -323,6 +338,7 @@
            .Case("hpp", TY_CXXHeader)
            .Case("hxx", TY_CXXHeader)
            .Case("iim", TY_PP_CXXModule)
+           .Case("iih", TY_PP_CXXHeaderUnit)
            .Case("lib", TY_Object)
            .Case("mii", TY_PP_ObjCXX)
            .Case("obj", TY_Object)
diff --git a/clang/lib/Frontend/FrontendOptions.cpp b/clang/lib/Frontend/FrontendOptions.cpp
--- a/clang/lib/Frontend/FrontendOptions.cpp
+++ b/clang/lib/Frontend/FrontendOptions.cpp
@@ -27,7 +27,7 @@
       .Cases("C", "cc", "cp", Language::CXX)
       .Cases("cpp", "CPP", "c++", "cxx", "hpp", "hxx", Language::CXX)
       .Case("cppm", Language::CXX)
-      .Case("iim", InputKind(Language::CXX).getPreprocessed())
+      .Cases("iim", "iih", InputKind(Language::CXX).getPreprocessed())
       .Case("cl", Language::OpenCL)
       .Case("clcpp", Language::OpenCLCXX)
       .Cases("cu", "cuh", Language::CUDA)
diff --git a/clang/test/Driver/Inputs/header-unit-01.hh b/clang/test/Driver/Inputs/header-unit-01.hh
new file mode 100644
diff --git a/clang/test/Driver/cxx20-header-units-01.cpp b/clang/test/Driver/cxx20-header-units-01.cpp
new file mode 100644
--- /dev/null
+++ b/clang/test/Driver/cxx20-header-units-01.cpp
@@ -0,0 +1,19 @@
+// RUN: %clang -### -std=c++20 -xc++-user-header foo.h 2>&1 | \
+// RUN:   FileCheck -check-prefix=CHECK-USER %s
+
+// RUN: %clang -### -std=c++20 -xc++-system-header vector 2>&1 | \
+// RUN:   FileCheck -check-prefix=CHECK-SYSTEM %s
+
+// RUN: %clang -### -std=c++20 \
+// RUN:   -xc++-header-unit-header %/S/Inputs/header-unit-01.hh 2>&1 | \
+// RUN:   FileCheck -check-prefix=CHECK-ABS %s -DTDIR=%/S/Inputs
+
+// CHECK-USER: "-emit-header-unit"
+// CHECK-USER-SAME: "-o" "foo.pcm"
+// CHECK-USER-SAME: "-x" "c++-user-header" "foo.h"
+// CHECK-SYSTEM: "-emit-header-unit"
+// CHECK-SYSTEM-SAME: "-o" "vector.pcm"
+// CHECK-SYSTEM-SAME: "-x" "c++-system-header" "vector"
+// CHECK-ABS: "-emit-header-unit"
+// CHECK-ABS-SAME: "-o" "header-unit-01.pcm"
+// CHECK-ABS-SAME: "-x" "c++-header-unit-header" "[[TDIR]]/header-unit-01.hh"