Index: include/clang/Basic/LangOptions.h
===================================================================
--- include/clang/Basic/LangOptions.h
+++ include/clang/Basic/LangOptions.h
@@ -126,6 +126,10 @@
   /// host code generation.
   std::string OMPHostIRFile;
 
+  /// \brief Indicates whether the front-end is explicitly told that the
+  /// input is a header file (i.e. -x c-header).
+  bool IsHeaderFile = false;
+
   LangOptions();
 
   // Define accessors/mutators for language options of enumeration type.
Index: lib/Frontend/CompilerInvocation.cpp
===================================================================
--- lib/Frontend/CompilerInvocation.cpp
+++ lib/Frontend/CompilerInvocation.cpp
@@ -1056,7 +1056,8 @@
 }
 
 static InputKind ParseFrontendArgs(FrontendOptions &Opts, ArgList &Args,
-                                   DiagnosticsEngine &Diags) {
+                                   DiagnosticsEngine &Diags,
+                                   bool &IsHeaderFile) {
   using namespace options;
   Opts.ProgramAction = frontend::ParseSyntaxOnly;
   if (const Arg *A = Args.getLastArg(OPT_Action_Group)) {
@@ -1296,6 +1297,14 @@
     if (DashX == IK_None)
       Diags.Report(diag::err_drv_invalid_value)
         << A->getAsString(Args) << A->getValue();
+
+    IsHeaderFile = llvm::StringSwitch<bool>(A->getValue())
+        .Case("c-header", true)
+        .Case("cl-header", true)
+        .Case("objective-c-header", true)
+        .Case("c++-header", true)
+        .Case("objective-c++-header", true)
+        .Default(false);
   }
 
   // '-' is the default input if none is given.
@@ -2291,7 +2300,8 @@
   ParseCommentArgs(LangOpts.CommentOpts, Args);
   ParseFileSystemArgs(Res.getFileSystemOpts(), Args);
   // FIXME: We shouldn't have to pass the DashX option around here
-  InputKind DashX = ParseFrontendArgs(Res.getFrontendOpts(), Args, Diags);
+  InputKind DashX = ParseFrontendArgs(Res.getFrontendOpts(), Args, Diags,
+                                      LangOpts.IsHeaderFile);
   ParseTargetArgs(Res.getTargetOpts(), Args, Diags);
   Success &= ParseCodeGenArgs(Res.getCodeGenOpts(), Args, DashX, Diags,
                               Res.getTargetOpts());
Index: lib/Lex/PPDirectives.cpp
===================================================================
--- lib/Lex/PPDirectives.cpp
+++ lib/Lex/PPDirectives.cpp
@@ -1895,7 +1895,10 @@
   // diagnostic.
   const DirectoryLookup *Lookup = CurDirLookup;
   const FileEntry *LookupFromFile = nullptr;
-  if (isInPrimaryFile()) {
+  if (isInPrimaryFile() && LangOpts.IsHeaderFile) {
+    // If the main file is a header, then it's either for PCH/AST generation,
+    // or libclang opened it. Either way, handle it as a normal include below.
+  } else if (isInPrimaryFile()) {
     Lookup = nullptr;
     Diag(IncludeNextTok, diag::pp_include_next_in_primary);
   } else if (CurSubmodule) {
Index: lib/Lex/PPMacroExpansion.cpp
===================================================================
--- lib/Lex/PPMacroExpansion.cpp
+++ lib/Lex/PPMacroExpansion.cpp
@@ -1400,7 +1400,10 @@
   // Preprocessor::HandleIncludeNextDirective.
   const DirectoryLookup *Lookup = PP.GetCurDirLookup();
   const FileEntry *LookupFromFile = nullptr;
-  if (PP.isInPrimaryFile()) {
+  if (PP.isInPrimaryFile() && PP.getLangOpts().IsHeaderFile) {
+    // If the main file is a header, then it's either for PCH/AST generation,
+    // or libclang opened it. Either way, handle it as a normal include below.
+  } else if (PP.isInPrimaryFile()) {
     Lookup = nullptr;
     PP.Diag(Tok, diag::pp_include_next_in_primary);
   } else if (PP.getCurrentSubmodule()) {
Index: lib/Lex/Pragma.cpp
===================================================================
--- lib/Lex/Pragma.cpp
+++ lib/Lex/Pragma.cpp
@@ -354,7 +354,9 @@
 /// HandlePragmaOnce - Handle \#pragma once.  OnceTok is the 'once'.
 ///
 void Preprocessor::HandlePragmaOnce(Token &OnceTok) {
-  if (isInPrimaryFile()) {
+  // If the main file is a header, then it's either for PCH/AST generation,
+  // or libclang opened it. Allow #pragma once either way.
+  if (isInPrimaryFile() && !getLangOpts().IsHeaderFile) {
     Diag(OnceTok, diag::pp_pragma_once_in_main_file);
     return;
   }
Index: test/Preprocessor/header_is_main_file.c
===================================================================
--- /dev/null
+++ test/Preprocessor/header_is_main_file.c
@@ -0,0 +1,8 @@
+// RUN: %clang_cc1 -x c-header -ffreestanding -Eonly -verify %s
+// expected-no-diagnostics
+
+#pragma once
+#include_next "stdint.h"
+#if !__has_include_next("stdint.h")
+#error "__has_include_next failed"
+#endif