Fix syntax of extension methods

Allow leading using parameter clauses. This aligns spec and actual parser.

Fixes #13027

Author: odersky

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

@@ -3627,7 +3627,7 @@ object Parsers {
       finalizeDef(gdef, mods1, start)
     }
 
-    /** Extension  ::=  ‘extension’ [DefTypeParamClause] ‘(’ DefParam ‘)’
+    /** Extension  ::=  ‘extension’ [DefTypeParamClause] {UsingParamClause} ‘(’ DefParam ‘)’
      *                  {UsingParamClause} ExtMethods
      */
     def extension(): ExtMethods =

docs/docs/internals/syntax.md

@@ -414,8 +414,8 @@ EnumDef           ::=  id ClassConstr InheritClauses EnumBody
 GivenDef          ::=  [GivenSig] (AnnotType [‘=’ Expr] | StructuralInstance)
 GivenSig          ::=  [id] [DefTypeParamClause] {UsingParamClause} ‘:’         -- one of `id`, `DefParamClause`, `UsingParamClause` must be present
 StructuralInstance ::=  ConstrApp {‘with’ ConstrApp} [‘with’ TemplateBody]
-Extension         ::=  ‘extension’ [DefTypeParamClause] ‘(’ DefParam ‘)’
-                       {UsingParamClause} ExtMethods
+Extension         ::=  ‘extension’ [DefTypeParamClause] {UsingParamClause}
+                       ‘(’ DefParam ‘)’ {UsingParamClause} ExtMethods
 ExtMethods        ::=  ExtMethod | [nl] <<< ExtMethod {semi ExtMethod} >>>
 ExtMethod         ::=  {Annotation [nl]} {Modifier} ‘def’ DefDef
 Template          ::=  InheritClauses [TemplateBody]

docs/docs/reference/contextual/extension-methods.md

@@ -288,8 +288,8 @@ to the [current syntax](../syntax.md).
 BlockStat         ::=  ... | Extension
 TemplateStat      ::=  ... | Extension
 TopStat           ::=  ... | Extension
-Extension         ::=  ‘extension’ [DefTypeParamClause] ‘(’ DefParam ‘)’
-                       {UsingParamClause} ExtMethods
+Extension         ::=  ‘extension’ [DefTypeParamClause] {UsingParamClause}
+                       ‘(’ DefParam ‘)’ {UsingParamClause} ExtMethods
 ExtMethods        ::=  ExtMethod | [nl] <<< ExtMethod {semi ExtMethod} >>>
 ExtMethod         ::=  {Annotation [nl]} {Modifier} ‘def’ DefDef
 ```

docs/docs/reference/syntax.md

@@ -400,8 +400,8 @@ EnumDef           ::=  id ClassConstr InheritClauses EnumBody
 GivenDef          ::=  [GivenSig] (AnnotType [‘=’ Expr] | StructuralInstance)
 GivenSig          ::=  [id] [DefTypeParamClause] {UsingParamClause} ‘:’         -- one of `id`, `DefParamClause`, `UsingParamClause` must be present
 StructuralInstance ::=  ConstrApp {‘with’ ConstrApp} [‘with’ TemplateBody]
-Extension         ::=  ‘extension’ [DefTypeParamClause] ‘(’ DefParam ‘)’
-                       {UsingParamClause} ExtMethods
+Extension         ::=  ‘extension’ [DefTypeParamClause] {UsingParamClause}
+                       ‘(’ DefParam ‘)’ {UsingParamClause} ExtMethods
 ExtMethods        ::=  ExtMethod | [nl] <<< ExtMethod {semi ExtMethod} >>>
 ExtMethod         ::=  {Annotation [nl]} {Modifier} ‘def’ DefDef
 Template          ::=  InheritClauses [TemplateBody]