Restrict given instance syntax

If a type is implemented, it must be an `AnnotType`, which is the only
kind of type allowed in a ConstrApp or extends clause.

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] (Type [‘=’ Expr] | ConstrApps TemplateBody)
+    /** GivenDef          ::=  [GivenSig] (AnnotType [‘=’ Expr] | ConstrApps TemplateBody)
      *  GivenSig          ::=  [id] [DefTypeParamClause] {UsingParamClauses} ‘:’
      */
     def givenDef(start: Offset, mods: Modifiers, givenMod: Mod) = atSpan(start, nameStart) {
@@ -3567,7 +3567,9 @@ object Parsers {
         newLinesOpt()
         val noParams = tparams.isEmpty && vparamss.isEmpty
         if !(name.isEmpty && noParams) then accept(COLON)
-        val parents = if isSimpleLiteral then toplevelTyp() :: Nil else constrApps()
+        val parents =
+          if isSimpleLiteral then rejectWildcardType(annotType()) :: Nil
+          else constrApps()
         val parentsIsType = parents.length == 1 && parents.head.isType
         newLineOptWhenFollowedBy(LBRACE)
         if in.token == EQUALS && parentsIsType then

docs/docs/internals/syntax.md

@@ -385,7 +385,7 @@ ClassConstr       ::=  [ClsTypeParamClause] [ConstrMods] ClsParamClauses
 ConstrMods        ::=  {Annotation} [AccessModifier]
 ObjectDef         ::=  id [Template]                                            ModuleDef(mods, name, template)  // no constructor
 EnumDef           ::=  id ClassConstr InheritClauses EnumBody
-GivenDef          ::=  [GivenSig] (Type [‘=’ Expr] | ConstrApps TemplateBody)
+GivenDef          ::=  [GivenSig] (AnnotType [‘=’ Expr] | ConstrApps TemplateBody)
 GivenSig          ::=  [id] [DefTypeParamClause] {UsingParamClause} ‘:’         -- one of `id`, `DefParamClause`, `UsingParamClause` must be present
 Extension         ::=  ‘extension’ [DefTypeParamClause] ‘(’ DefParam ‘)’
                        {UsingParamClause}] ExtMethods

docs/docs/reference/contextual/givens.md

@@ -154,8 +154,8 @@ Here is the syntax for given instances:
 TmplDef             ::=  ...
                      |   ‘given’ GivenDef
 GivenDef            ::=  [GivenSig] ConstrApps TemplateBody
-                     |   [GivenSig] Type ‘=’ Expr
-                     |   [GivenSig] Type
+                     |   [GivenSig] AnnotType ‘=’ Expr
+                     |   [GivenSig] AnnotType
 GivenSig            ::=  [id] [DefTypeParamClause] {UsingParamClause} ‘:’
 ```