Rename EM_ConstantExpressionUnevaluated to EM_ConstantFoldUnevaluated, which is actually what the code is currently doing.

[jyknight]

Summary

And, configure it to explicitly request equivalent behavior to
ConstantFold, instead of getting that behavior accidentally.

While it seems that diagnose_if and enable_if were intended to require
a constexpr argument, the code was actually only checking whether it
could be constant-folded, due to the confusing way the constant
evaluator code currently works.

It _probably_ should be fixed to actually check for
constexpr-ness. However, r290584 now depends on it NOT doing so -- and
it's possible that some users are, too.

I'd like to land this, to unblock the follow-on cleanup -- without
changing the semantics first -- and maybe loop back to convert it to
require a constexpr later, if it looks like users aren't depending on
the semantics it has now.

This change is a prerequisite for fixing the confusion in the
ExprConstant code which led to this situation.

[jyknight]

https://reviews.llvm.org/D52926

clang/lib/AST/ExprConstant.cpp

enum EvaluationMode {
       /// issues. Don't worry about side-effects, and try to visit all
       /// subexpressions.
       EM_EvaluateForOverflow,
 
       /// Evaluate in any way we know how. Don't worry about side-effects that
       /// can't be modeled.
       EM_IgnoreSideEffects,
 
-      /// Evaluate as a constant expression. Stop if we find that the expression
-      /// is not a constant expression. Some expressions can be retried in the
+      /// Evaluate as a constant fold. Some expressions can be retried in the
       /// optimizer if we don't constant fold them here, but in an unevaluated
-      /// context we try to fold them immediately since the optimizer never
-      /// gets a chance to look at it.
-      EM_ConstantExpressionUnevaluated,
+      /// context we try to fold them immediately since the optimizer never gets
+      /// a chance to look at it.
+      EM_ConstantFoldUnevaluated,
 
       /// Evaluate as a potential constant expression. Keep going if we hit a
       /// construct that we can't evaluate yet (because we don't yet know the
       /// value of something) but stop if we hit something that could never be
       /// a constant expression. Some expressions can be retried in the
       /// optimizer if we don't constant fold them here, but in an unevaluated
       /// context we try to fold them immediately since the optimizer never
       /// gets a chance to look at it.
OptionalDiagnostic Diag(SourceLocation Loc, diag::kind DiagId,
         //
         // FIXME: We might want to show both diagnostics to the user in
         // EM_ConstantFold mode.
         if (!EvalStatus.Diag->empty()) {
           switch (EvalMode) {
           case EM_ConstantFold:
           case EM_IgnoreSideEffects:
           case EM_EvaluateForOverflow:
+          case EM_ConstantFoldUnevaluated:
             if (!HasFoldFailureDiagnostic)
               break;
             // We've already failed to fold something. Keep that diagnostic.
             LLVM_FALLTHROUGH;
           case EM_ConstantExpression:
           case EM_PotentialConstantExpression:
-          case EM_ConstantExpressionUnevaluated:
           case EM_PotentialConstantExpressionUnevaluated:
             HasActiveDiagnostic = false;
             return OptionalDiagnostic();
           }
         }
 
         unsigned CallStackNotes = CallStackDepth - 1;
         unsigned Limit = Ctx.getDiagnostics().getConstexprBacktraceLimit();
bool keepEvaluatingAfterSideEffect() {
       switch (EvalMode) {
       case EM_PotentialConstantExpression:
       case EM_PotentialConstantExpressionUnevaluated:
       case EM_EvaluateForOverflow:
       case EM_IgnoreSideEffects:
         return true;
 
       case EM_ConstantExpression:
-      case EM_ConstantExpressionUnevaluated:
+      case EM_ConstantFoldUnevaluated:
       case EM_ConstantFold:
         return false;
       }
       llvm_unreachable("Missed EvalMode case");
     }
 
     /// Note that we have had a side-effect, and determine whether we should
     /// keep evaluating.
     bool noteSideEffect() {
       EvalStatus.HasSideEffects = true;
       return keepEvaluatingAfterSideEffect();
     }
 
     /// Should we continue evaluation after encountering undefined behavior?
     bool keepEvaluatingAfterUndefinedBehavior() {
       switch (EvalMode) {
       case EM_EvaluateForOverflow:
       case EM_IgnoreSideEffects:
       case EM_ConstantFold:
+      case EM_ConstantFoldUnevaluated:
         return true;
 
       case EM_PotentialConstantExpression:
       case EM_PotentialConstantExpressionUnevaluated:
       case EM_ConstantExpression:
-      case EM_ConstantExpressionUnevaluated:
         return false;
       }
       llvm_unreachable("Missed EvalMode case");
     }
 
     /// Note that we hit something that was technically undefined behavior, but
     /// that we can evaluate past it (such as signed overflow or floating-point
     /// division by zero.)
bool keepEvaluatingAfterFailure() {
 
       switch (EvalMode) {
       case EM_PotentialConstantExpression:
       case EM_PotentialConstantExpressionUnevaluated:
       case EM_EvaluateForOverflow:
         return true;
 
       case EM_ConstantExpression:
-      case EM_ConstantExpressionUnevaluated:
+      case EM_ConstantFoldUnevaluated:
       case EM_ConstantFold:
       case EM_IgnoreSideEffects:
         return false;
       }
       llvm_unreachable("Missed EvalMode case");
     }
 
