Fixes #12536 (#12568) [backport]

compiler/commands.nim

@@ -627,21 +627,19 @@ proc processSwitch*(switch, arg: string, pass: TCmdLinePass, info: TLineInfo;
     processOnOffSwitchG(conf, {optGenMapping}, arg, pass, info)
   of "os":
     expectArg(conf, switch, arg, pass, info)
-    if pass in {passCmd1, passPP}:
     let theOS = platform.nameToOS(arg)
     if theOS == osNone:
       let osList = platform.listOSnames().join(", ")
       localError(conf, info, "unknown OS: '$1'. Available options are: $2" % [arg, $osList])
-      elif theOS != conf.target.hostOS:
+    else:
       setTarget(conf.target, theOS, conf.target.targetCPU)
   of "cpu":
     expectArg(conf, switch, arg, pass, info)
-    if pass in {passCmd1, passPP}:
     let cpu = platform.nameToCPU(arg)
     if cpu == cpuNone:
       let cpuList = platform.listCPUnames().join(", ")
       localError(conf, info, "unknown CPU: '$1'. Available options are: $2" % [ arg, cpuList])
-      elif cpu != conf.target.hostCPU:
+    else:
       setTarget(conf.target, conf.target.targetOS, cpu)
   of "run", "r":
     processOnOffSwitchG(conf, {optRun}, arg, pass, info)

tests/compiler/tcmdlinecpuamd64.nim

@@ -0,0 +1,10 @@
+discard """
+  cmd: "nim $target $options --cpu:amd64 $file"
+  disabled: "32bit"
+"""
+
+import strutils
+
+static:
+  #cpu is set to "i386" in tcpuamd64.nim.cfg, but --cpu:amd64 in command line should override it.
+  doAssert cmpIgnoreCase(hostCPU, "amd64") == 0

tests/compiler/tcmdlinecpuamd64.nim.cfg

@@ -0,0 +1 @@
+cpu:i386

tests/compiler/tcmdlineoswin.nim

@@ -0,0 +1,14 @@
+discard """
+  cmd: "nim $target $options --os:windows $file"
+  disabled: "linux"
+  disabled: "bsd"
+  disabled: "macosx"
+  disabled: "unix"
+  disabled: "posix"
+"""
+
+import strutils
+
+static:
+  #os is set to "linux" in toswin.nim.cfg, but --os:windows in command line should override it.
+  doAssert cmpIgnoreCase(hostOS, "windows") == 0

tests/compiler/tcmdlineoswin.nim.cfg

@@ -0,0 +1 @@
+os:linux