fix: disallow toplevel infix definitions for vals, vars, givens, methods and implicits

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

@@ -199,6 +199,7 @@ enum ErrorMessageID(val isActive: Boolean = true) extends java.lang.Enum[ErrorMe
   case ClosureCannotHaveInternalParameterDependenciesID // errorNumber: 183
   case MatchTypeNoCasesID // errorNumber: 184
   case UnimportedAndImportedID // errorNumber: 185
+  case ToplevelDefCantBeInfixID // errorNumber: 186
 
   def errorNumber = ordinal - 1
 

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

@@ -2938,3 +2938,13 @@ class ClosureCannotHaveInternalParameterDependencies(mt: Type)(using Context)
       i"""cannot turn method type $mt into closure
          |because it has internal parameter dependencies"""
     def explain(using Context) = ""
+
+class ToplevelDefCantBeInfix(sym: Symbol)(using Context)
+  extends SyntaxMsg(ToplevelDefCantBeInfixID):
+    def msg(using Context) = i"a toplevel $defName cannot be infix"
+    def explain(using Context) = ""
+    private val defName = 
+      if sym.flags.is(Method) then "def"
+      else if sym.flags.is(Mutable) then "var"
+      else if sym.flags.is(Given) then "given"
+      else "val"

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

@@ -561,6 +561,9 @@ 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(ToplevelDefCantBeInfix(sym))
     checkApplicable(Erased,
       !sym.isOneOf(MutableOrLazy, butNot = Given) && !sym.isType || sym.isClass)
     checkCombination(Final, Open)

tests/neg/i17738-toplevel-infix.check

@@ -0,0 +1,20 @@
+-- [E186] Syntax Error: tests/neg/i17738-toplevel-infix.scala:13:10 ----------------------------------------------------
+13 |infix val toplevelVal = ??? // error
+   |          ^
+   |          a toplevel val cannot be infix
+-- [E186] Syntax Error: tests/neg/i17738-toplevel-infix.scala:14:10 ----------------------------------------------------
+14 |infix var toplevelVar = ??? // error
+   |          ^
+   |          a toplevel var cannot be infix
+-- [E186] Syntax Error: tests/neg/i17738-toplevel-infix.scala:15:10 ----------------------------------------------------
+15 |infix def toplevelDef = ??? // error
+   |          ^
+   |          a toplevel def cannot be infix
+-- [E186] Syntax Error: tests/neg/i17738-toplevel-infix.scala:16:12 ----------------------------------------------------
+16 |infix given toplevelGiven: Int = ??? // error
+   |            ^
+   |            a toplevel given cannot be infix
+-- [E186] Syntax Error: tests/neg/i17738-toplevel-infix.scala:17:19 ----------------------------------------------------
+17 |infix implicit val topevelImplicit: Int = ??? // error
+   |                   ^
+   |                   a toplevel val cannot be infix

tests/neg/i17738-toplevel-infix.scala

@@ -0,0 +1,17 @@
+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 topevelImplicit: Int = ??? // error