Fix #6460: less forcing in checkNonCyclic

checkNonCyclic was forcing too many infos because it didn't account for
the case where the prefix is a ModuleVal and not a ModuleClass. Fixing
this solves the incremental compilation issue in the Dotty build
reported in #6460 were the root cause was a completion cycle when
jointly compiling SymbolLoaders and Types that looked like this:

Complete `object tpd`
 Complete its parent `Trees.Instance`
  Complete its type parameter `T >: Untyped <: Type`
    run `checkNonCyclic` on the the type parameter definition
      Complete `Type`
        Complete `import ast.tpd._` in `Types.scala`
          Eventually this requires unpickling the selection `Trees.Instance#T` which
          gets a `NoDenotation` since `T` hasn't been entered yet.

As a side-effect this also means that the neg/toplevel-cyclic
test no longer leads to an "illegal cyclic reference" error, instead it
now causes "recursion limit exceeded" errors, which are less nice but
still acceptable.

Author: smarter

compiler/src/dotty/tools/dotc/core/tasty/TreeUnpickler.scala

@@ -1018,6 +1018,8 @@ class TreeUnpickler(reader: TastyReader,
         val qual = readTerm()(localCtx)
         var qualType = qual.tpe.widenIfUnstable
         val denot = accessibleDenot(qualType, name, sig)
+        assert(denot.exists,
+          s"Unpickling selection with qualifier $qual of type $qualType, name $name and signature $sig failed: No denotation found.")
         val owner = denot.symbol.maybeOwner
         if (owner.isPackageObject && qualType.termSymbol.is(Package))
           qualType = qualType.select(owner.sourceModule)

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

@@ -240,7 +240,7 @@ object Checking {
               )
             case prefix: NamedType =>
               (!sym.is(Private) && prefix.derivesFrom(sym.owner)) ||
-              (!prefix.symbol.isStaticOwner && isInteresting(prefix.prefix))
+              (!prefix.symbol.moduleClass.isStaticOwner && isInteresting(prefix.prefix))
             case SuperType(thistp, _) => isInteresting(thistp)
             case AndType(tp1, tp2) => isInteresting(tp1) || isInteresting(tp2)
             case OrType(tp1, tp2) => isInteresting(tp1) && isInteresting(tp2)

compiler/test/dotty/tools/dotc/BootstrappedOnlyCompilationTests.scala

@@ -48,7 +48,15 @@ class BootstrappedOnlyCompilationTests extends ParallelTesting {
       compileDir("compiler/src/dotty/tools/dotc/reporting", withCompilerOptions),
       compileDir("compiler/src/dotty/tools/dotc/typer", withCompilerOptions),
       compileDir("compiler/src/dotty/tools/dotc/util", withCompilerOptions),
-      compileDir("compiler/src/dotty/tools/io", withCompilerOptions)
+      compileDir("compiler/src/dotty/tools/io", withCompilerOptions),
+      compileList(
+        "testIssue6460",
+        List(
+          "compiler/src/dotty/tools/dotc/core/SymbolLoaders.scala",
+          "compiler/src/dotty/tools/dotc/core/Types.scala"
+        ),
+        withCompilerOptions
+      ),
     ).checkCompile()
   }
 

tests/neg/toplevel-cyclic/defs_1.scala

@@ -1,2 +1,2 @@
-type A = B  // error: illegal cyclic reference
+type A = B  // error: recursion limit exceeded
 

tests/neg/toplevel-cyclic/moredefs_1.scala

@@ -1 +1 @@
-type B = A
+type B = A // error: recursion limit exceeded // error: recursion limit exceeded