[clang][Interp] PointerToBoolean casts

[tbaeder]

Summary

Just emit p != nullptr for this.

[aaron.ballman]

LGTM!

clang/lib/AST/Interp/ByteCodeExprGen.cpp

case CK_IntegralCast: {
 
     if (!this->visit(SubExpr))
       return false;
 
     // TODO: Emit this only if FromT != ToT.
     return this->emitCast(*FromT, *ToT, CE);
   }
 
+  case CK_PointerToBoolean: {
+    // Just emit p != nullptr for this.
+    if (!this->visit(SubExpr))
+      return false;
+
+    if (!this->emitNullPtr(CE))
+      return false;
+
+    return this->emitNEPtr(CE);
+  }
+
   case CK_ToVoid:
     return discard(SubExpr);
 
   default:
     assert(false && "Cast not implemented");
   }
   llvm_unreachable("Unhandled clang::CastKind enum");
 }

clang/test/AST/Interp/literals.cpp

 static_assert(getIntPointer() == &m, "");
 static_assert(*getIntPointer() == 10, "");
 
 constexpr int gimme(int k) {
   return k;
 }
 static_assert(gimme(5) == 5, "");
 
+namespace PointerToBool {
+
+  constexpr void *N = nullptr;
+  constexpr bool B = N;
+  static_assert(!B, "");
+  static_assert(!N, "");
+
+  constexpr float F = 1.0;
+  constexpr const float *FP = &F;
+  static_assert(FP, "");
+  static_assert(!!FP, "");
+}
+
 namespace SizeOf {
   constexpr int soint = sizeof(int);
   constexpr int souint = sizeof(unsigned int);
   static_assert(soint == souint, "");
 
   static_assert(sizeof(&soint) == sizeof(void*), "");
   static_assert(sizeof(&soint) == sizeof(nullptr), "");