[tree] Propagate IMT settings of TChain to internal TTrees

Fixes issue #8720

tree/tree/src/TChain.cxx

@@ -1344,6 +1344,7 @@ Long64_t TChain::LoadTree(Long64_t entry)
       // (the friends of the chain will be updated in the
       // next loop).
       fTree->LoadTree(treeReadEntry);
+
       if (fFriends) {
          // The current tree has not changed but some of its friends might.
          //
@@ -1528,6 +1529,10 @@ Long64_t TChain::LoadTree(Long64_t entry)
       } else if (!fGlobalRegistration) {
          fTree->ResetBit(kMustCleanup);
       }
+      // Propagate the IMT settings
+      if (fTree) {
+         fTree->SetImplicitMT(fIMTEnabled);
+      }
    }
 
    fTreeNumber = treenum;

tree/tree/test/ImplicitMT.cxx

@@ -1,3 +1,4 @@
+#include "TChain.h"
 #include "TFile.h"
 #include "TROOT.h"
 #include "TSystem.h"
@@ -28,4 +29,32 @@ TEST(TTreeImplicitMT, flushBaskets)
    gSystem->Unlink(ofileName);
 }
 
+//# 8720
+TEST(TChainImplicitMT, propagateToTTree)
+{
+   ROOT::DisableImplicitMT();
+
+   const auto fname0 = "propagateToTTree0.root";
+   const auto fname1 = "propagateToTTree1.root";
+   for (const auto fname : {fname0, fname1}) {
+      TFile f(fname, "RECREATE");
+      TTree t("e", "e");
+      t.Fill();
+      t.Write();
+      EXPECT_FALSE(t.GetImplicitMT());
+   }
+
+   TChain c("e");
+   c.SetImplicitMT(true);
+   c.Add(fname0);
+   c.Add(fname1);
+   c.LoadTree(0);
+   EXPECT_TRUE(c.GetTree()->GetImplicitMT());
+   c.LoadTree(1);
+   EXPECT_TRUE(c.GetTree()->GetImplicitMT());
+
+   gSystem->Unlink(fname0);
+   gSystem->Unlink(fname1);
+}
+
 #endif // R__USE_IMT