[clang][Interp] Allow ltor casts for null pointers

Summary: We can't read from them but we special-case them later.

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Differential Revision: https://phabricator.intern.facebook.com/D60251516

Author: tbaederr

clang/lib/AST/Interp/EvalEmitter.cpp

@@ -152,7 +152,7 @@ template <> bool EvalEmitter::emitRet<PT_Ptr>(const SourceInfo &Info) {
 
   // Implicitly convert lvalue to rvalue, if requested.
   if (ConvertResultToRValue) {
-    if (!Ptr.isDereferencable())
+    if (!Ptr.isZero() && !Ptr.isDereferencable())
       return false;
     // Never allow reading from a non-const pointer, unless the memory
     // has been created in this evaluation.

clang/test/CodeGenCXX/nullptr.cpp

@@ -1,4 +1,5 @@
 // RUN: %clang_cc1 -std=c++11 -triple x86_64-apple-darwin10 -I%S -emit-llvm -o - %s | FileCheck %s
+// RUN: %clang_cc1 -std=c++11 -triple x86_64-apple-darwin10 -I%S -emit-llvm -o - %s -fexperimental-new-constant-interpreter | FileCheck %s
 
 #include <typeinfo>