     /// Notes that we failed to evaluate an expression that other expressions
struct FoldConstant {
 
     explicit FoldConstant(EvalInfo &Info, bool Enabled)
       : Info(Info),
         Enabled(Enabled),
         HadNoPriorDiags(Info.EvalStatus.Diag &&
                         Info.EvalStatus.Diag->empty() &&
                         !Info.EvalStatus.HasSideEffects),
         OldMode(Info.EvalMode) {
-      if (Enabled &&
-          (Info.EvalMode == EvalInfo::EM_ConstantExpression ||
-           Info.EvalMode == EvalInfo::EM_ConstantExpressionUnevaluated))
+      if (Enabled && Info.EvalMode == EvalInfo::EM_ConstantExpression)
         Info.EvalMode = EvalInfo::EM_ConstantFold;
     }
     void keepDiagnostics() { Enabled = false; }
     ~FoldConstant() {
       if (Enabled && HadNoPriorDiags && !Info.EvalStatus.Diag->empty() &&
           !Info.EvalStatus.HasSideEffects)
         Info.EvalStatus.Diag->clear();
       Info.EvalMode = OldMode;
case Builtin::BI__builtin_object_size: {
     switch (Info.EvalMode) {
     case EvalInfo::EM_ConstantExpression:
     case EvalInfo::EM_PotentialConstantExpression:
     case EvalInfo::EM_ConstantFold:
     case EvalInfo::EM_EvaluateForOverflow:
     case EvalInfo::EM_IgnoreSideEffects:
       // Leave it to IR generation.
       return Error(E);
-    case EvalInfo::EM_ConstantExpressionUnevaluated:
+    case EvalInfo::EM_ConstantFoldUnevaluated:
     case EvalInfo::EM_PotentialConstantExpressionUnevaluated:
       // Reduce it to a constant now.
       return Success((Type & 2) ? 0 : -1, E);
     }
 
     llvm_unreachable("unexpected EvalMode");
   }
 
bool Expr::isCXX11ConstantExpr(const ASTContext &Ctx, APValue *Result,
   return IsConstExpr;
 }
 
 bool Expr::EvaluateWithSubstitution(APValue &Value, ASTContext &Ctx,
                                     const FunctionDecl *Callee,
                                     ArrayRef<const Expr*> Args,
                                     const Expr *This) const {
   Expr::EvalStatus Status;
-  EvalInfo Info(Ctx, Status, EvalInfo::EM_ConstantExpressionUnevaluated);
+  EvalInfo Info(Ctx, Status, EvalInfo::EM_ConstantFoldUnevaluated);
 
   LValue ThisVal;
   const LValue *ThisPtr = nullptr;
   if (This) {
 #ifndef NDEBUG
     auto *MD = dyn_cast<CXXMethodDecl>(Callee);
     assert(MD && "Don't provide `this` for non-methods.");
     assert(!MD->isStatic() && "Don't provide `this` for static methods.");
 #endif
     if (EvaluateObjectArgument(Info, This, ThisVal))
       ThisPtr = &ThisVal;
-    if (Info.EvalStatus.HasSideEffects)
+    if (Info.EvalStatus.HasSideEffects || Info.EvalStatus.HasUndefinedBehavior)
       return false;
   }
 
   ArgVector ArgValues(Args.size());
   for (ArrayRef<const Expr*>::iterator I = Args.begin(), E = Args.end();
        I != E; ++I) {
     if ((*I)->isValueDependent() ||
         !Evaluate(ArgValues[I - Args.begin()], Info, *I))
       // If evaluation fails, throw away the argument entirely.
       ArgValues[I - Args.begin()] = APValue();
-    if (Info.EvalStatus.HasSideEffects)
+    if (Info.EvalStatus.HasSideEffects || Info.EvalStatus.HasUndefinedBehavior)
       return false;
   }
 
   // Build fake call to Callee.
   CallStackFrame Frame(Info, Callee->getLocation(), Callee, ThisPtr,
                        ArgValues.data());
-  return Evaluate(Value, Info, this) && !Info.EvalStatus.HasSideEffects;
+  return Evaluate(Value, Info, this) && !Info.EvalStatus.HasSideEffects && !Info.EvalStatus.HasUndefinedBehavior;
 }
 
 bool Expr::isPotentialConstantExpr(const FunctionDecl *FD,
                                    SmallVectorImpl<
                                      PartialDiagnosticAt> &Diags) {
   // FIXME: It would be useful to check constexpr function templates, but at the
   // moment the constant expression evaluator cannot cope with the non-rigorous
   // ASTs which we build for dependent expressions.

clang/test/Sema/enable_if.c

void variadic_test() {
   variadic_enable_if(1);
   variadic_enable_if(1, m);
   variadic_enable_if(1, m, "c");
 
   variadic_enable_if(0); // expected-error{{no matching}}
   variadic_enable_if(0, m); // expected-error{{no matching}}
   variadic_enable_if(0, m, 3); // expected-error{{no matching}}
 }
+
+// Test of the current weird semantics of enable_if (and diagnose_if).
+// There's two levels of checking for the expression:
+// 1. Whether it could potentially be a constexpr, without access to the parameters.
+// 2. Given the parameters, whether it can be evaluated in any way, regardless
+//    of whether it's a proper constexpr.
+void non_constexpr() __attribute__((enable_if((int*)(char*)0 == 0, ""))); // expected-error{{'enable_if' attribute expression never produces a constant expression}} expected-note{{cast that performs the conversions of a reinterpret_cast is not allowed in a constant expression}}
+void non_constexpr_call() {
+    non_constexpr();
+}
+
+void non_constexpr_conditional(long x) __attribute__((enable_if(x?1: (int*)(char*)0 == 0, "")));
+void non_constexpr_conditional_call() {
+    non_constexpr_conditional(0);
+}
 #endif