diff --git a/compiler/nim.cfg b/compiler/nim.cfg
index 853a657b39..926cb4c28f 100644
--- a/compiler/nim.cfg
+++ b/compiler/nim.cfg
@@ -6,6 +6,7 @@ define:booting
 define:nimcore
 define:nimPreviewFloatRoundtrip
 define:nimPreviewSlimSystem
+threads:off
 
 #import:"$projectpath/testability"
 
diff --git a/compiler/nim.nim b/compiler/nim.nim
index 2a6b3bc1b7..b98a514d73 100644
--- a/compiler/nim.nim
+++ b/compiler/nim.nim
@@ -97,6 +97,10 @@ proc handleCmdLine(cache: IdentCache; conf: ConfigRef) =
   if not self.loadConfigsAndProcessCmdLine(cache, conf, graph):
     return
 
+  if conf.cmd == cmdCheck and optWasNimscript notin conf.globalOptions and
+       conf.backend == backendInvalid:
+    conf.backend = backendC
+
   if conf.selectedGC == gcUnselected:
     if conf.backend in {backendC, backendCpp, backendObjc}:
       initOrcDefines(conf)
diff --git a/tests/arc/t20456.nim b/tests/arc/t20456.nim
new file mode 100644
index 0000000000..ace79255a0
--- /dev/null
+++ b/tests/arc/t20456.nim
@@ -0,0 +1,7 @@
+discard """
+  cmd: "nim check $file"
+  action: "compile"
+"""
+
+when not defined(gcOrc):
+  {.error: "orc".}
diff --git a/tests/collections/ttables.nim b/tests/collections/ttables.nim
index 638f4241b6..95f9418a00 100644
--- a/tests/collections/ttables.nim
+++ b/tests/collections/ttables.nim
@@ -177,16 +177,14 @@ block tableconstr:
 block ttables2:
   proc TestHashIntInt() =
     var tab = initTable[int,int]()
-    let n = 100_000
+    let n = 10
     for i in 1..n:
       tab[i] = i
     for i in 1..n:
       var x = tab[i]
       if x != i : echo "not found ", i
 
-  proc run1() =
-    for i in 1 .. 50:
-      TestHashIntInt()
+  TestHashIntInt()
 
   # bug #2107
 
@@ -198,7 +196,6 @@ block ttables2:
   delTab[5] = 5
 
 
-  run1()
   echo "2"
 
 block tablesref:
diff --git a/tests/errmsgs/tsigmatch.nim b/tests/errmsgs/tsigmatch.nim
index 8f32ef9e66..49b3559829 100644
--- a/tests/errmsgs/tsigmatch.nim
+++ b/tests/errmsgs/tsigmatch.nim
@@ -1,5 +1,5 @@
 discard """
-  cmd: "nim check --showAllMismatches:on --hints:off $file"
+  cmd: "nim check --mm:refc --showAllMismatches:on --hints:off $file"
   nimout: '''
 tsigmatch.nim(111, 4) Error: type mismatch: got <A, string>
 but expected one of:
diff --git a/tests/generics/tgeneric3.nim b/tests/generics/tgeneric3.nim
index 07ab822ae4..29a73afc68 100644
--- a/tests/generics/tgeneric3.nim
+++ b/tests/generics/tgeneric3.nim
@@ -1,9 +1,9 @@
 discard """
 output: '''
 312
-1000000
-1000000
-500000
+1000
+1000
+500
 0
 '''
 """
@@ -445,7 +445,7 @@ proc test() =
   var it1 = internalFind(root, 312)
   echo it1.value
 
-  for i in 1..1_000_000:
+  for i in 1..1_000:
     root = internalPut(root, i, i, oldvalue)
 
   var cnt = 0
diff --git a/tests/misc/m20456.nims b/tests/misc/m20456.nims
new file mode 100644
index 0000000000..8a93131295
--- /dev/null
+++ b/tests/misc/m20456.nims
@@ -0,0 +1 @@
+echo 123
\ No newline at end of file
diff --git a/tests/misc/t20456_2.nim b/tests/misc/t20456_2.nim
new file mode 100644
index 0000000000..37e52c4522
--- /dev/null
+++ b/tests/misc/t20456_2.nim
@@ -0,0 +1,14 @@
+discard """
+  joinable: false
+"""
+
+import std/[osproc, os, strformat]
+from stdtest/specialpaths import testsDir
+
+when defined(nimPreviewSlimSystem):
+  import std/assertions
+
+const
+  nim = getCurrentCompilerExe()
+  file = testsDir / "misc" / "m20456.nims"
+doAssert execCmd(fmt"{nim} check {file}") == 0
diff --git a/tests/objects/tobject.nim b/tests/objects/tobject.nim
index 543a863765..207cda0bc0 100644
--- a/tests/objects/tobject.nim
+++ b/tests/objects/tobject.nim
@@ -1,4 +1,3 @@
-import unittest
 
 type Obj = object
   foo: int
@@ -10,9 +9,9 @@ block: # object basic methods
   block: # it should convert an object to a string
     var obj = makeObj(1)
     # Should be "obj: (foo: 1)" or similar.
-    check($obj == "(foo: 1)")
+    doAssert($obj == "(foo: 1)")
   block: # it should test equality based on fields
-    check(makeObj(1) == makeObj(1))
+    doAssert(makeObj(1) == makeObj(1))
 
 # bug #10203