Merge pull request #12755 from dotty-staging/better-implicit-error

Preserve tvar instantiation in implicit error messages

Author: odersky

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

@@ -1120,12 +1120,20 @@ trait Implicits:
         SearchFailure(new DivergingImplicit(cand.ref, wideProto, argument), span)
       else {
         val history = ctx.searchHistory.nest(cand, pt)
-        val result =
-          typedImplicit(cand, pt, argument, span)(using nestedContext().setNewTyperState().setFreshGADTBounds.setSearchHistory(history))
+        val typingCtx =
+          nestedContext().setNewTyperState().setFreshGADTBounds.setSearchHistory(history)
+        val result = typedImplicit(cand, pt, argument, span)(using typingCtx)
         result match {
           case res: SearchSuccess =>
             ctx.searchHistory.defineBynameImplicit(wideProto, res)
           case _ =>
+            // Since the search failed, the local typerstate will be discarded
+            // without being committed, but type variables local to that state
+            // might still appear in an error message, so we run `gc()` here to
+            // make sure we don't forget their instantiation. This leads to more
+            // precise error messages in tests/neg/missing-implicit3.check and
+            // tests/neg/implicitSearch.check
+            typingCtx.typerState.gc()
             result
         }
       }

tests/neg/implicitSearch.check

@@ -4,7 +4,7 @@
    |   no implicit argument of type Test.Ord[List[List[T]]] was found for parameter o of method sort in object Test.
    |   I found:
    |
-   |       Test.listOrd[T](Test.listOrd[T](/* missing */summon[Test.Ord[T]]))
+   |       Test.listOrd[List[T]](Test.listOrd[T](/* missing */summon[Test.Ord[T]]))
    |
    |   But no implicit values were found that match type Test.Ord[T].
 -- Error: tests/neg/implicitSearch.scala:15:38 -------------------------------------------------------------------------

tests/neg/missing-implicit3.check

@@ -4,6 +4,6 @@
    |no implicit argument of type ord.Ord[ord.Foo] was found for an implicit parameter of method sort in package ord.
    |I found:
    |
-   |    ord.Ord.ordered[A](/* missing */summon[ord.Foo => Comparable[? >: ord.Foo]])
+   |    ord.Ord.ordered[ord.Foo](/* missing */summon[ord.Foo => Comparable[? >: ord.Foo]])
    |
    |But no implicit values were found that match type ord.Foo => Comparable[? >: ord.Foo].