Fix #12260: Add underscore to match type syntax

Author: OlivierBlanvillain

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

@@ -2594,12 +2594,19 @@ object Parsers {
       })
     }
 
-    /** TypeCaseClause     ::= ‘case’ InfixType ‘=>’ Type [nl]
+    /** TypeCaseClause     ::= ‘case’ (InfixType | ‘_’) ‘=>’ Type [nl]
      */
     def typeCaseClause(): CaseDef = atSpan(in.offset) {
       val pat = inSepRegion(InCase) {
         accept(CASE)
-        infixType()
+        in.token match {
+          case USCORE if in.lookahead.isArrow =>
+            val start = in.skipToken()
+            typeBounds().withSpan(Span(start, in.lastOffset, start))
+
+          case _ =>
+            infixType()
+        }
       }
       CaseDef(pat, EmptyTree, atSpan(accept(ARROW)) {
         val t = typ()

docs/docs/internals/syntax.md

@@ -286,7 +286,7 @@ CaseClauses       ::=  CaseClause { CaseClause }
 CaseClause        ::=  ‘case’ Pattern [Guard] ‘=>’ Block                        CaseDef(pat, guard?, block)   // block starts at =>
 ExprCaseClause    ::=  ‘case’ Pattern [Guard] ‘=>’ Expr
 TypeCaseClauses   ::=  TypeCaseClause { TypeCaseClause }
-TypeCaseClause    ::=  ‘case’ InfixType ‘=>’ Type [nl]
+TypeCaseClause    ::=  ‘case’ (InfixType | ‘_’) ‘=>’ Type [nl]
 
 Pattern           ::=  Pattern1 { ‘|’ Pattern1 }                                Alternative(pats)
 Pattern1          ::=  Pattern2 [‘:’ RefinedType]                               Bind(name, Typed(Ident(wildcard), tpe))

tests/pos/unify-wildcard-patterns.scala

@@ -0,0 +1,13 @@
+// `case _ => expr` in a match expression should be equivalant to
+// `case _: Any => expr`. Likewise, in a match type, `case _ => T`
+// should be equivalant to `case Any => T`.
+
+object Test0 {
+  type M[X] =            X match { case String => Int   case Any => String }
+  def m[X](x: X): M[X] = x match { case _: String => 1  case _: Any => "s" }
+}
+
+object Test2 {
+  type M[X] =            X match { case String => Int   case _ => String   }
+  def m[X](x: X): M[X] = x match { case _: String => 1  case _: Any => "s" }
+}