Fix nullptr value printing (ROOT-7092).

lib/Interpreter/ValueExtractionSynthesizer.cpp

@@ -318,6 +318,7 @@ namespace {
     else if (desugaredTy->isIntegralOrEnumerationType()
              || desugaredTy->isReferenceType()
              || desugaredTy->isPointerType()
+             || desugaredTy->isNullPtrType()
              || desugaredTy->isFloatingType()) {
       if (desugaredTy->isIntegralOrEnumerationType()) {
         // 1)  enum, integral, float, double, referece, pointer types :
@@ -349,6 +350,10 @@ namespace {
           = m_Sema->BuildCStyleCastExpr(noLoc, TSI, noLoc, E).get();
         CallArgs.push_back(castedE);
       }
+      else if (desugaredTy->isNullPtrType()) {
+        // nullptr should decay to void* just fine.
+        CallArgs.push_back(E);
+      }
       else if (desugaredTy->isFloatingType()) {
         // floats and double will fall naturally in the correct
         // case, because of the overload resolution.

test/Prompt/ValuePrinter/Regression.C

@@ -46,3 +46,5 @@ q // CHECK: (const int *) 0x123
 
 // PR ROOT-5467
 &A::someFunc // CHECK: (int (class A::*)(float)) @0x{{[0-9a-f]+}}
+
+nullptr // CHECK: (nullptr_t) @0x0