Contextual varargs parameters (#18186)

fixes #18090

Author: nicolasstucki

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

@@ -3334,11 +3334,13 @@ object Parsers {
 
       def checkVarArgsRules(vparams: List[ValDef]): Unit = vparams match {
         case Nil =>
-        case _ :: Nil if !prefix =>
         case vparam :: rest =>
           vparam.tpt match {
             case PostfixOp(_, op) if op.name == tpnme.raw.STAR =>
-              syntaxError(VarArgsParamMustComeLast(), vparam.tpt.span)
+              if vparam.mods.isOneOf(GivenOrImplicit) then
+                syntaxError(VarArgsParamCannotBeGiven(vparam.mods.is(Given)), vparam.tpt.span)
+              if rest.nonEmpty then
+                syntaxError(VarArgsParamMustComeLast(), vparam.tpt.span)
             case _ =>
           }
           checkVarArgsRules(rest)

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

@@ -201,6 +201,7 @@ enum ErrorMessageID(val isActive: Boolean = true) extends java.lang.Enum[ErrorMe
   case UnimportedAndImportedID // errorNumber: 185
   case ImplausiblePatternWarningID // erorNumber: 186
   case SynchronizedCallOnBoxedClassID // errorNumber: 187
+  case VarArgsParamCannotBeGivenID // erorNumber: 188
 
   def errorNumber = ordinal - 1
 

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

@@ -1319,6 +1319,14 @@ extends SyntaxMsg(VarArgsParamMustComeLastID) {
         |"""
 }
 
+class VarArgsParamCannotBeGiven(isGiven: Boolean)(using Context)
+extends SyntaxMsg(VarArgsParamCannotBeGivenID) {
+  def msg(using Context) = i"repeated parameters are not allowed in a ${if isGiven then "using" else "implicit"} clause"
+  def explain(using Context) =
+    "It is not possible to define a given with a repeated parameter type. This hypothetical given parameter could always be satisfied by providing 0 arguments, which defeats the purpose of a given argument."
+}
+
+
 import typer.Typer.BindingPrec
 
 class ConstrProxyShadows(proxy: TermRef, shadowed: Type, shadowedIsApply: Boolean)(using Context)

tests/neg/i18090.check

@@ -0,0 +1,42 @@
+-- [E188] Syntax Error: tests/neg/i18090.scala:2:18 --------------------------------------------------------------------
+2 |def foo(using xs: Int*) = xs // error
+  |                  ^^^^
+  |                  repeated parameters are not allowed in a using clause
+  |
+  | longer explanation available when compiling with `-explain`
+-- [E188] Syntax Error: tests/neg/i18090.scala:3:27 --------------------------------------------------------------------
+3 |def foo5(using d: Int, xs: Int*) = xs // error
+  |                           ^^^^
+  |                           repeated parameters are not allowed in a using clause
+  |
+  | longer explanation available when compiling with `-explain`
+-- [E188] Syntax Error: tests/neg/i18090.scala:4:22 --------------------------------------------------------------------
+4 |def foo2(implicit xs: Int*) = xs // error
+  |                      ^^^^
+  |                      repeated parameters are not allowed in a implicit clause
+  |
+  | longer explanation available when compiling with `-explain`
+-- [E188] Syntax Error: tests/neg/i18090.scala:5:35 --------------------------------------------------------------------
+5 |def foo3(u: Int)(using d: Int, xs: Int*) = xs // error
+  |                                   ^^^^
+  |                                   repeated parameters are not allowed in a using clause
+  |
+  | longer explanation available when compiling with `-explain`
+-- [E188] Syntax Error: tests/neg/i18090.scala:6:38 --------------------------------------------------------------------
+6 |def foo4(u: Int)(implicit d: Int, xs: Int*) = xs // error
+  |                                      ^^^^
+  |                                      repeated parameters are not allowed in a implicit clause
+  |
+  | longer explanation available when compiling with `-explain`
+-- [E188] Syntax Error: tests/neg/i18090.scala:9:20 --------------------------------------------------------------------
+9 |  def bar(using xs: Float*) = ??? // error
+  |                    ^^^^^^
+  |                    repeated parameters are not allowed in a using clause
+  |
+  | longer explanation available when compiling with `-explain`
+-- [E188] Syntax Error: tests/neg/i18090.scala:10:33 -------------------------------------------------------------------
+10 |  def bar2(using d: Boolean, xs: Float*) = ??? // error
+   |                                 ^^^^^^
+   |                                 repeated parameters are not allowed in a using clause
+   |
+   | longer explanation available when compiling with `-explain`

tests/neg/i18090.scala

@@ -0,0 +1,10 @@
+
+def foo(using xs: Int*) = xs // error
+def foo5(using d: Int, xs: Int*) = xs // error
+def foo2(implicit xs: Int*) = xs // error
+def foo3(u: Int)(using d: Int, xs: Int*) = xs // error
+def foo4(u: Int)(implicit d: Int, xs: Int*) = xs // error
+
+extension (i: Int)
+  def bar(using xs: Float*) = ??? // error
+  def bar2(using d: Boolean, xs: Float*) = ??? // error