Hooks to check that comparisons with == / != make sense

Also, check that pattern matching against idents/selects/literals makes
sense.

The hooks perform an implicit search for an instance of `Eq[L, R]`, where
`L`, `R` are the argument types. So far this always succeeeds because Eq.eqAny
matches all such types. A separate commit will check the returned
search term for validity.

1 files changed, 11 insertions, 1 deletions

diff --git a/src/dotty/tools/dotc/typer/Applications.scala b/src/dotty/tools/dotc/typer/Applications.scala
index 6ce98606b..2b0cc4033 100644
--- a/src/dotty/tools/dotc/typer/Applications.scala
+++ b/src/dotty/tools/dotc/typer/Applications.scala
@@ -603,7 +603,17 @@ trait Applications extends Compatibility { self: Typer =>
           failedVal
         }
       }
-    else realApply
+    else {
+      val app = realApply
+      app match {
+        case Apply(fn @ Select(left, _), right :: Nil) if fn.hasType =>
+          val op = fn.symbol
+          if (op == defn.Any_== || op == defn.Any_!=)
+            checkCanEqual(left.tpe.widen, right.tpe.widen, app.pos)
+        case _ =>
+      }
+      app
+    }
   }
 
   /** Overridden in ReTyper to handle primitive operations that can be generated after erasure */