From 12c90507f772a6d4554a928747081b40c238187e Mon Sep 17 00:00:00 2001
From: Guillaume Martres <smarter@ubuntu.com>
Date: Tue, 8 Jun 2021 21:41:14 +0200
Subject: [PATCH] Preserve tvar instantiation in implicit error messages

---
 compiler/src/dotty/tools/dotc/typer/Implicits.scala | 12 ++++++++++--
 tests/neg/implicitSearch.check                      |  2 +-
 tests/neg/missing-implicit3.check                   |  2 +-
 3 files changed, 12 insertions(+), 4 deletions(-)

diff --git a/compiler/src/dotty/tools/dotc/typer/Implicits.scala b/compiler/src/dotty/tools/dotc/typer/Implicits.scala
index ae3b04d37657..ed8b347679b5 100644
--- a/compiler/src/dotty/tools/dotc/typer/Implicits.scala
+++ b/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
         }
       }
diff --git a/tests/neg/implicitSearch.check b/tests/neg/implicitSearch.check
index efc3ac002739..9ff67e2c2638 100644
--- a/tests/neg/implicitSearch.check
+++ b/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 -------------------------------------------------------------------------
diff --git a/tests/neg/missing-implicit3.check b/tests/neg/missing-implicit3.check
index dec05cecdad6..d58216b9173e 100644
--- a/tests/neg/missing-implicit3.check
+++ b/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].