diff --git a/clang/include/clang/Basic/Diagnostic.h b/clang/include/clang/Basic/Diagnostic.h
--- a/clang/include/clang/Basic/Diagnostic.h
+++ b/clang/include/clang/Basic/Diagnostic.h
@@ -16,6 +16,7 @@
 
 #include "clang/Basic/DiagnosticIDs.h"
 #include "clang/Basic/DiagnosticOptions.h"
+#include "clang/Basic/Sarif.h"
 #include "clang/Basic/SourceLocation.h"
 #include "clang/Basic/Specifiers.h"
 #include "llvm/ADT/ArrayRef.h"
@@ -183,6 +184,45 @@
   DiagnosticStorage() = default;
 };
 
+/// Abstract interface for all errors, warnings, and remarks. This allows us to
+/// use the visitor pattern. This is something that will be concretely defined
+/// in the short-term, as we experiment, but will be migrated over to TableGen
+/// after a few diagnostics have been implemented.
+class BasicDiagnostic {
+public:
+  virtual ~BasicDiagnostic() = default;
+
+  /// Emits the diagnostic as Clang's original unstructured text format.
+  virtual void EmitTraditionalDiagnostic() const = 0;
+
+  /// Emits the diagnostic as SARIF.
+  virtual void EmitSarifDiagnostic(SarifDocumentWriter &) const = 0;
+
+protected:
+  using DiagID = unsigned;
+
+  explicit BasicDiagnostic(DiagID K) : ID(K) {}
+
+  DiagID getID() const { return ID; }
+
+private:
+  DiagID ID;
+};
+
+/// Abstract interface for diagnostic contexts (traditionally: notes).
+/// A BasicDiagnostic may have as many different contexts as required to provide
+/// users with a complete picture.
+class DiagnosticContext {
+public:
+  virtual ~DiagnosticContext() = default;
+
+  /// Emits the diagnostic as an unstructured text note.
+  virtual void EmitTraditionalNote() const = 0;
+
+  /// Emits the diagnostic as SARIF.
+  virtual void EmitSarifContext(SarifDocumentWriter &) const = 0;
+};
+
 /// Concrete class used by the front-end to report problems and issues.
 ///
 /// This massages the diagnostics (e.g. handling things like "report warnings
@@ -295,6 +335,7 @@
   DiagnosticConsumer *Client = nullptr;
   std::unique_ptr<DiagnosticConsumer> Owner;
   SourceManager *SourceMgr = nullptr;
+  SarifDocumentWriter *SarifWriter = nullptr;
 
   /// Mapping information for diagnostics.
   ///
@@ -979,6 +1020,9 @@
   /// Return the value associated with this diagnostic flag.
   StringRef getFlagValue() const { return FlagValue; }
 
+  void setSarifWriter(SarifDocumentWriter *W) { SarifWriter = W; }
+  SarifDocumentWriter *getSarifWriter() const { return SarifWriter; }
+
 private:
   // This is private state used by DiagnosticBuilder.  We put it here instead of
   // in DiagnosticBuilder in order to keep DiagnosticBuilder a small lightweight
@@ -1343,6 +1387,15 @@
     return *this;
   }
 
+  const DiagnosticBuilder &operator<<(const BasicDiagnostic &Diag) const {
+    assert(isActive() && "Clients must not add to cleared diagnostic!");
+    if (SarifDocumentWriter *W = DiagObj->getSarifWriter())
+      Diag.EmitSarifDiagnostic(*W);
+    else
+      Diag.EmitTraditionalDiagnostic();
+    return *this;
+  }
+
   // It is necessary to limit this to rvalue reference to avoid calling this
   // function with a bitfield lvalue argument since non-const reference to
   // bitfield is not allowed.
diff --git a/clang/include/clang/Sema/Sema.h b/clang/include/clang/Sema/Sema.h
--- a/clang/include/clang/Sema/Sema.h
+++ b/clang/include/clang/Sema/Sema.h
@@ -38,6 +38,7 @@
 #include "clang/Basic/BitmaskEnum.h"
 #include "clang/Basic/Builtins.h"
 #include "clang/Basic/DarwinSDKInfo.h"
+#include "clang/Basic/Diagnostic.h"
 #include "clang/Basic/ExpressionTraits.h"
 #include "clang/Basic/Module.h"
 #include "clang/Basic/OpenCLOptions.h"
@@ -1772,6 +1773,14 @@
       return Diag;
     }
 
+    const SemaDiagnosticBuilder &operator<<(const BasicDiagnostic &Diag) const {
+      if (SarifDocumentWriter *W = S.Diags.getSarifWriter())
+        Diag.EmitSarifDiagnostic(*W);
+      else
+        Diag.EmitTraditionalDiagnostic();
+      return *this;
+    }
+
     // It is necessary to limit this to rvalue reference to avoid calling this
     // function with a bitfield lvalue argument since non-const reference to
     // bitfield is not allowed.
@@ -13583,7 +13592,6 @@
                                                  PragmaMsStackAction Action,
                                                  llvm::StringRef StackSlotLabel,
                                                  AlignPackInfo Value);
-
 } // end namespace clang
 
 namespace llvm {
diff --git a/clang/unittests/Basic/SarifTest.cpp b/clang/unittests/Basic/SarifTest.cpp
--- a/clang/unittests/Basic/SarifTest.cpp
+++ b/clang/unittests/Basic/SarifTest.cpp
@@ -317,4 +317,20 @@
   ASSERT_THAT(Output, ::testing::StrEq(ExpectedOutput));
 }
 
+// Check that we can set/get a SarifDocumentWriter from the DiagnosticsEngine
+// Note that there doesn't appear to be any tests for the DiagnosticsEngine, so
+// we test here.
+TEST_F(SarifDocumentWriterTest, checkDiagnosticsEngineSetAndGet) {
+  ASSERT_FALSE(Diags.getSarifWriter());
+
+  // GIVEN:
+  SarifDocumentWriter Writer(SourceMgr);
+
+  // WHEN: the DiagnosticsEngine is set
+  Diags.setSarifWriter(&Writer);
+
+  // THEN: the DiagnosticsEngine points to the local SarifDocumentWriter
+  EXPECT_TRUE(Diags.getSarifWriter() == &Writer);
+}
+
 } // namespace