fixes #10765 (#10993) [backport]

lib/system/excpt.nim

@@ -468,19 +468,36 @@ when defined(endb):
 when defined(cpp) and appType != "lib" and
     not defined(js) and not defined(nimscript) and
     hostOS != "standalone" and not defined(noCppExceptions):
+      
+  type 
+    StdException {.importcpp: "std::exception", header: "<exception>".} = object
+      
+  proc what(ex: StdException): cstring {.importcpp: "((char *)#.what())".}
+
   proc setTerminate(handler: proc() {.noconv.})
     {.importc: "std::set_terminate", header: "<exception>".}
+
   setTerminate proc() {.noconv.} =
     # Remove ourself as a handler, reinstalling the default handler.
     setTerminate(nil)
 
+    var msg = "Unknown error in unexpected exception handler"
+    try:
+      raise
+    except Exception:
+      msg = currException.getStackTrace() & "Error: unhandled exception: " &
+        currException.msg & " [" & $currException.name & "]"
+    except StdException as e:
+      msg = "Error: unhandled cpp exception: " & $e.what()
+    except:
+      msg = "Error: unhandled unknown cpp exception"
+      
     when defined(genode):
       # stderr not available by default, use the LOG session
-      echo currException.getStackTrace() & "Error: unhandled exception: " &
-              currException.msg & " [" & $currException.name & "]\n"
+      echo msg
     else:
-      writeToStdErr currException.getStackTrace() & "Error: unhandled exception: " &
-              currException.msg & " [" & $currException.name & "]\n"
+      writeToStdErr msg & "\n"
+
     quit 1
 
 when not defined(noSignalHandler) and not defined(useNimRtl):

tests/cpp/tterminate_handler.nim

@@ -0,0 +1,9 @@
+discard """
+  targets: "cpp"
+  outputsub: "Error: unhandled unknown cpp exception"
+  exitcode: 1
+"""
+type Crap {.importcpp: "int".} = object
+
+var c: Crap
+raise c