fixes nim check with orc (#20456)

* fixes nim check with orc

* fixes tests

* add tests

* fixes tests

* Update tests/arc/t20456.nim

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)

tests/arc/t20456.nim

@@ -0,0 +1,7 @@
+discard """
+  cmd: "nim check $file"
+  action: "compile"
+"""
+
+when not defined(gcOrc):
+  {.error: "orc".}

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:

tests/misc/m20456.nims

@@ -0,0 +1 @@
+echo 123

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