Fix #8096: Change syntax of single-case try

Author: odersky

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

@@ -1863,6 +1863,7 @@ object Parsers {
      *  Ascription        ::=  `:' InfixType
      *                      |  `:' Annotation {Annotation}
      *                      |  `:' `_' `*'
+     *  Catches           ::=  ‘catch’ (Expr | ExprCaseClause)
      */
     val exprInParens: () => Tree = () => expr(Location.InParens)
 
@@ -1946,7 +1947,7 @@ object Parsers {
               if in.token == CATCH then
                 val span = in.offset
                 in.nextToken()
-                (if in.token == CASE then Match(EmptyTree, caseClause() :: Nil)
+                (if in.token == CASE then Match(EmptyTree, caseClause(exprOnly = true) :: Nil)
                  else subExpr(),
                  span)
               else (EmptyTree, -1)
@@ -2072,7 +2073,7 @@ object Parsers {
     def matchClause(t: Tree): Match =
       in.endMarkerScope(MATCH) {
         atSpan(t.span.start, in.skipToken()) {
-          Match(t, inBracesOrIndented(caseClauses(caseClause)))
+          Match(t, inBracesOrIndented(caseClauses(() => caseClause())))
         }
       }
 
@@ -2366,7 +2367,7 @@ object Parsers {
     def blockExpr(): Tree = atSpan(in.offset) {
       val simplify = in.token == INDENT
       inDefScopeBraces {
-        if (in.token == CASE) Match(EmptyTree, caseClauses(caseClause))
+        if (in.token == CASE) Match(EmptyTree, caseClauses(() => caseClause()))
         else block(simplify)
       }
     }
@@ -2537,18 +2538,21 @@ object Parsers {
     }
 
     /** CaseClause         ::= ‘case’ Pattern [Guard] `=>' Block
+     *  ExprCaseClause    ::=  ‘case’ Pattern [Guard] ‘=>’ Expr
      */
-    val caseClause: () => CaseDef = () => atSpan(in.offset) {
+    def caseClause(exprOnly: Boolean = false): CaseDef = atSpan(in.offset) {
       val (pat, grd) = inSepRegion(LPAREN, RPAREN) {
         accept(CASE)
         (pattern(), guard())
       }
-      CaseDef(pat, grd, atSpan(accept(ARROW)) { block() })
+      CaseDef(pat, grd, atSpan(accept(ARROW)) {
+        if exprOnly then expr() else block()
+      })
     }
 
     /** TypeCaseClause     ::= ‘case’ InfixType ‘=>’ Type [nl]
      */
-    val typeCaseClause: () => CaseDef = () => atSpan(in.offset) {
+    def typeCaseClause(): CaseDef = atSpan(in.offset) {
       val pat = inSepRegion(LPAREN, RPAREN) {
         accept(CASE)
         infixType()

docs/docs/internals/syntax.md

@@ -203,7 +203,7 @@ Expr1             ::=  [‘inline’] ‘if’ ‘(’ Expr ‘)’ {nl} Expr [[
                     |  ‘inline’ InfixExpr MatchClause
 Ascription        ::=  ‘:’ InfixType                                            Typed(expr, tp)
                     |  ‘:’ Annotation {Annotation}                              Typed(expr, Annotated(EmptyTree, annot)*)
-Catches           ::=  ‘catch’ (Expr | CaseClause)
+Catches           ::=  ‘catch’ (Expr | ExprCaseClause)
 PostfixExpr       ::=  InfixExpr [id]                                           PostfixOp(expr, op)
 InfixExpr         ::=  PrefixExpr
                     |  InfixExpr id [nl] InfixExpr                              InfixOp(expr, op, expr)
@@ -256,7 +256,8 @@ Generator         ::=  [‘case’] Pattern1 ‘<-’ Expr
 Guard             ::=  ‘if’ PostfixExpr
 
 CaseClauses       ::=  CaseClause { CaseClause }                                Match(EmptyTree, cases)
-CaseClause        ::=  ‘case’ (Pattern [Guard] ‘=>’ Block | INT)                CaseDef(pat, guard?, block)   // block starts at =>
+CaseClause        ::=  ‘case’ Pattern [Guard] ‘=>’ Block                        CaseDef(pat, guard?, block)   // block starts at =>
+ExprCaseClause    ::=  ‘case’ Pattern [Guard] ‘=>’ Expr
 ImplicitCaseClauses ::=  ImplicitCaseClause { ImplicitCaseClause }
 ImplicitCaseClause  ::=  ‘case’ PatVar [‘:’ RefinedType] [Guard] ‘=>’ Block
 TypeCaseClauses   ::=  TypeCaseClause { TypeCaseClause }

tests/run/i8096.scala

@@ -0,0 +1,10 @@
+def k: String =
+  try
+    "WUT"
+  catch case t =>
+    println(s"Caught a case of $t")
+  "OK"
+
+@main def Test =
+  assert(k == "OK")
+