From 081f98232b9ff5c7c7798b807d0a82cf4592f4ca Mon Sep 17 00:00:00 2001
From: Wesley Wigham <wwigham@gmail.com>
Date: Wed, 16 Sep 2020 13:31:13 -0700
Subject: [PATCH] Handle the mapping between Array and ReadonlyArray in
 isTypeDerivedFrom

---
 src/compiler/checker.ts                       |  2 +-
 .../instanceofNarrowReadonlyArray.js          | 21 +++++++++++++++++++
 .../instanceofNarrowReadonlyArray.symbols     | 19 +++++++++++++++++
 .../instanceofNarrowReadonlyArray.types       | 21 +++++++++++++++++++
 .../compiler/instanceofNarrowReadonlyArray.ts |  9 ++++++++
 5 files changed, 71 insertions(+), 1 deletion(-)
 create mode 100644 tests/baselines/reference/instanceofNarrowReadonlyArray.js
 create mode 100644 tests/baselines/reference/instanceofNarrowReadonlyArray.symbols
 create mode 100644 tests/baselines/reference/instanceofNarrowReadonlyArray.types
 create mode 100644 tests/cases/compiler/instanceofNarrowReadonlyArray.ts

diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index a8f6e129f9c..6e80d32111b 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -15349,7 +15349,7 @@ namespace ts {
                 source.flags & TypeFlags.InstantiableNonPrimitive ? isTypeDerivedFrom(getBaseConstraintOfType(source) || unknownType, target) :
                 target === globalObjectType ? !!(source.flags & (TypeFlags.Object | TypeFlags.NonPrimitive)) :
                 target === globalFunctionType ? !!(source.flags & TypeFlags.Object) && isFunctionObjectType(source as ObjectType) :
-                hasBaseType(source, getTargetType(target));
+                hasBaseType(source, getTargetType(target)) || (isArrayType(target) && !isReadonlyArrayType(target) && isTypeDerivedFrom(source, globalReadonlyArrayType));
         }
 
         /**
diff --git a/tests/baselines/reference/instanceofNarrowReadonlyArray.js b/tests/baselines/reference/instanceofNarrowReadonlyArray.js
new file mode 100644
index 00000000000..a6976ff98f1
--- /dev/null
+++ b/tests/baselines/reference/instanceofNarrowReadonlyArray.js
@@ -0,0 +1,21 @@
+//// [instanceofNarrowReadonlyArray.ts]
+// @strict
+
+function narrow(x: readonly number[] | number): readonly number[] {
+    if (x instanceof Array) {
+        return x;
+    } else {
+        return [x];
+    }
+}
+
+//// [instanceofNarrowReadonlyArray.js]
+// @strict
+function narrow(x) {
+    if (x instanceof Array) {
+        return x;
+    }
+    else {
+        return [x];
+    }
+}
diff --git a/tests/baselines/reference/instanceofNarrowReadonlyArray.symbols b/tests/baselines/reference/instanceofNarrowReadonlyArray.symbols
new file mode 100644
index 00000000000..03d6923f7e2
--- /dev/null
+++ b/tests/baselines/reference/instanceofNarrowReadonlyArray.symbols
@@ -0,0 +1,19 @@
+=== tests/cases/compiler/instanceofNarrowReadonlyArray.ts ===
+// @strict
+
+function narrow(x: readonly number[] | number): readonly number[] {
+>narrow : Symbol(narrow, Decl(instanceofNarrowReadonlyArray.ts, 0, 0))
+>x : Symbol(x, Decl(instanceofNarrowReadonlyArray.ts, 2, 16))
+
+    if (x instanceof Array) {
+>x : Symbol(x, Decl(instanceofNarrowReadonlyArray.ts, 2, 16))
+>Array : Symbol(Array, Decl(lib.es5.d.ts, --, --), Decl(lib.es5.d.ts, --, --))
+
+        return x;
+>x : Symbol(x, Decl(instanceofNarrowReadonlyArray.ts, 2, 16))
+
+    } else {
+        return [x];
+>x : Symbol(x, Decl(instanceofNarrowReadonlyArray.ts, 2, 16))
+    }
+}
diff --git a/tests/baselines/reference/instanceofNarrowReadonlyArray.types b/tests/baselines/reference/instanceofNarrowReadonlyArray.types
new file mode 100644
index 00000000000..53b4adac41e
--- /dev/null
+++ b/tests/baselines/reference/instanceofNarrowReadonlyArray.types
@@ -0,0 +1,21 @@
+=== tests/cases/compiler/instanceofNarrowReadonlyArray.ts ===
+// @strict
+
+function narrow(x: readonly number[] | number): readonly number[] {
+>narrow : (x: readonly number[] | number) => readonly number[]
+>x : number | readonly number[]
+
+    if (x instanceof Array) {
+>x instanceof Array : boolean
+>x : number | readonly number[]
+>Array : ArrayConstructor
+
+        return x;
+>x : readonly number[]
+
+    } else {
+        return [x];
+>[x] : number[]
+>x : number
+    }
+}
diff --git a/tests/cases/compiler/instanceofNarrowReadonlyArray.ts b/tests/cases/compiler/instanceofNarrowReadonlyArray.ts
new file mode 100644
index 00000000000..dbf9d9b409f
--- /dev/null
+++ b/tests/cases/compiler/instanceofNarrowReadonlyArray.ts
@@ -0,0 +1,9 @@
+// @strict
+
+function narrow(x: readonly number[] | number): readonly number[] {
+    if (x instanceof Array) {
+        return x;
+    } else {
+        return [x];
+    }
+}
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