diff --git a/llvm/lib/Transforms/Scalar/JumpThreading.cpp b/llvm/lib/Transforms/Scalar/JumpThreading.cpp
--- a/llvm/lib/Transforms/Scalar/JumpThreading.cpp
+++ b/llvm/lib/Transforms/Scalar/JumpThreading.cpp
@@ -1097,14 +1097,21 @@
 
   // If the terminator of this block is branching on a constant, simplify the
   // terminator to an unconditional branch.  This can occur due to threading in
-  // other blocks.
-  if (getKnownConstant(Condition, Preference)) {
+  // other blocks.  We can only convert blocks that aren't LoopHeaders.  It's
+  // non-trivial to determine if BB remains a LoopHeader and if a recurisve
+  // successor of BB is also a new LoopHeader.
+  if (!LoopHeaders.count(BB) && getKnownConstant(Condition, Preference)) {
     LLVM_DEBUG(dbgs() << "  In block '" << BB->getName()
-                      << "' folding terminator: " << *BB->getTerminator()
+                      << "' trying to fold terminator: " << *BB->getTerminator()
                       << '\n');
-    ++NumFolds;
-    ConstantFoldTerminator(BB, true, nullptr, DTU);
-    return true;
+    if (ConstantFoldTerminator(BB, true, nullptr, DTU)) {
+      LLVM_DEBUG(dbgs() << "  Successfully folded block '" << BB->getName()
+                        << "' to new terminator: " << *BB->getTerminator()
+                        << '\n');
+      ++NumFolds;
+      return true;
+    }
+    return false;
   }
 
   Instruction *CondInst = dyn_cast<Instruction>(Condition);
diff --git a/llvm/test/Transforms/JumpThreading/pr42085-thread-over-loopheader.ll b/llvm/test/Transforms/JumpThreading/pr42085-thread-over-loopheader.ll
new file mode 100644
--- /dev/null
+++ b/llvm/test/Transforms/JumpThreading/pr42085-thread-over-loopheader.ll
@@ -0,0 +1,35 @@
+; RUN: opt -S < %s -jump-threading | FileCheck %s
+
+; PR42085: Do not thread over 'body' from 'pre_body' to 'latch'. In this
+;          case 'body' and 'head' are the correct loop headers and must
+;          not be threaded over. There are no JumpThread opportunities
+;          in this test case.
+
+define i32 @test(i1 %ARG) {
+; CHECK-NOT: {{.+}}.thread:
+entry:
+  br label %pre_body
+
+head:
+  %inc_head = phi i32 [ %inc_pre_body, %pre_body ], [ %inc, %latch ]
+  %cmp1 = icmp sgt i32 %inc_head, 1
+  br i1 %cmp1, label %body, label %pre_body
+
+pre_body:
+  %inc_pre_body = phi i32 [ 1, %entry ], [ %inc_head, %head ]
+  br i1 false, label %head, label %body
+
+body:
+  %res = phi i32 [ %inc_head, %head ], [ %inc_pre_body, %pre_body ]
+  %tmp = phi i32 [ %inc_head, %head ], [ %inc_pre_body, %pre_body ]
+  %inc = add i32 %tmp, 1
+  br i1 %ARG, label %exit, label %latch
+
+latch:
+  %cmp2 = icmp sgt i32 %inc, 2
+  br i1 %cmp2, label %exit, label %head
+
+exit:
+  %rc = phi i32 [ %res, %latch ], [ 0, %body ]
+  ret i32 %rc
+}