Change to new syntax in run-custom-args

And fine tune distinction of ^ as postfix or infix operator

Author: odersky

compiler/src/dotty/tools/dotc/parsing/Parsers.scala

@@ -1711,10 +1711,11 @@ object Parsers {
         })
       else if in.isIdent(nme.UPARROW)
           && (in.lookahead.token == LBRACE
-              || !canStartInfixTypeTokens.contains(in.lookahead.token))
+              || !canStartInfixTypeTokens.contains(in.lookahead.token)
+              || in.lookahead.lineOffset > 0)
         // Disambiguation: a `^` is treated as a postfix operator meaning `^{any}`
-        // if followed by `{` or newline, or any other token that cannot start
-        // an infix type. Otherwise it is treated as an infix operator.
+        // if followed by `{` or a new line (significant or not), or a token that
+        // cannot start an infix type. Otherwise it is treated as an infix operator.
       then
         val upArrowStart = in.offset
         in.nextToken()

tests/pos-custom-args/captures/trickyTrailingUpArrow.scala

@@ -0,0 +1,6 @@
+object Test:
+  var x = 0
+  type FreshContext = String^
+  x += 1
+
+  inline def ctx(using c: String) = c

tests/run-custom-args/captures/minicheck.scala

@@ -83,7 +83,7 @@ abstract class Ctx:
   def run: Run
   def detached: DetachedContext
 
-type Context = {*} Ctx
+type Context = Ctx^
 
 abstract class DetachedContext extends Ctx:
   def outer: DetachedContext
@@ -110,9 +110,9 @@ object NoContext extends FreshCtx(-1):
   owner = NoSymbol
   scope = EmptyScope
 
-type FreshContext = {*} FreshCtx
+type FreshContext = FreshCtx^
 
-inline def ctx(using c: Context): {c} Ctx = c
+inline def ctx(using c: Context): Ctx^{c} = c
 
 // !cc! it does not work if ctxStack is an Array[FreshContext] instead.
 var ctxStack = Array.tabulate(16)(new FreshCtx(_))