Fix #10074: Disallow given as syntax

Author: odersky

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

@@ -3544,7 +3544,7 @@ object Parsers {
           then paramClauses(givenOnly = true)
           else Nil
         newLinesOpt()
-        if isIdent(nme.as) || !name.isEmpty || !tparams.isEmpty || !vparamss.isEmpty then
+        if !name.isEmpty || !tparams.isEmpty || !vparamss.isEmpty then
           accept(nme.as)
         val parents = constrApps(commaOK = true, templateCanFollow = true)
         if in.token == EQUALS && parents.length == 1 && parents.head.isType then

docs/docs/internals/syntax.md

@@ -401,7 +401,7 @@ ObjectDef         ::=  id [Template]
 EnumDef           ::=  id ClassConstr InheritClauses EnumBody                   EnumDef(mods, name, tparams, template)
 GivenDef          ::=  [GivenSig] Type ‘=’ Expr
                     |  [GivenSig] ConstrApps [TemplateBody]
-GivenSig          ::=  [id] [DefTypeParamClause] {UsingParamClause} ‘as’
+GivenSig          ::=  [id] [DefTypeParamClause] {UsingParamClause} ‘as’       -- one of `id`, `DefParamClause`, `UsingParamClause` must appear
 Extension         ::=  ‘extension’ [DefTypeParamClause] ‘(’ DefParam ‘)’
                        {UsingParamClause}] ExtMethods
 ExtMethods        ::=  ExtMethod | [nl] ‘{’ ExtMethod {semi ExtMethod ‘}’

tests/neg/i10074.scala

@@ -0,0 +1,3 @@
+class Context
+given as as Context = ???  // ok
+given as Context = ???     // error // error // error