add testcase for #6499 #12229 #7172 (#18547)

tests/magics/tmagics.nim

@@ -55,3 +55,11 @@ block t9442:
   GC_unref(v2)
   GC_ref(v3)
   GC_unref(v3)
+
+block: # bug #6499
+  let x = (chr, 0)
+  doAssert x[1] == 0
+
+block: # bug #12229
+  proc foo(T: typedesc) = discard
+  foo(ref)

tests/threads/t7172.nim

@@ -0,0 +1,32 @@
+discard """
+  output: '''
+In doStuff()
+In initProcess()
+initProcess() done
+TEST
+Crashes before getting here!
+'''
+  joinable: false
+"""
+
+import std/os
+
+proc whatever() {.thread, nimcall.} =
+  echo("TEST")
+
+proc initProcess(): void =
+  echo("In initProcess()")
+  var thread: Thread[void]
+  createThread(thread, whatever)
+  echo("initProcess() done")
+  joinThread(thread)
+
+proc doStuff(): void =
+  echo("In doStuff()")
+  # ...
+  initProcess()
+  sleep(500)
+  # ...
+  echo("Crashes before getting here!")
+
+doStuff()