Revert "Adapt given instance definitions"

This reverts commit b0933d3.

# Conflicts:
#	compiler/src/dotty/tools/dotc/parsing/Parsers.scala
#	docs/docs/internals/syntax.md
#	docs/docs/reference/contextual/givens.md

Author: odersky

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

@@ -3549,7 +3549,7 @@ object Parsers {
         syntaxError(i"extension clause can only define methods", stat.span)
     }
 
-    /** GivenDef          ::=  [GivenSig] (AnnotType [‘=’ Expr] | ConstrApps TemplateBody)
+    /** GivenDef          ::=  [GivenSig] (AnnotType [‘=’ Expr] | StructuralInstance)
      *  GivenSig          ::=  [id] [DefTypeParamClause] {UsingParamClauses} ‘:’
      */
     def givenDef(start: Offset, mods: Modifiers, givenMod: Mod) = atSpan(start, nameStart) {
@@ -3569,9 +3569,8 @@ object Parsers {
         if !(name.isEmpty && noParams) then accept(COLON)
         val parents =
           if isSimpleLiteral then rejectWildcardType(annotType()) :: Nil
-          else constrApps()
+          else constrApp() :: withConstrApps()
         val parentsIsType = parents.length == 1 && parents.head.isType
-        newLineOptWhenFollowedBy(LBRACE)
         if in.token == EQUALS && parentsIsType then
           accept(EQUALS)
           mods1 |= Final
@@ -3580,17 +3579,17 @@ object Parsers {
             ValDef(name, parents.head, subExpr())
           else
             DefDef(name, joinParams(tparams, vparamss), parents.head, subExpr())
-        else if isTemplateBodyStart then
+        else if in.token != WITH && parentsIsType then
+          if name.isEmpty then
+            syntaxError(em"anonymous given cannot be abstract")
+          DefDef(name, joinParams(tparams, vparamss), parents.head, EmptyTree)
+        else
           val tparams1 = tparams.map(tparam => tparam.withMods(tparam.mods | PrivateLocal))
           val vparamss1 = vparamss.map(_.map(vparam =>
             vparam.withMods(vparam.mods &~ Param | ParamAccessor | Protected)))
-          val templ = templateBodyOpt(makeConstructor(tparams1, vparamss1), parents, Nil)
+          val templ = withTemplate(makeConstructor(tparams1, vparamss1), parents)
           if noParams then ModuleDef(name, templ)
           else TypeDef(name.toTypeName, templ)
-        else
-          if name.isEmpty then
-            syntaxError(em"anonymous given cannot be abstract")
-          DefDef(name, joinParams(tparams, vparamss), parents.head, EmptyTree)
       end gdef
       finalizeDef(gdef, mods1, start)
     }
@@ -3780,6 +3779,14 @@ object Parsers {
       vd
     }
 
+    /** with Template, with EOL <indent> interpreted */
+    def withTemplate(constr: DefDef, parents: List[Tree]): Template =
+      if in.token != WITH then syntaxError(em"`with` expected")
+      possibleTemplateStart() // consumes a WITH token
+      val (self, stats) = templateBody()
+      Template(constr, parents, Nil, self, stats)
+        .withSpan(Span(constr.span.orElse(parents.head.span).start, in.lastOffset))
+
 /* -------- STATSEQS ------------------------------------------- */
 
     /** Create a tree representing a packaging */

docs/docs/internals/syntax.md

@@ -385,8 +385,9 @@ ClassConstr       ::=  [ClsTypeParamClause] [ConstrMods] ClsParamClauses
 ConstrMods        ::=  {Annotation} [AccessModifier]
 ObjectDef         ::=  id [Template]                                            ModuleDef(mods, name, template)  // no constructor
 EnumDef           ::=  id ClassConstr InheritClauses EnumBody
-GivenDef          ::=  [GivenSig] (AnnotType [‘=’ Expr] | ConstrApps TemplateBody)
+GivenDef          ::=  [GivenSig] (AnnotType [‘=’ Expr] | StructuralInstance)
 GivenSig          ::=  [id] [DefTypeParamClause] {UsingParamClause} ‘:’         -- one of `id`, `DefParamClause`, `UsingParamClause` must be present
+StructuralInstance ::=  ConstrApp {‘with’ ConstrApp} TemplateBody
 Extension         ::=  ‘extension’ [DefTypeParamClause] ‘(’ DefParam ‘)’
                        {UsingParamClause}] ExtMethods
 ExtMethods        ::=  ExtMethod | [nl] ‘{’ ExtMethod {semi ExtMethod ‘}’

docs/docs/reference/contextual/givens.md

@@ -153,10 +153,11 @@ Here is the syntax for given instances:
 ```
 TmplDef             ::=  ...
                      |   ‘given’ GivenDef
-GivenDef            ::=  [GivenSig] ConstrApps TemplateBody
+GivenDef            ::=  [GivenSig] StructuralInstance
                      |   [GivenSig] AnnotType ‘=’ Expr
                      |   [GivenSig] AnnotType
 GivenSig            ::=  [id] [DefTypeParamClause] {UsingParamClause} ‘:’
+StructuralInstance  ::=  ConstrApp {‘with’ ConstrApp} ‘with’ TemplateBody
 ```
 
 A given instance starts with the reserved word `given` and an optional _signature_. The signature

tests/pos-custom-args/erased/i7868.scala

@@ -12,14 +12,13 @@ object Coproduct {
 
   object At {
 
-    given atHead[Head, Tail]: At[Head +: Tail, Head, 0] {
+    given atHead[Head, Tail]: At[Head +: Tail, Head, 0] with {
       def cast: Head <:< Head +: Tail = summon[Head <:< Head +: Tail]
     }
 
     given atTail[Head, Tail, Value, NextIndex <: Int]
           (using atNext: At[Tail, Value, NextIndex])
-      : At[Head +: Tail, Value, S[NextIndex]]
-    with
+      : At[Head +: Tail, Value, S[NextIndex]] with
       val cast: Value <:< Head +: Tail = atNext.cast
 
     given [A](using A): (() => A) = { () => summon[A]}