From 66a931b8a1e9102a90a68cc8aa2da8b12a7df20a Mon Sep 17 00:00:00 2001
From: Martin Odersky <odersky@gmail.com>
Date: Wed, 26 May 2021 19:05:59 +0200
Subject: [PATCH 1/3] Improve display of references for ambiguous implicit
 errors

If the first attempt yields equal strings, print the full path to the implicits instead.

Fixes #12591
---
 compiler/src/dotty/tools/dotc/typer/Implicits.scala |  7 ++++++-
 tests/pos/i12591/Inner.scala                        | 13 +++++++++++++
 tests/pos/i12591/Outer.scala                        |  3 +++
 3 files changed, 22 insertions(+), 1 deletion(-)
 create mode 100644 tests/pos/i12591/Inner.scala
 create mode 100644 tests/pos/i12591/Outer.scala

diff --git a/compiler/src/dotty/tools/dotc/typer/Implicits.scala b/compiler/src/dotty/tools/dotc/typer/Implicits.scala
index 7b743aaab49f..9306a6ac50fe 100644
--- a/compiler/src/dotty/tools/dotc/typer/Implicits.scala
+++ b/compiler/src/dotty/tools/dotc/typer/Implicits.scala
@@ -492,7 +492,12 @@ object Implicits:
   /** An ambiguous implicits failure */
   class AmbiguousImplicits(val alt1: SearchSuccess, val alt2: SearchSuccess, val expectedType: Type, val argument: Tree) extends SearchFailureType {
     def explanation(using Context): String =
-      em"both ${err.refStr(alt1.ref)} and ${err.refStr(alt2.ref)} $qualify"
+      var str1 = err.refStr(alt1.ref)
+      var str2 = err.refStr(alt2.ref)
+      if str1 == str2 then
+        str1 = ctx.printer.toTextRef(alt1.ref).show
+        str2 = ctx.printer.toTextRef(alt2.ref).show
+      em"both $str1 and $str2 $qualify"
     override def whyNoConversion(using Context): String =
       if !argument.isEmpty && argument.tpe.widen.isRef(defn.NothingClass) then
         ""
diff --git a/tests/pos/i12591/Inner.scala b/tests/pos/i12591/Inner.scala
new file mode 100644
index 000000000000..aae9bd5b9234
--- /dev/null
+++ b/tests/pos/i12591/Inner.scala
@@ -0,0 +1,13 @@
+package outer
+package inner
+
+sealed trait Foo
+object Foo:
+  trait TC[T]
+  given ofFoo[T <: Foo]: TC[T] = ???
+  trait Bar extends Foo
+
+import Foo.TC
+//Adding import Foo.Bar resolves the issue
+val badSummon = summon[TC[Bar]] // error here
+
diff --git a/tests/pos/i12591/Outer.scala b/tests/pos/i12591/Outer.scala
new file mode 100644
index 000000000000..5c810ec785fe
--- /dev/null
+++ b/tests/pos/i12591/Outer.scala
@@ -0,0 +1,3 @@
+package outer
+export inner.Foo
+export Foo.Bar

From 6b36e32a3e6bf35d6fcf547d0937b64bc7ea39a9 Mon Sep 17 00:00:00 2001
From: Martin Odersky <odersky@gmail.com>
Date: Wed, 26 May 2021 20:39:45 +0200
Subject: [PATCH 2/3] Reclassify test

---
 tests/{pos => neg}/i12591/Inner.scala | 0
 tests/{pos => neg}/i12591/Outer.scala | 0
 2 files changed, 0 insertions(+), 0 deletions(-)
 rename tests/{pos => neg}/i12591/Inner.scala (100%)
 rename tests/{pos => neg}/i12591/Outer.scala (100%)

diff --git a/tests/pos/i12591/Inner.scala b/tests/neg/i12591/Inner.scala
similarity index 100%
rename from tests/pos/i12591/Inner.scala
rename to tests/neg/i12591/Inner.scala
diff --git a/tests/pos/i12591/Outer.scala b/tests/neg/i12591/Outer.scala
similarity index 100%
rename from tests/pos/i12591/Outer.scala
rename to tests/neg/i12591/Outer.scala

From 0c9d8c0c70863b7031288b90a845217df553325b Mon Sep 17 00:00:00 2001
From: Martin Odersky <odersky@gmail.com>
Date: Fri, 28 May 2021 11:22:01 +0200
Subject: [PATCH 3/3] Add check file

---
 tests/neg/i12591.check | 4 ++++
 1 file changed, 4 insertions(+)
 create mode 100644 tests/neg/i12591.check

diff --git a/tests/neg/i12591.check b/tests/neg/i12591.check
new file mode 100644
index 000000000000..694eb68e3299
--- /dev/null
+++ b/tests/neg/i12591.check
@@ -0,0 +1,4 @@
+-- Error: tests/neg/i12591/Inner.scala:12:31 ---------------------------------------------------------------------------
+12 |val badSummon = summon[TC[Bar]] // error here
+   |                               ^
+   |ambiguous implicit arguments: both outer.inner.Foo.ofFoo and outer.Foo.ofFoo match type outer.inner.Foo.TC[outer.Bar] of parameter x of method summon in object Predef