Fix #15514 in Parser

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

@@ -3957,8 +3957,8 @@ object Parsers {
      */
     def selfType(): ValDef =
       if (in.isIdent || in.token == THIS)
-            && in.lookahead.isColon && followingIsSelfType()
-            || in.lookahead.token == ARROW
+        && (in.lookahead.isColon && followingIsSelfType()
+            || in.lookahead.token == ARROW)
       then
         atSpan(in.offset) {
           val selfName =

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

@@ -662,7 +662,7 @@ object Scanners {
           true
         else token == COLONfollow && (inTemplate || fewerBracesEnabled)
       if enabled then
-        lookAhead()
+        peekAhead()
         val atEOL = isAfterLineEnd || token == EOF
         reset()
         if atEOL then token = COLONeol
@@ -690,15 +690,14 @@ object Scanners {
         insert(OUTDENT, offset)
       case _ =>
 
-    def lookAhead() =
+    def peekAhead() =
       prev.copyFrom(this)
       getNextToken(token)
       if token == END && !isEndMarker then token = IDENTIFIER
 
-    def reset() = {
+    def reset() =
       next.copyFrom(this)
       this.copyFrom(prev)
-    }
 
     def closeIndented() = currentRegion match
       case r: Indented if !r.isOutermost => insert(OUTDENT, offset)
@@ -717,12 +716,12 @@ object Scanners {
       }
       (token: @switch) match {
         case CASE =>
-          lookAhead()
+          peekAhead()
           if (token == CLASS) fuse(CASECLASS)
           else if (token == OBJECT) fuse(CASEOBJECT)
           else reset()
         case SEMI =>
-          lookAhead()
+          peekAhead()
           if (token != ELSE) reset()
         case COMMA =>
           def isEnclosedInParens(r: Region): Boolean = r match
@@ -733,7 +732,7 @@ object Scanners {
             case r: Indented if isEnclosedInParens(r.outer) =>
               insert(OUTDENT, offset)
             case _ =>
-              lookAhead()
+              peekAhead()
               if isAfterLineEnd
                  && currentRegion.commasExpected
                  && (token == RPAREN || token == RBRACKET || token == RBRACE || token == OUTDENT)
@@ -1083,11 +1082,14 @@ object Scanners {
      *  The token is computed via fetchToken, so complex two word
      *  tokens such as CASECLASS are not recognized.
      *  Newlines and indent/unindent tokens are skipped.
-     *
+     *  Restriction: `lookahead` is illegal if the current token is INTERPOLATIONID
      */
-     def lookahead: TokenData =
+    def lookahead: TokenData =
       if next.token == EMPTY then
-        lookAhead()
+        assert(token != INTERPOLATIONID)
+          // INTERPOLATONIDs are followed by a string literal, which can set next
+          // in peekAhead(). In that case, the following reset() would forget that token.
+        peekAhead()
         reset()
       next
 

tests/pos/i15514.scala

@@ -0,0 +1,4 @@
+
+object Main { s"Hello $Main.toStr!" }
+
+object Alt { s"Hello ${Alt}.toStr!" }