Merge pull request #7834 from dotty-staging/fix-#7822

Fix #7822: Avoid crash in liftToThis

Author: odersky

compiler/src/dotty/tools/dotc/core/TypeComparer.scala

@@ -1114,7 +1114,7 @@ class TypeComparer(initctx: Context) extends ConstraintHandling[AbsentContext] w
         findEnclosingThis(tp.symbol.moduleClass, ctx.owner)
       case tp: TypeRef =>
         val pre1 = liftToThis(tp.prefix)
-        if (pre1 ne tp.prefix) tp.withPrefix(pre1) else tp
+        if ((pre1 ne tp.prefix) && pre1.exists) tp.withPrefix(pre1) else tp
       case tp: ThisType if tp.cls.is(Package) =>
         findEnclosingThis(tp.cls, ctx.owner)
       case tp: AppliedType =>

tests/pos/i7822.scala

@@ -0,0 +1,5 @@
+class Code {
+  type Ctx
+}
+def foo(o0:Code,o1:Code) = new Code { type Ctx = o0.Ctx & o1.Ctx }
+def foo0(o0:Code,o1:Code): Code { type Ctx = o0.Ctx & o1.Ctx } = foo(o0,o1)