diff --git a/clang/lib/AST/Interp/State.cpp b/clang/lib/AST/Interp/State.cpp
index 2512979257605..f67bde1082fa0 100644
--- a/clang/lib/AST/Interp/State.cpp
+++ b/clang/lib/AST/Interp/State.cpp
@@ -11,6 +11,7 @@
 #include "Program.h"
 #include "clang/AST/ASTContext.h"
 #include "clang/AST/CXXInheritance.h"
+#include "clang/AST/OptionalDiagnostic.h"
 
 using namespace clang;
 using namespace clang::interp;
diff --git a/clang/lib/AST/Interp/State.h b/clang/lib/AST/Interp/State.h
index 131fbcf3cffc4..d897b7c202756 100644
--- a/clang/lib/AST/Interp/State.h
+++ b/clang/lib/AST/Interp/State.h
@@ -15,9 +15,9 @@
 
 #include "clang/AST/ASTDiagnostic.h"
 #include "clang/AST/Expr.h"
-#include "clang/AST/OptionalDiagnostic.h"
 
 namespace clang {
+class OptionalDiagnostic;
 
 /// Kinds of access we can perform on an object, for diagnostics. Note that
 /// we consider a member function call to be a kind of access, even though
@@ -36,7 +36,7 @@ enum AccessKinds {
   AK_Destroy,
 };
 
-// The order of this enum is important for diagnostics.
+/// The order of this enum is important for diagnostics.
 enum CheckSubobjectKind {
   CSK_Base,
   CSK_Derived,
@@ -72,7 +72,7 @@ class State {
 
 public:
   State() : InConstantContext(false) {}
-  // Diagnose that the evaluation could not be folded (FF => FoldFailure)
+  /// Diagnose that the evaluation could not be folded (FF => FoldFailure)
   OptionalDiagnostic
   FFDiag(SourceLocation Loc,
          diag::kind DiagId = diag::note_invalid_subexpr_in_const_expr,