From 1b8243daba4c94290ac45a849e34286827dc3b05 Mon Sep 17 00:00:00 2001
From: Oleksandr T <oleksandr.tarasiuk@outlook.com>
Date: Thu, 24 Dec 2020 21:24:43 +0200
Subject: [PATCH] fix(42088): fix crash on unreachability condition in for
 statement

---
 src/compiler/checker.ts                       |  6 ++++-
 .../reference/reachabilityChecks8.errors.txt  | 17 +++++++++++++
 .../reference/reachabilityChecks8.js          | 15 +++++++++++
 .../reference/reachabilityChecks8.symbols     | 10 ++++++++
 .../reference/reachabilityChecks8.types       | 25 +++++++++++++++++++
 tests/cases/compiler/reachabilityChecks8.ts   |  9 +++++++
 6 files changed, 81 insertions(+), 1 deletion(-)
 create mode 100644 tests/baselines/reference/reachabilityChecks8.errors.txt
 create mode 100644 tests/baselines/reference/reachabilityChecks8.js
 create mode 100644 tests/baselines/reference/reachabilityChecks8.symbols
 create mode 100644 tests/baselines/reference/reachabilityChecks8.types
 create mode 100644 tests/cases/compiler/reachabilityChecks8.ts

diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 1c3bc622b0c75..66c3817196d0e 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -21769,8 +21769,12 @@ namespace ts {
                     return some((<FlowLabel>flow).antecedents, f => isReachableFlowNodeWorker(f, /*noCacheCheck*/ false));
                 }
                 else if (flags & FlowFlags.LoopLabel) {
+                    const antecedents = (<FlowLabel>flow).antecedents;
+                    if (antecedents === undefined || antecedents.length === 0) {
+                        return false;
+                    }
                     // A loop is reachable if the control flow path that leads to the top is reachable.
-                    flow = (<FlowLabel>flow).antecedents![0];
+                    flow = antecedents[0];
                 }
                 else if (flags & FlowFlags.SwitchClause) {
                     // The control flow path representing an unmatched value in a switch statement with
diff --git a/tests/baselines/reference/reachabilityChecks8.errors.txt b/tests/baselines/reference/reachabilityChecks8.errors.txt
new file mode 100644
index 0000000000000..839c650d1142d
--- /dev/null
+++ b/tests/baselines/reference/reachabilityChecks8.errors.txt
@@ -0,0 +1,17 @@
+tests/cases/compiler/reachabilityChecks8.ts(4,24): error TS7027: Unreachable code detected.
+tests/cases/compiler/reachabilityChecks8.ts(5,24): error TS7027: Unreachable code detected.
+
+
+==== tests/cases/compiler/reachabilityChecks8.ts (2 errors) ====
+    try {
+        for (
+            (function () { throw "1"; })();
+            (function () { throw "2"; })();
+                           ~~~~~~~~~~
+!!! error TS7027: Unreachable code detected.
+            (function () { throw "3"; })()
+                           ~~~~~~~~~~
+!!! error TS7027: Unreachable code detected.
+        ) {}
+    } catch (e) {}
+    
\ No newline at end of file
diff --git a/tests/baselines/reference/reachabilityChecks8.js b/tests/baselines/reference/reachabilityChecks8.js
new file mode 100644
index 0000000000000..fe33f8fe2b446
--- /dev/null
+++ b/tests/baselines/reference/reachabilityChecks8.js
@@ -0,0 +1,15 @@
+//// [reachabilityChecks8.ts]
+try {
+    for (
+        (function () { throw "1"; })();
+        (function () { throw "2"; })();
+        (function () { throw "3"; })()
+    ) {}
+} catch (e) {}
+
+
+//// [reachabilityChecks8.js]
+try {
+    for ((function () { throw "1"; })(); (function () { throw "2"; })(); (function () { throw "3"; })()) { }
+}
+catch (e) { }
diff --git a/tests/baselines/reference/reachabilityChecks8.symbols b/tests/baselines/reference/reachabilityChecks8.symbols
new file mode 100644
index 0000000000000..9e95d0979207e
--- /dev/null
+++ b/tests/baselines/reference/reachabilityChecks8.symbols
@@ -0,0 +1,10 @@
+=== tests/cases/compiler/reachabilityChecks8.ts ===
+try {
+    for (
+        (function () { throw "1"; })();
+        (function () { throw "2"; })();
+        (function () { throw "3"; })()
+    ) {}
+} catch (e) {}
+>e : Symbol(e, Decl(reachabilityChecks8.ts, 6, 9))
+
diff --git a/tests/baselines/reference/reachabilityChecks8.types b/tests/baselines/reference/reachabilityChecks8.types
new file mode 100644
index 0000000000000..0e595a2404cb3
--- /dev/null
+++ b/tests/baselines/reference/reachabilityChecks8.types
@@ -0,0 +1,25 @@
+=== tests/cases/compiler/reachabilityChecks8.ts ===
+try {
+    for (
+        (function () { throw "1"; })();
+>(function () { throw "1"; })() : never
+>(function () { throw "1"; }) : () => never
+>function () { throw "1"; } : () => never
+>"1" : "1"
+
+        (function () { throw "2"; })();
+>(function () { throw "2"; })() : never
+>(function () { throw "2"; }) : () => never
+>function () { throw "2"; } : () => never
+>"2" : "2"
+
+        (function () { throw "3"; })()
+>(function () { throw "3"; })() : never
+>(function () { throw "3"; }) : () => never
+>function () { throw "3"; } : () => never
+>"3" : "3"
+
+    ) {}
+} catch (e) {}
+>e : any
+
diff --git a/tests/cases/compiler/reachabilityChecks8.ts b/tests/cases/compiler/reachabilityChecks8.ts
new file mode 100644
index 0000000000000..5f59df0f17d5f
--- /dev/null
+++ b/tests/cases/compiler/reachabilityChecks8.ts
@@ -0,0 +1,9 @@
+// @allowUnreachableCode: false
+
+try {
+    for (
+        (function () { throw "1"; })();
+        (function () { throw "2"; })();
+        (function () { throw "3"; })()
+    ) {}
+} catch (e) {}