Merge pull request #9991 from dotty-staging/fix-eql

Fix multiversal equality checks for pattern matching

Author: abgruszecki

compiler/src/dotty/tools/dotc/ast/Trees.scala

@@ -1594,7 +1594,9 @@ object Trees {
 
     def resolveConstructor(atp: Type, args: List[Tree])(using Context): tpd.Tree = {
       val targs = atp.argTypes
-      applyOverloaded(tpd.New(atp.typeConstructor), nme.CONSTRUCTOR, args, targs, atp)
+      withoutMode(Mode.PatternOrTypeBits) {
+        applyOverloaded(tpd.New(atp.typeConstructor), nme.CONSTRUCTOR, args, targs, atp)
+      }
     }
   }
 }

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

@@ -3608,20 +3608,18 @@ class Typer extends Namer
    *  Overwritten to no-op in ReTyper.
    */
   protected def checkEqualityEvidence(tree: tpd.Tree, pt: Type)(using Context) : Unit =
-    tree match {
+    tree match
       case _: RefTree | _: Literal
-      if !isVarPattern(tree)
-         && !(pt <:< tree.tpe)
-         && !withMode(Mode.GadtConstraintInference) {
-              TypeComparer.constrainPatternType(tree.tpe, pt)
-            } =>
+      if !isVarPattern(tree) && !(pt <:< tree.tpe) =>
+        withMode(Mode.GadtConstraintInference) {
+          TypeComparer.constrainPatternType(tree.tpe, pt)
+        }
         val cmp =
           untpd.Apply(
             untpd.Select(untpd.TypedSplice(tree), nme.EQ),
             untpd.TypedSplice(dummyTreeOfType(pt)))
         typedExpr(cmp, defn.BooleanType)
       case _ =>
-    }
 
   private def checkStatementPurity(tree: tpd.Tree)(original: untpd.Tree, exprOwner: Symbol)(using Context): Unit =
     if !tree.tpe.isErroneous

tests/neg/eql.scala

@@ -0,0 +1,25 @@
+object lst:
+  opaque type Lst[+T] = Any
+  object Lst:
+    given lstEql[T, U] as Eql[Lst[T], Lst[U]] = Eql.derived
+    val Empty: Lst[Nothing] = ???
+end lst
+
+import lst.Lst
+
+val xs: Lst[Int] = ???
+val ys: List[Int] = ???
+
+def test =
+  xs == ys                  // error: cannot be compared
+  ys == xs                  // error
+  xs == Nil                 // error
+  Nil == xs                 // error
+  ys == Lst.Empty           // error
+  Lst.Empty == ys           // error
+  xs match
+    case Nil => ???         // error, used to pass
+  ys match
+    case Lst.Empty => ???   // error, used to pass
+
+