Make noexcept check for constant expressions.

[Eelis]

Summary

This fixes PR15481.

[ismailp]

Thanks for looking into this! I have submitted a very small patch to fix this bug in 2013, in D538. It has been abandoned, because Richard was planning to implement Core DR 1351.

include/clang/AST/ExprCXX.h

 
 public:
   CXXNoexceptExpr(QualType Ty, Expr *Operand, CanThrowResult Val,
-                  SourceLocation Keyword, SourceLocation RParen)
+                  bool IsConstexpr, SourceLocation Keyword,
+                  SourceLocation RParen)
     : Expr(CXXNoexceptExprClass, Ty, VK_RValue, OK_Ordinary,
            /*TypeDependent*/false,
-           /*ValueDependent*/Val == CT_Dependent,
+           /*ValueDependent*/Val == CT_Dependent && !IsConstexpr,
            Val == CT_Dependent || Operand->isInstantiationDependent(),
            Operand->containsUnexpandedParameterPack()),
-      Value(Val == CT_Cannot), Operand(Operand), Range(Keyword, RParen)
+      Value(Val == CT_Cannot || IsConstexpr),
+      Operand(Operand), Range(Keyword, RParen)
   { }
 
   CXXNoexceptExpr(EmptyShell Empty)

lib/Sema/SemaExprCXX.cpp

   }
 
   CanThrowResult CanThrow = canThrow(Operand);
+  bool IsConstexpr = Operand->isCXX11ConstantExpr(Context);
   return new (Context)
-      CXXNoexceptExpr(Context.BoolTy, Operand, CanThrow, KeyLoc, RParen);
+      CXXNoexceptExpr(Context.BoolTy, Operand, CanThrow, IsConstexpr,
+                      KeyLoc, RParen);
 }
 
 ExprResult Sema::ActOnNoexceptExpr(SourceLocation KeyLoc, SourceLocation,

test/CXX/expr/expr.unary/expr.unary.noexcept/sema.cpp

 void nothrowattr() __attribute__((nothrow));
 void noexcept_true() noexcept;
 void noexcept_false() noexcept(false);
+constexpr int constexprspec() { return 3; }
 
 void call() {
   N(nospec());

   P(nothrowattr());
   P(noexcept_true());
   N(noexcept_false());
+  P(constexprspec());
 }
 
 void (*pnospec)();