Re-lub also hard union types in simplify

Simplify had some elaborate condition that prevented hard union types to be
recomputed with a lub. I am not sure why that was. In the concrete scenario
of i10693.scala, we had an explicitly union result type `B | A` where `A` and `B` are
type parameters. So that is a hard union type. Then `A` was instantiated by `Int | String`
and `B` was instantiated by `String | Int`. Re-forming the lub of that union would
have eliminated one pair, but since the union type was hard tyat was not done. On the
other hand I see no reason why hard unions should not be re-lubbed. Hard unions are
about preventing the widening of or types with a join. I don't see a connection with
avoiding re-lubbing.

Fixes scala#10693

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

@@ -157,15 +157,8 @@ object TypeOps:
         tp.derivedAlias(simplify(tp.alias, theMap))
       case AndType(l, r) if !ctx.mode.is(Mode.Type) =>
         simplify(l, theMap) & simplify(r, theMap)
-      case tp @ OrType(l, r)
-      if !ctx.mode.is(Mode.Type)
-         && (tp.isSoft || l.isBottomType || r.isBottomType) =>
-        // Normalize A | Null and Null | A to A even if the union is hard (i.e.
-        // explicitly declared), but not if -Yexplicit-nulls is set. The reason is
-        // that in this case the normal asSeenFrom machinery is not prepared to deal
-        // with Nulls (which have no base classes). Under -Yexplicit-nulls, we take
-        // corrective steps, so no widening is wanted.
-        simplify(l, theMap) | simplify(r, theMap)
+      case tp @ OrType(l, r) if !ctx.mode.is(Mode.Type) =>
+        TypeComparer.lub(simplify(l, theMap), simplify(r, theMap), isSoft = tp.isSoft)
       case tp @ CapturingType(parent, refs) =>
         if !ctx.mode.is(Mode.Type)
             && refs.subCaptures(parent.captureSet, frozen = true).isOK

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

@@ -1961,7 +1961,8 @@ class Namer { typer: Typer =>
       else
         // don't strip @uncheckedVariance annot for default getters
         TypeOps.simplify(tp.widenTermRefExpr,
-            if defaultTp.exists then TypeOps.SimplifyKeepUnchecked() else null) match
+            if defaultTp.exists then TypeOps.SimplifyKeepUnchecked() else null)
+        match
           case ctp: ConstantType if sym.isInlineVal => ctp
           case tp => TypeComparer.widenInferred(tp, pt, widenUnions = true)
 

tests/pos/i10693.scala

@@ -0,0 +1,8 @@
+def test[A, B](a: A, b: B): A | B = a
+val v0 = test("string", 1)
+val v1 = test(1, "string")
+val v2 = test(v0, v1)
+val v3 = test(v1, v0)
+val v4 = test(v2, v3)
+val v5 = test(v3, v2)
+val v6 = test(v4, v5)