Disallow toplevel infix vals, vars, givens, methods and implicits (#17994)

Disallow infix toplevel definitions for anything that is not a `class`,
`typealias`, `match type`, `extension method` and a `trait` (and
`objects` as well but these are handled in their own PR).

Part of #17738
Continuation of #17966

---------

Co-authored-by: Matt Bovel <[email protected]>

Author: arainko

compiler/src/dotty/tools/dotc/reporting/messages.scala

@@ -2608,10 +2608,10 @@ class AnonymousInstanceCannotBeEmpty(impl:  untpd.Template)(using Context)
         |"""
 }
 
-class ModifierNotAllowedForDefinition(flag: Flag)(using Context)
+class ModifierNotAllowedForDefinition(flag: Flag, explanation: String = "")(using Context)
   extends SyntaxMsg(ModifierNotAllowedForDefinitionID) {
   def msg(using Context) = i"Modifier ${hl(flag.flagsString)} is not allowed for this definition"
-  def explain(using Context) = ""
+  def explain(using Context) = explanation
 }
 
 class RedundantModifier(flag: Flag)(using Context)

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

@@ -561,6 +561,8 @@ object Checking {
         fail(CannotHaveSameNameAs(sym, cls, CannotHaveSameNameAs.CannotBeOverridden))
         sym.setFlag(Private) // break the overriding relationship by making sym Private
       }
+    if sym.isWrappedToplevelDef && !sym.isType && sym.flags.is(Infix, butNot = Extension) then
+      fail(ModifierNotAllowedForDefinition(Flags.Infix, s"A top-level ${sym.showKind} cannot be infix."))
     checkApplicable(Erased,
       !sym.isOneOf(MutableOrLazy, butNot = Given) && !sym.isType || sym.isClass)
     checkCombination(Final, Open)

tests/neg/i17738-toplevel-infix.check

@@ -0,0 +1,45 @@
+-- [E156] Syntax Error: tests/neg/i17738-toplevel-infix.scala:14:10 ----------------------------------------------------
+14 |infix val toplevelVal = ??? // error
+   |          ^
+   |          Modifier infix is not allowed for this definition
+   |--------------------------------------------------------------------------------------------------------------------
+   | Explanation (enabled by `-explain`)
+   |- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
+   | A top-level value cannot be infix.
+    --------------------------------------------------------------------------------------------------------------------
+-- [E156] Syntax Error: tests/neg/i17738-toplevel-infix.scala:15:10 ----------------------------------------------------
+15 |infix var toplevelVar = ??? // error
+   |          ^
+   |          Modifier infix is not allowed for this definition
+   |--------------------------------------------------------------------------------------------------------------------
+   | Explanation (enabled by `-explain`)
+   |- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
+   | A top-level variable cannot be infix.
+    --------------------------------------------------------------------------------------------------------------------
+-- [E156] Syntax Error: tests/neg/i17738-toplevel-infix.scala:16:10 ----------------------------------------------------
+16 |infix def toplevelDef = ??? // error
+   |          ^
+   |          Modifier infix is not allowed for this definition
+   |--------------------------------------------------------------------------------------------------------------------
+   | Explanation (enabled by `-explain`)
+   |- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
+   | A top-level method cannot be infix.
+    --------------------------------------------------------------------------------------------------------------------
+-- [E156] Syntax Error: tests/neg/i17738-toplevel-infix.scala:17:12 ----------------------------------------------------
+17 |infix given toplevelGiven: Int = ??? // error
+   |            ^
+   |            Modifier infix is not allowed for this definition
+   |--------------------------------------------------------------------------------------------------------------------
+   | Explanation (enabled by `-explain`)
+   |- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
+   | A top-level given instance cannot be infix.
+    --------------------------------------------------------------------------------------------------------------------
+-- [E156] Syntax Error: tests/neg/i17738-toplevel-infix.scala:18:19 ----------------------------------------------------
+18 |infix implicit val toplevelImplicit: Int = ??? // error
+   |                   ^
+   |                   Modifier infix is not allowed for this definition
+   |--------------------------------------------------------------------------------------------------------------------
+   | Explanation (enabled by `-explain`)
+   |- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
+   | A top-level value cannot be infix.
+    --------------------------------------------------------------------------------------------------------------------

tests/neg/i17738-toplevel-infix.scala

@@ -0,0 +1,18 @@
+// scalac: -explain
+infix type A[b, a] = Nothing
+
+infix type B[b, a] = b match {
+  case Int => a
+}
+
+infix class C[A, B]
+infix trait D[A, B]
+
+extension (x: Boolean)
+  infix def or (y: => Boolean) = x || y
+
+infix val toplevelVal = ??? // error
+infix var toplevelVar = ??? // error
+infix def toplevelDef = ??? // error
+infix given toplevelGiven: Int = ??? // error
+infix implicit val toplevelImplicit: Int = ??? // error