--- a/lldb/source/Expression/LLVMUserExpression.cpp	Mon May 25 11:50:15 2020 +0900
+++ b/lldb/source/Expression/LLVMUserExpression.cpp	Mon May 25 11:55:54 2020 +0900
@@ -134,6 +134,10 @@
       return lldb::eExpressionSetupError;
     }
 
+    // Store away the thread ID for error reporting, in case it exits
+    // during execution:
+    lldb::tid_t expr_thread_id = exe_ctx.GetThreadRef().GetID();
+
     Address wrapper_address(m_jit_start_addr);
 
     std::vector<lldb::addr_t> args;
@@ -223,6 +227,14 @@
           "Use \"thread return -x\" to return to the state before expression "
           "evaluation.");
       return execution_result;
+    } else if (execution_result == lldb::eExpressionThreadVanished) {
+      diagnostic_manager.Printf(
+          eDiagnosticSeverityError,
+          "Couldn't complete execution; the thread "
+          "on which the expression was being run: 0x%" PRIx64
+          " exited during its execution.", 
+          expr_thread_id);
+      return execution_result;
     } else if (execution_result != lldb::eExpressionCompleted) {
       diagnostic_manager.Printf(
           eDiagnosticSeverityError, "Couldn't execute function; result was %s",