fix: disallow infix objects

Disallow `infix` objects by checking for the `infix` modifier in
`Checking.scala`.

Part of #17738

Author: arainko

compiler/src/dotty/tools/dotc/typer/Checking.scala

@@ -584,6 +584,8 @@ object Checking {
       report.error(ModifierNotAllowedForDefinition(Sealed), flagSourcePos(Sealed))
     if mods.is(Final, butNot = Synthetic) then
       report.warning(RedundantModifier(Final), flagSourcePos(Final))
+    if mods.is(Infix) then
+      report.error(ModifierNotAllowedForDefinition(Infix), flagSourcePos(Infix))
 
   /** Check the type signature of the symbol `M` defined by `tree` does not refer
    *  to a private type or value which is invisible at a point where `M` is still

tests/neg/i17738-infix-object/NestedInfixObject.scala

@@ -0,0 +1,2 @@
+object ToplevelObject:
+  infix object NestedInfixObject // error

tests/neg/i17738-infix-object/ToplevelInfixObject.scala

@@ -0,0 +1 @@
+infix object ToplevelInfixObject // error