From a3a91216b9395cd12cc3e96491640f5ebf840d49 Mon Sep 17 00:00:00 2001
From: Martin Odersky <odersky@gmail.com>
Date: Thu, 15 Jun 2017 13:41:15 +0200
Subject: [PATCH 1/3] Harden TreeAccumulator for IDE

---
 compiler/src/dotty/tools/dotc/ast/Trees.scala | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/compiler/src/dotty/tools/dotc/ast/Trees.scala b/compiler/src/dotty/tools/dotc/ast/Trees.scala
index f7df30b404c6..74453570678c 100644
--- a/compiler/src/dotty/tools/dotc/ast/Trees.scala
+++ b/compiler/src/dotty/tools/dotc/ast/Trees.scala
@@ -1305,8 +1305,10 @@ object Trees {
             this(this(x, arg), annot)
           case Thicket(ts) =>
             this(x, ts)
-          case _ if ctx.reporter.errorsReported =>
+          case _ if ctx.mode.is(Mode.Interactive) =>
             // in case of errors it may be that typed trees point to untyped ones.
+            // The can still traverse inside such trees, either in the run where errors
+            // are reported, or in subsequent ones.
             x
         }
       }

From 23d0a491c445c05e7212c79a2714107e1e9d48d3 Mon Sep 17 00:00:00 2001
From: Martin Odersky <odersky@gmail.com>
Date: Sun, 18 Jun 2017 18:27:17 +0200
Subject: [PATCH 2/3] Avoid another source of stale symbol errors in IDE

---
 compiler/src/dotty/tools/dotc/core/Types.scala | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/compiler/src/dotty/tools/dotc/core/Types.scala b/compiler/src/dotty/tools/dotc/core/Types.scala
index f8fe33e3e33f..d8c6dd814a8a 100644
--- a/compiler/src/dotty/tools/dotc/core/Types.scala
+++ b/compiler/src/dotty/tools/dotc/core/Types.scala
@@ -1538,7 +1538,8 @@ object Types {
               else recomputeMember(d) // symbol could have been overridden, recompute membership
             else {
               val newd = loadDenot
-              if (newd.exists) newd else d.staleSymbolError
+              if (newd.exists || ctx.mode.is(Mode.Interactive)) newd
+              else d.staleSymbolError
             }
           case d =>
             if (d.validFor.runId != ctx.period.runId) loadDenot

From f43af9d74b4c39490a01c6705535b177586ebe70 Mon Sep 17 00:00:00 2001
From: Martin Odersky <odersky@gmail.com>
Date: Thu, 22 Jun 2017 13:10:44 +0200
Subject: [PATCH 3/3] Address reviewers comments

---
 compiler/src/dotty/tools/dotc/ast/Trees.scala  | 4 ++--
 compiler/src/dotty/tools/dotc/core/Types.scala | 3 ++-
 2 files changed, 4 insertions(+), 3 deletions(-)

diff --git a/compiler/src/dotty/tools/dotc/ast/Trees.scala b/compiler/src/dotty/tools/dotc/ast/Trees.scala
index 74453570678c..2d120eca16ce 100644
--- a/compiler/src/dotty/tools/dotc/ast/Trees.scala
+++ b/compiler/src/dotty/tools/dotc/ast/Trees.scala
@@ -1306,8 +1306,8 @@ object Trees {
           case Thicket(ts) =>
             this(x, ts)
           case _ if ctx.mode.is(Mode.Interactive) =>
-            // in case of errors it may be that typed trees point to untyped ones.
-            // The can still traverse inside such trees, either in the run where errors
+            // In case of errors it may be that typed trees point to untyped ones.
+            // The IDE can still traverse inside such trees, either in the run where errors
             // are reported, or in subsequent ones.
             x
         }
diff --git a/compiler/src/dotty/tools/dotc/core/Types.scala b/compiler/src/dotty/tools/dotc/core/Types.scala
index d8c6dd814a8a..cc11bc4f0bd5 100644
--- a/compiler/src/dotty/tools/dotc/core/Types.scala
+++ b/compiler/src/dotty/tools/dotc/core/Types.scala
@@ -1538,7 +1538,8 @@ object Types {
               else recomputeMember(d) // symbol could have been overridden, recompute membership
             else {
               val newd = loadDenot
-              if (newd.exists || ctx.mode.is(Mode.Interactive)) newd
+              if (newd.exists) newd
+              else if (ctx.mode.is(Mode.Interactive)) d
               else d.staleSymbolError
             }
           case d =>