Fix #5032: Make sure that erroneous empty idents have a position

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

@@ -550,18 +550,23 @@ object Parsers {
       }
 
     /** Accept identifier and return Ident with its name as a term name. */
-    def termIdent(): Ident = atSpan(in.offset) {
-      makeIdent(in.token, ident())
-    }
+    def termIdent(): Ident =
+      makeIdent(in.token, in.offset, ident())
 
     /** Accept identifier and return Ident with its name as a type name. */
-    def typeIdent(): Ident = atSpan(in.offset) {
-      makeIdent(in.token, ident().toTypeName)
-    }
+    def typeIdent(): Ident =
+      makeIdent(in.token, in.offset, ident().toTypeName)
 
-    private def makeIdent(tok: Token, name: Name) =
-      if (tok == BACKQUOTED_IDENT) BackquotedIdent(name)
-      else Ident(name)
+    private def makeIdent(tok: Token, offset: Offset, name: Name) = {
+      val tree =
+        if (tok == BACKQUOTED_IDENT) BackquotedIdent(name)
+        else Ident(name)
+
+      // Make sure that even trees with parsing errors have a offset that is within the offset
+      val errorOffset = offset min (in.lastOffset - 1)
+      if (tree.name == nme.ERROR && tree.span == NoSpan) tree.withSpan(Span(errorOffset, errorOffset))
+      else atSpan(offset)(tree)
+    }
 
     def wildcardIdent(): Ident =
       atSpan(accept(USCORE)) { Ident(nme.WILDCARD) }

tests/neg/i5032a.scala

@@ -0,0 +1,2 @@
+class i1@
+// error

tests/neg/i5032b.scala

@@ -0,0 +1,2 @@
+class a@
+class b@ // error // error