From 98a0a49fbfdfc1933e1f40cce630d7f4bccf5afa Mon Sep 17 00:00:00 2001
From: Richard Smith <richard-llvm@metafoo.co.uk>
Date: Tue, 14 Feb 2012 21:38:30 +0000
Subject: [PATCH] Pending clear answer from WG21 on whether core issue 903 is
 intended to apply to C++11 or just C++17, restrict the set of null pointer
 constants in C++11 mode back to those which were considered null in C++98.

llvm-svn: 150510
---
 clang/include/clang/AST/Expr.h |  4 ++++
 clang/lib/AST/Expr.cpp         | 15 +++++++++++----
 clang/lib/AST/ExprConstant.cpp |  5 +++++
 clang/test/SemaCXX/nullptr.cpp | 11 +++++++++++
 4 files changed, 31 insertions(+), 4 deletions(-)

diff --git a/clang/include/clang/AST/Expr.h b/clang/include/clang/AST/Expr.h
index adf12c1b2d08..8a80803f01c2 100644
--- a/clang/include/clang/AST/Expr.h
+++ b/clang/include/clang/AST/Expr.h
@@ -423,6 +423,10 @@ public:
                              bool isEvaluated = true) const;
   bool isIntegerConstantExpr(ASTContext &Ctx, SourceLocation *Loc = 0) const;
 
+  /// isCXX98IntegralConstantExpr - Return true if this expression is an
+  /// integral constant expression in C++98. Can only be used in C++.
+  bool isCXX98IntegralConstantExpr(ASTContext &Ctx) const;
+
   /// isCXX11ConstantExpr - Return true if this expression is a constant
   /// expression in C++11. Can only be used in C++.
   ///
diff --git a/clang/lib/AST/Expr.cpp b/clang/lib/AST/Expr.cpp
index 5d22d144f48c..5b511cc88315 100644
--- a/clang/lib/AST/Expr.cpp
+++ b/clang/lib/AST/Expr.cpp
@@ -2728,11 +2728,18 @@ Expr::isNullPointerConstant(ASTContext &Ctx,
     return NPCK_NotNull;
 
   // If we have an integer constant expression, we need to *evaluate* it and
-  // test for the value 0.
-  llvm::APSInt Result;
-  bool IsNull = isIntegerConstantExpr(Result, Ctx) && Result == 0;
+  // test for the value 0. Don't use the C++11 constant expression semantics
+  // for this, for now; once the dust settles on core issue 903, we might only
+  // allow a literal 0 here in C++11 mode.
+  if (Ctx.getLangOptions().CPlusPlus0x) {
+    if (!isCXX98IntegralConstantExpr(Ctx))
+      return NPCK_NotNull;
+  } else {
+    if (!isIntegerConstantExpr(Ctx))
+      return NPCK_NotNull;
+  }
 
-  return (IsNull ? NPCK_ZeroInteger : NPCK_NotNull);
+  return (EvaluateKnownConstInt(Ctx) == 0) ? NPCK_ZeroInteger : NPCK_NotNull;
 }
 
 /// \brief If this expression is an l-value for an Objective C
diff --git a/clang/lib/AST/ExprConstant.cpp b/clang/lib/AST/ExprConstant.cpp
index c0fff5e4781e..787e722a7cb4 100644
--- a/clang/lib/AST/ExprConstant.cpp
+++ b/clang/lib/AST/ExprConstant.cpp
@@ -6435,12 +6435,17 @@ bool Expr::isIntegerConstantExpr(llvm::APSInt &Value, ASTContext &Ctx,
   return true;
 }
 
+bool Expr::isCXX98IntegralConstantExpr(ASTContext &Ctx) const {
+  return CheckICE(this, Ctx).Val == 0;
+}
+
 bool Expr::isCXX11ConstantExpr(ASTContext &Ctx, APValue *Result,
                                SourceLocation *Loc) const {
   // We support this checking in C++98 mode in order to diagnose compatibility
   // issues.
   assert(Ctx.getLangOptions().CPlusPlus);
 
+  // Build evaluation settings.
   Expr::EvalStatus Status;
   llvm::SmallVector<PartialDiagnosticAt, 8> Diags;
   Status.Diag = &Diags;
diff --git a/clang/test/SemaCXX/nullptr.cpp b/clang/test/SemaCXX/nullptr.cpp
index 6f660366e998..0e6771b57fcb 100644
--- a/clang/test/SemaCXX/nullptr.cpp
+++ b/clang/test/SemaCXX/nullptr.cpp
@@ -161,3 +161,14 @@ namespace templates {
   
   X2<nullptr, nullptr, nullptr, nullptr> x2;
 }
+
+namespace null_pointer_constant {
+
+// Pending implementation of core issue 903, ensure we don't allow any of the
+// C++11 constant evaluation semantics in null pointer constants.
+struct S { int n; };
+constexpr int null() { return 0; }
+void *p = S().n; // expected-error {{cannot initialize}}
+void *q = null(); // expected-error {{cannot initialize}}
+
+}