Fix #9746: Require then in new conditional syntax

Author: odersky

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

@@ -357,9 +357,6 @@ object Parsers {
       offset
     }
 
-    def reportMissing(expected: Token): Unit =
-      syntaxError(ExpectedTokenButFound(expected, in.token))
-
     /** semi = nl {nl} | `;'
      *  nl  = `\n' // where allowed
      */
@@ -1825,7 +1822,7 @@ object Parsers {
      *  the initially parsed (...) region?
      */
     def toBeContinued(altToken: Token): Boolean =
-      if in.token == altToken || in.isNewLine || migrateTo3 then
+      if in.isNewLine || migrateTo3 then
         false // a newline token means the expression is finished
       else if !in.canStartStatTokens.contains(in.token)
               || in.isLeadingInfixOperator(inConditional = true)
@@ -1835,37 +1832,27 @@ object Parsers {
         followedByToken(altToken) // scan ahead to see whether we find a `then` or `do`
 
     def condExpr(altToken: Token): Tree =
-      if in.token == LPAREN then
-        var t: Tree = atSpan(in.offset) { Parens(inParens(exprInParens())) }
-        val enclosedInParens = !toBeContinued(altToken)
-        if !enclosedInParens then
-          t = inSepRegion(InCond) {
-            expr1Rest(postfixExprRest(simpleExprRest(t)), Location.ElseWhere)
-          }
-        if in.token == altToken then
-          if rewriteToOldSyntax() then revertToParens(t)
-          in.nextToken()
+      val t: Tree =
+        if in.token == LPAREN then
+          var t: Tree = atSpan(in.offset) { Parens(inParens(exprInParens())) }
+          if in.token != altToken then
+            if toBeContinued(altToken) then
+              t = inSepRegion(InCond) {
+                expr1Rest(postfixExprRest(simpleExprRest(t)), Location.ElseWhere)
+              }
+            else
+              if rewriteToNewSyntax(t.span) then
+                dropParensOrBraces(t.span.start, s"${tokenString(altToken)}")
+              in.observeIndented()
+              return t
+          t
+        else if in.isNestedStart then
+          try expr() finally newLinesOpt()
         else
-          if (altToken == THEN || enclosedInParens) && in.isNewLine then
-            in.observeIndented()
-          if !enclosedInParens && in.token != INDENT then reportMissing(altToken)
-          if (rewriteToNewSyntax(t.span))
-            dropParensOrBraces(t.span.start, s"${tokenString(altToken)}")
-        t
-      else
-        val t =
-          if in.isNestedStart then
-            try expr() finally newLinesOpt()
-          else
-            inSepRegion(InCond)(expr())
-        if rewriteToOldSyntax(t.span.startPos) then
-          revertToParens(t)
-        if altToken == THEN && in.isNewLine then
-          // don't require a `then` at the end of a line
-          in.observeIndented()
-        if in.token != INDENT then accept(altToken)
-        t
-    end condExpr
+          inSepRegion(InCond)(expr())
+      if rewriteToOldSyntax(t.span.startPos) then revertToParens(t)
+      accept(altToken)
+      t
 
     /** Expr              ::=  [`implicit'] FunParams (‘=>’ | ‘?=>’) Expr
      *                      |  Expr1

compiler/src/dotty/tools/dotc/semanticdb/ExtractSemanticDB.scala

@@ -546,7 +546,7 @@ class ExtractSemanticDB extends Phase:
 
     extension (sym: Symbol):
       private def adjustIfCtorTyparam(using Context) =
-        if sym.isType && sym.owner.exists && sym.owner.isConstructor
+        if sym.isType && sym.owner.exists && sym.owner.isConstructor then
           matchingMemberType(sym, sym.owner.owner)
         else
           sym
@@ -557,20 +557,20 @@ class ExtractSemanticDB extends Phase:
     /**Necessary because not all of the eventual flags are propagated from the Tree to the symbol yet.
      */
     private def symbolKinds(tree: NamedDefTree)(using Context): Set[SymbolKind] =
-      if tree.symbol.isSelfSym
+      if tree.symbol.isSelfSym then
         Set.empty
       else
         val symkinds = mutable.HashSet.empty[SymbolKind]
         tree match
         case tree: ValDef =>
-          if !tree.symbol.is(Param)
+          if !tree.symbol.is(Param) then
             symkinds += (if tree.mods is Mutable then SymbolKind.Var else SymbolKind.Val)
-          if tree.rhs.isEmpty && !tree.symbol.isOneOf(TermParam | CaseAccessor | ParamAccessor)
+          if tree.rhs.isEmpty && !tree.symbol.isOneOf(TermParam | CaseAccessor | ParamAccessor) then
             symkinds += SymbolKind.Abstract
         case tree: DefDef =>
-          if tree.isSetterDef
+          if tree.isSetterDef then
             symkinds += SymbolKind.Setter
-          else if tree.rhs.isEmpty
+          else if tree.rhs.isEmpty then
             symkinds += SymbolKind.Abstract
         case tree: Bind =>
           symkinds += SymbolKind.Val
@@ -584,7 +584,7 @@ class ExtractSemanticDB extends Phase:
         vparams <- vparamss
         vparam  <- vparams
       do
-        if !excludeSymbol(vparam.symbol)
+        if !excludeSymbol(vparam.symbol) then
           traverseAnnotsOfDefinition(vparam.symbol)
           val symkinds =
             getters.get(vparam.name).fold(SymbolKind.emptySet)(getter =>

