fix(#19402): error message for missing using in given params

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

@@ -4005,13 +4005,17 @@ object Parsers {
     def givenDef(start: Offset, mods: Modifiers, givenMod: Mod) = atSpan(start, nameStart) {
       var mods1 = addMod(mods, givenMod)
       val nameStart = in.offset
-      val name = if isIdent && followingIsGivenSig() then ident() else EmptyTermName
 
+      val givenSigPart =
+        if isIdent && followingIsGivenSig() then Some(ident())
+        else if followingIsGivenSig() then Some(EmptyTermName)
+        else None
+      val name = givenSigPart.getOrElse(EmptyTermName)
       val gdef =
         val tparams = typeParamClauseOpt(ParamOwner.Given)
         newLineOpt()
         val vparamss =
-          if in.token == LPAREN && in.lookahead.isIdent(nme.using)
+          if in.token == LPAREN && givenSigPart.nonEmpty
           then termParamClauses(ParamOwner.Given)
           else Nil
         newLinesOpt()

tests/neg/i19402.check

@@ -0,0 +1,4 @@
+-- Error: tests/neg/i19402.scala:9:12 ----------------------------------------------------------------------------------
+9 |  given bar(foo: Foo): Bar = Bar(foo) // error
+  |            ^^^
+  |            `using` expected

tests/neg/i19402.scala

@@ -0,0 +1,11 @@
+object Example {
+
+  class Bar(foo: Foo)
+
+  class Foo
+
+  given Foo = Foo()
+
+  given bar(foo: Foo): Bar = Bar(foo) // error
+
+}

tests/neg/i8150.check

@@ -0,0 +1,16 @@
+-- Error: tests/neg/i8150.scala:3:16 -----------------------------------------------------------------------------------
+3 |type T = {given A with A} // error
+  |          ^^^^^^^^^^^^^^
+  |          refinement cannot be `given`
+-- [E040] Syntax Error: tests/neg/i8150.scala:4:23 ---------------------------------------------------------------------
+4 |type T = {given(using a: A) as B} // error // error
+  |                       ^
+  |                       an identifier expected, but ':' found
+-- [E040] Syntax Error: tests/neg/i8150.scala:4:28 ---------------------------------------------------------------------
+4 |type T = {given(using a: A) as B} // error // error
+  |                            ^^
+  |                            'with' expected, but identifier found
+-- [E040] Syntax Error: tests/neg/i8150.scala:5:8 ----------------------------------------------------------------------
+5 |// error
+  |        ^
+  |        '}' expected, but eof found

tests/neg/i8150.scala

@@ -1,3 +1,5 @@
 trait A
 trait B
-type T = {given(using a: A) as B} // error: refinement cannot be `given`
+type T = {given A with A} // error
+type T = {given(using a: A) as B} // error // error
+// error