[hist] Request dictionaries for common RHistEngine<T>

This results in proper error messages when trying to stream. Other
instantiations will be caught by the RAxes member.

Author: hahnjo

hist/histv7/inc/LinkDef.h

@@ -1,3 +1,11 @@
+// For RHistEngine, we request dictionaries for the most commonly used bin content types. This results in proper error
+// messages when trying to stream. Other instantiations will be caught by the RAxes member.
+#pragma link C++ class ROOT::Experimental::RHistEngine<int>-;
+#pragma link C++ class ROOT::Experimental::RHistEngine<long>-;
+#pragma link C++ class ROOT::Experimental::RHistEngine<long long>-;
+#pragma link C++ class ROOT::Experimental::RHistEngine<float>-;
+#pragma link C++ class ROOT::Experimental::RHistEngine<double>-;
+
 #pragma link C++ class ROOT::Experimental::RRegularAxis-;
 #pragma link C++ class ROOT::Experimental::RVariableBinAxis-;
 #pragma link C++ class ROOT::Experimental::Internal::RAxes-;

hist/histv7/test/hist_io.cxx

@@ -50,3 +50,29 @@ TEST(RAxes, Streamer)
    const RAxes axes({regularAxis, variableBinAxis});
    ExpectThrowOnWriteObject(axes);
 }
+
+TEST(RHistEngine, Streamer)
+{
+   static constexpr std::size_t Bins = 20;
+   const RRegularAxis axis(Bins, 0, Bins);
+
+   // We don't request a dictionary for RHistEngine<unsigned char>, and we generally don't recommend such narrow bin
+   // content types. If used, the RAxes member will prevent streaming.
+   const RHistEngine<unsigned char> engineC({axis});
+   ExpectThrowOnWriteObject(engineC);
+
+   const RHistEngine<int> engineI({axis});
+   ExpectThrowOnWriteObject(engineI);
+
+   const RHistEngine<long> engineL({axis});
+   ExpectThrowOnWriteObject(engineL);
+
+   const RHistEngine<long long> engineLL({axis});
+   ExpectThrowOnWriteObject(engineLL);
+
+   const RHistEngine<float> engineF({axis});
+   ExpectThrowOnWriteObject(engineF);
+
+   const RHistEngine<double> engineD({axis});
+   ExpectThrowOnWriteObject(engineD);
+}