compiler/src/dotty/tools/dotc/semanticdb/Scala3.scala

@@ -58,9 +58,9 @@ object Scala3:
         displaySymbol(symbol.owner)
       else if symbol.is(ModuleClass) then
         displaySymbol(symbol.sourceModule)
-      else if symbol == defn.RootPackage
+      else if symbol == defn.RootPackage then
         RootPackageName
-      else if symbol.isEmptyPackage
+      else if symbol.isEmptyPackage then
         EmptyPackageName
       else
         symbol.name.show
@@ -131,7 +131,7 @@ object Scala3:
     def unapply(symbolInfo: SymbolInformation): Option[Int] = symbolInfo.symbol match
       case locals(ints) =>
         val bi = BigInt(ints)
-        if bi.isValidInt
+        if bi.isValidInt then
           Some(bi.toInt)
         else
           None
@@ -242,14 +242,14 @@ object Scala3:
       while i < len do
         val a = o1.charAt(i)
         val b = o2.charAt(i)
-        if a.isDigit && b.isDigit
+        if a.isDigit && b.isDigit then
           val byDigit = Integer.compare(toDigit(o1, i), toDigit(o2, i))
           if (byDigit != 0) return byDigit
           else
             i = seekNonDigit(o1, i)
         else
           val result = Character.compare(a, b)
-          if result != 0
+          if result != 0 then
             return result
           i += 1
       end while

compiler/src/dotty/tools/dotc/transform/Erasure.scala

@@ -647,7 +647,7 @@ object Erasure {
           if defn.specialErasure.contains(owner) then
             assert(sym.isConstructor, s"${sym.showLocated}")
             defn.specialErasure(owner)
-          else if defn.isSyntheticFunctionClass(owner)
+          else if defn.isSyntheticFunctionClass(owner) then
             defn.erasedFunctionClass(owner)
           else
             owner

compiler/src/dotty/tools/dotc/typer/Applications.scala

@@ -1471,7 +1471,7 @@ trait Applications extends Compatibility {
         case tp1: MethodType => // (1)
           tp1.paramInfos.isEmpty && tp2.isInstanceOf[LambdaType]
           || {
-            if tp1.isVarArgsMethod
+            if tp1.isVarArgsMethod then
               tp2.isVarArgsMethod
               && isApplicableMethodRef(alt2, tp1.paramInfos.map(_.repeatedToSingle), WildcardType)
             else

compiler/src/dotty/tools/dotc/typer/Typer.scala

@@ -2146,6 +2146,7 @@ class Typer extends Namer
          && !cls.isAllOf(PrivateLocal)
          && effectiveOwner.is(Trait)
          && !effectiveOwner.derivesFrom(defn.ObjectClass)
+      then
         report.error(i"$cls cannot be defined in universal $effectiveOwner", cdef.srcPos)
 
       // Temporarily set the typed class def as root tree so that we have at least some

tests/neg/i8731.scala

@@ -6,7 +6,7 @@ object test:
   case _ => () // error: missing parameter type
   end match
 
-  if 3 == 3
+  if 3 == 3 then
     ()
   end if
   else     // error: illegal start of definition

tests/pos/indent.scala

@@ -19,12 +19,12 @@ object Test:
         println("world")
         33
     val y1 =
-      if x > 0
+      if x > 0 then
         1
       else
         2
     val y2 =
-      if (y > 0) && y < 0
+      if (y > 0) && y < 0 then
         1
       else
         2