Formalize syntax changes

Also: Allow inline givens to be whitebox

Author: odersky

docs/docs/internals/syntax.md

@@ -386,10 +386,9 @@ ClassConstr       ::=  [ClsTypeParamClause] [ConstrMods] ClsParamClauses
 ConstrMods        ::=  {Annotation} [AccessModifier]
 ObjectDef         ::=  id [Template]                                            ModuleDef(mods, name, template)  // no constructor
 EnumDef           ::=  id ClassConstr InheritClauses EnumBody                   EnumDef(mods, name, tparams, template)
-GivenDef          ::=  [id] [DefTypeParamClause] {GivenParamClause} GivenBody
-GivenBody         ::=  [‘as’ ConstrApp {‘,’ ConstrApp }] [TemplateBody]
-                    |  ‘as’ Type ‘=’ Expr
-                    |  ‘(’ DefParam ‘)’ TemplateBody
+GivenDef          ::=  [GivenSig (‘:’ | <:)] Type ‘=’ Expr
+                    |  [GivenSig ‘:’] [ConstrApp {‘,’ ConstrApp }] [TemplateBody]
+GivenSig          ::=  [id] [DefTypeParamClause] {GivenParamClause}
 Template          ::=  InheritClauses [TemplateBody]                            Template(constr, parents, self, stats)
 InheritClauses    ::=  [‘extends’ ConstrApps] [‘derives’ QualId {‘,’ QualId}]
 ConstrApps        ::=  ConstrApp {‘with’ ConstrApp}

docs/docs/reference/contextual/delegates.md

@@ -78,11 +78,9 @@ Here is the new syntax of given instances, seen as a delta from the [standard co
 ```
 TmplDef          ::=  ...
                   |   ‘given’ GivenDef
-GivenDef         ::=  GivenBody
-                  |   [id] [DefTypeParamClause] {GivenParamClause}
-                      (‘:’ GivenBody | ‘<:’ Type ‘=’ Expr)
-GivenBody        ::=  [ConstrApp {‘,’ ConstrApp }] [TemplateBody]
-                  |   Type ‘=’ Expr
+GivenDef         ::=  [GivenSig (‘:’ | <:)] Type ‘=’ Expr
+                  |   [GivenSig ‘:’] [ConstrApp {‘,’ ConstrApp }] [TemplateBody]
+GivenSig         ::=  [id] [DefTypeParamClause] {GivenParamClause}
 GivenParamClause ::=  ‘(’ ‘given’ (DefParams | GivenTypes) ‘)’
 GivenTypes       ::=  AnnotType {‘,’ AnnotType}
 ```