From 34eb5ddf75ad7401217175a4d623d34e9aaee534 Mon Sep 17 00:00:00 2001
From: Wesley Wigham <wewigham@microsoft.com>
Date: Tue, 25 Jun 2019 20:56:41 -0700
Subject: [PATCH] Fix tuple and array comparisons during identity checking
 (#32089)

---
 src/compiler/checker.ts                           |  9 ++++++++-
 ...uickinfoExpressionTypeNotChangedViaDeletion.ts | 15 +++++++++++++++
 2 files changed, 23 insertions(+), 1 deletion(-)
 create mode 100644 tests/cases/fourslash/quickinfoExpressionTypeNotChangedViaDeletion.ts

diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index a3abe1dc5b5..fb96ca0813a 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -13321,7 +13321,14 @@ namespace ts {
                         }
                     }
                     else if (isReadonlyArrayType(target) ? isArrayType(source) || isTupleType(source) : isArrayType(target) && isTupleType(source) && !source.target.readonly) {
-                        return isRelatedTo(getIndexTypeOfType(source, IndexKind.Number) || anyType, getIndexTypeOfType(target, IndexKind.Number) || anyType, reportErrors);
+                        if (relation !== identityRelation) {
+                            return isRelatedTo(getIndexTypeOfType(source, IndexKind.Number) || anyType, getIndexTypeOfType(target, IndexKind.Number) || anyType, reportErrors);
+                        }
+                        else {
+                            // By flags alone, we know that the `target` is a readonly array while the source is a normal array or tuple
+                            // or `target` is an array and source is a tuple - in both cases the types cannot be identical, by construction
+                            return Ternary.False;
+                        }
                     }
                     // Consider a fresh empty object literal type "closed" under the subtype relationship - this way `{} <- {[idx: string]: any} <- fresh({})`
                     // and not `{} <- fresh({}) <- {[idx: string]: any}`
diff --git a/tests/cases/fourslash/quickinfoExpressionTypeNotChangedViaDeletion.ts b/tests/cases/fourslash/quickinfoExpressionTypeNotChangedViaDeletion.ts
new file mode 100644
index 00000000000..01656aea566
--- /dev/null
+++ b/tests/cases/fourslash/quickinfoExpressionTypeNotChangedViaDeletion.ts
@@ -0,0 +1,15 @@
+/// <reference path="fourslash.ts" />
+////type TypeEq<A, B> = (<T>() => T extends A ? 1 : 2) extends (<T>() => T extends B ? 1 : 2) ? true : false;
+////
+////const /*2*/test1: TypeEq<number[], [number, ...number[]]> = false;
+////
+////declare const foo: [number, ...number[]];
+////declare const bar: number[];
+////
+////const /*1*/test2: TypeEq<typeof foo, typeof bar> = false;
+
+goTo.marker("1");
+verify.quickInfoIs("const test2: false");
+
+goTo.marker("2");
+verify.quickInfoIs("const test1: false");