Typer: Refine position when catching top-level TypeError

Ensures stackoverflows aren't hidden by errors, without needing #4511.

Also add relevant testcase from
#4385 (comment); I confirmed
this fails without the position fix.

Author: Blaisorblade

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

@@ -1857,7 +1857,13 @@ class Typer extends Namer
       assertPositioned(tree)
       try adapt(typedUnadapted(tree, pt, locked), pt, locked)
       catch {
-        case ex: TypeError => errorTree(tree, ex.toMessage)
+        case ex: TypeError =>
+          // This is equivalent to errorTree(tree, ex.toMessage) but
+          // uses tree.pos.focus instead of tree.pos. We use this because:
+          // - since tree can be a top-level definition, tree.pos can point to the whole definition
+          // - that would in turn hide all other type errors inside tree.
+          // TODO: might be even better to store positions inside TypeErrors.
+          tree withType errorType(ex.toMessage, tree.pos.focus)
       }
     }
 

tests/neg/i4385.scala

@@ -0,0 +1,7 @@
+class i0 { // error: stack overflow.
+  class i1 { type i2 }
+  type i3 = i1.i2 // error
+  type i4 = i0 { type i1 <: i4 } // this line triggers the overflow
+                                 // (and reporting it here would be better).
+  // This test ensure the above stack overflow is reported and not hidden by the earlier error.
+}