From 19f9ae72bb74c2e2575981aba5704302aa8d1612 Mon Sep 17 00:00:00 2001
From: Martin Odersky <odersky@gmail.com>
Date: Sat, 12 Jun 2021 16:05:29 +0200
Subject: [PATCH 1/2] Use `==` for instance tests against string constants

We used `eq` before, as for other objetc singletons. But this means
we are subject to the vagaries of string hashing.

Fixes #12796
---
 compiler/src/dotty/tools/dotc/ast/tpd.scala | 4 +++-
 tests/run/i12796/A_1.scala                  | 4 ++++
 tests/run/i12796/Test_2.scala               | 3 +++
 3 files changed, 10 insertions(+), 1 deletion(-)
 create mode 100644 tests/run/i12796/A_1.scala
 create mode 100644 tests/run/i12796/Test_2.scala

diff --git a/compiler/src/dotty/tools/dotc/ast/tpd.scala b/compiler/src/dotty/tools/dotc/ast/tpd.scala
index 990bbf4155e9..6555e2b3b1fb 100644
--- a/compiler/src/dotty/tools/dotc/ast/tpd.scala
+++ b/compiler/src/dotty/tools/dotc/ast/tpd.scala
@@ -963,8 +963,10 @@ object tpd extends Trees.Instance[Type] with TypedTreeInfo {
 
     /** `tree.isInstanceOf[tp]`, with special treatment of singleton types */
     def isInstance(tp: Type)(using Context): Tree = tp.dealias match {
+      case ConstantType(c) if c.tag == StringTag =>
+        singleton(tp).equal(tree)
       case tp: SingletonType =>
-        if (tp.widen.derivesFrom(defn.ObjectClass))
+        if tp.widen.derivesFrom(defn.ObjectClass) then
           tree.ensureConforms(defn.ObjectType).select(defn.Object_eq).appliedTo(singleton(tp))
         else
           singleton(tp).equal(tree)
diff --git a/tests/run/i12796/A_1.scala b/tests/run/i12796/A_1.scala
new file mode 100644
index 000000000000..46290e942335
--- /dev/null
+++ b/tests/run/i12796/A_1.scala
@@ -0,0 +1,4 @@
+object A:
+  type Timeframe = "1m" | "2m" | "1H"
+
+  def test(input: String) = input.isInstanceOf[Timeframe]
diff --git a/tests/run/i12796/Test_2.scala b/tests/run/i12796/Test_2.scala
new file mode 100644
index 000000000000..8274f8804063
--- /dev/null
+++ b/tests/run/i12796/Test_2.scala
@@ -0,0 +1,3 @@
+@main def Test =
+  val x = "1"
+  assert(A.test(x + "m"))

From 82b94e98d4cfabd8c318055f65806bba1b3c4824 Mon Sep 17 00:00:00 2001
From: Martin Odersky <odersky@gmail.com>
Date: Sat, 12 Jun 2021 17:24:38 +0200
Subject: [PATCH 2/2] Additional fixed issue test

---
 tests/run/i6996.check | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/tests/run/i6996.check b/tests/run/i6996.check
index a4387987de57..3e37e529877c 100644
--- a/tests/run/i6996.check
+++ b/tests/run/i6996.check
@@ -1,3 +1,3 @@
 an `a`
-false
-not `a`
+true
+an `a`