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diff --git a/lib/AST/Interp/State.h b/lib/AST/Interp/State.h
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+//===--- State.h - State chain for the VM and AST Walker --------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+// Defines the base class of the interpreter and evaluator state.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_CLANG_AST_INTERP_STATE_H
+#define LLVM_CLANG_AST_INTERP_STATE_H
+
+#include "clang/AST/ASTDiagnostic.h"
+#include "clang/AST/Expr.h"
+#include "clang/AST/OptionalDiagnostic.h"
+
+namespace clang {
+
+/// 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
+/// it is not formally an access of the object, because it has (largely) the
+/// same set of semantic restrictions.
+enum AccessKinds {
+  AK_Read,
+  AK_ReadObjectRepresentation,
+  AK_Assign,
+  AK_Increment,
+  AK_Decrement,
+  AK_MemberCall,
+  AK_DynamicCast,
+  AK_TypeId,
+  AK_Construct,
+  AK_Destroy,
+};
+
+// The order of this enum is important for diagnostics.
+enum CheckSubobjectKind {
+  CSK_Base,
+  CSK_Derived,
+  CSK_Field,
+  CSK_ArrayToPointer,
+  CSK_ArrayIndex,
+  CSK_Real,
+  CSK_Imag
+};
+
+namespace interp {
+class Frame;
+class SourceInfo;
+
+/// Interface for the VM to interact with the AST walker's context.
+class State {
+public:
+  virtual ~State();
+
+  virtual bool checkingForUndefinedBehavior() const = 0;
+  virtual bool checkingPotentialConstantExpression() const = 0;
+  virtual bool noteUndefinedBehavior() = 0;
+  virtual bool keepEvaluatingAfterFailure() const = 0;
+  virtual Frame *getCurrentFrame() = 0;
+  virtual const Frame *getBottomFrame() const = 0;
+  virtual bool hasActiveDiagnostic() = 0;
+  virtual void setActiveDiagnostic(bool Flag) = 0;
+  virtual void setFoldFailureDiagnostic(bool Flag) = 0;
+  virtual Expr::EvalStatus &getEvalStatus() const = 0;
+  virtual ASTContext &getCtx() const = 0;
+  virtual bool hasPriorDiagnostic() = 0;
+  virtual unsigned getCallStackDepth() = 0;
+
+public:
+  // Diagnose that the evaluation could not be folded (FF => FoldFailure)
+  OptionalDiagnostic
+  FFDiag(SourceLocation Loc,
+         diag::kind DiagId = diag::note_invalid_subexpr_in_const_expr,
+         unsigned ExtraNotes = 0);
+
+  OptionalDiagnostic
+  FFDiag(const Expr *E,
+         diag::kind DiagId = diag::note_invalid_subexpr_in_const_expr,
+         unsigned ExtraNotes = 0);
+
+  OptionalDiagnostic
+  FFDiag(const SourceInfo &SI,
+         diag::kind DiagId = diag::note_invalid_subexpr_in_const_expr,
+         unsigned ExtraNotes = 0);
+
+  /// Diagnose that the evaluation does not produce a C++11 core constant
+  /// expression.
+  ///
+  /// FIXME: Stop evaluating if we're in EM_ConstantExpression or
+  /// EM_PotentialConstantExpression mode and we produce one of these.
+  OptionalDiagnostic
+  CCEDiag(SourceLocation Loc,
+          diag::kind DiagId = diag::note_invalid_subexpr_in_const_expr,
+          unsigned ExtraNotes = 0);
+
+  OptionalDiagnostic
+  CCEDiag(const Expr *E,
+          diag::kind DiagId = diag::note_invalid_subexpr_in_const_expr,
+          unsigned ExtraNotes = 0);
+
+  OptionalDiagnostic
+  CCEDiag(const SourceInfo &SI,
+          diag::kind DiagId = diag::note_invalid_subexpr_in_const_expr,
+          unsigned ExtraNotes = 0);
+
+  /// Add a note to a prior diagnostic.
+  OptionalDiagnostic Note(SourceLocation Loc, diag::kind DiagId);
+
+  /// Add a stack of notes to a prior diagnostic.
+  void addNotes(ArrayRef<PartialDiagnosticAt> Diags);
+
+  /// Directly reports a diagnostic message.
+  DiagnosticBuilder report(SourceLocation Loc, diag::kind DiagId);
+
+  const LangOptions &getLangOpts() const;
+
+private:
+  void addCallStack(unsigned Limit);
+
+  PartialDiagnostic &addDiag(SourceLocation Loc, diag::kind DiagId);
+
+  OptionalDiagnostic diag(SourceLocation Loc, diag::kind DiagId,
+                          unsigned ExtraNotes, bool IsCCEDiag);
+};
+
+} // namespace interp
+} // namespace clang
+
+#endif