Index: include/clang/Frontend/ASTUnit.h
===================================================================
--- include/clang/Frontend/ASTUnit.h
+++ include/clang/Frontend/ASTUnit.h
@@ -438,6 +438,8 @@
   void setASTContext(ASTContext *ctx) { Ctx = ctx; }
   void setPreprocessor(std::shared_ptr<Preprocessor> pp);
 
+  void beginSourceFile();
+
   bool hasSema() const { return (bool)TheSema; }
 
   Sema &getSema() const { 
Index: lib/Frontend/ASTUnit.cpp
===================================================================
--- lib/Frontend/ASTUnit.cpp
+++ lib/Frontend/ASTUnit.cpp
@@ -274,6 +274,12 @@
   this->PP = std::move(PP);
 }
 
+void ASTUnit::beginSourceFile() {
+  assert(getDiagnostics().getClient() && PP && Ctx &&
+      "Bad context for source file");
+  getDiagnostics().getClient()->BeginSourceFile(Ctx->getLangOpts(), PP.get());
+}
+
 /// Determine the set of code-completion contexts in which this
 /// declaration should be shown.
 static unsigned getDeclShowContexts(const NamedDecl *ND,
Index: unittests/AST/ASTImporterTest.cpp
===================================================================
--- unittests/AST/ASTImporterTest.cpp
+++ unittests/AST/ASTImporterTest.cpp
@@ -98,6 +98,9 @@
   ASTContext &FromCtx = FromAST->getASTContext(),
       &ToCtx = ToAST->getASTContext();
 
+  FromAST->beginSourceFile();
+  ToAST->beginSourceFile();
+
   ASTImporter Importer(ToCtx, ToAST->getFileManager(),
                        FromCtx, FromAST->getFileManager(), false);
 
@@ -172,7 +175,9 @@
         : Code(Code), FileName(FileName),
           Unit(tooling::buildASTFromCodeWithArgs(this->Code, Args,
                                                  this->FileName)),
-          TUDecl(Unit->getASTContext().getTranslationUnitDecl()) {}
+          TUDecl(Unit->getASTContext().getTranslationUnitDecl()) {
+      Unit->beginSourceFile();
+    }
   };
 
   // We may have several From contexts and related translation units. In each
@@ -214,6 +219,7 @@
     ToCode = ToSrcCode;
     assert(!ToAST);
     ToAST = tooling::buildASTFromCodeWithArgs(ToCode, ToArgs, OutputFileName);
+    ToAST->beginSourceFile();
 
     ASTContext &FromCtx = FromTU.Unit->getASTContext(),
                &ToCtx = ToAST->getASTContext();
@@ -261,6 +267,7 @@
     ToCode = ToSrcCode;
     assert(!ToAST);
     ToAST = tooling::buildASTFromCodeWithArgs(ToCode, ToArgs, OutputFileName);
+    ToAST->beginSourceFile();
 
     return ToAST->getASTContext().getTranslationUnitDecl();
   }
@@ -274,6 +281,7 @@
       // Build the AST from an empty file.
       ToAST =
           tooling::buildASTFromCodeWithArgs(/*Code=*/"", ToArgs, "empty.cc");
+      ToAST->beginSourceFile();
     }
 
     // Create a virtual file in the To Ctx which corresponds to the file from