From c3807802371a89998887740182ee4a348b400b30 Mon Sep 17 00:00:00 2001
From: Prathmesh Ravindra Salunkhe <salunkheprathmesh0@gmail.com>
Date: Wed, 1 Oct 2025 12:58:40 +0530
Subject: [PATCH] Add test for discriminant property order-independence and
 update baselines

---
 .../discriminantOrderIndependence.js          |  61 +++++++
 .../discriminantOrderIndependence.symbols     | 106 ++++++++++++
 .../discriminantOrderIndependence.types       | 162 ++++++++++++++++++
 .../compiler/discriminantOrderIndependence.ts |  41 +++++
 4 files changed, 370 insertions(+)
 create mode 100644 tests/baselines/reference/discriminantOrderIndependence.js
 create mode 100644 tests/baselines/reference/discriminantOrderIndependence.symbols
 create mode 100644 tests/baselines/reference/discriminantOrderIndependence.types
 create mode 100644 tests/cases/compiler/discriminantOrderIndependence.ts

diff --git a/tests/baselines/reference/discriminantOrderIndependence.js b/tests/baselines/reference/discriminantOrderIndependence.js
new file mode 100644
index 0000000000000..0dcd9850e352f
--- /dev/null
+++ b/tests/baselines/reference/discriminantOrderIndependence.js
@@ -0,0 +1,61 @@
+//// [tests/cases/compiler/discriminantOrderIndependence.ts] ////
+
+//// [discriminantOrderIndependence.ts]
+interface A {
+    subType: "b";
+    type: "a";
+}
+
+declare let order1:
+    | { type: "1" }
+    | A
+    | { type: "2" }
+    | { type: "3" }
+    | undefined;
+
+// Should NOT error: 'order1' is possibly 'undefined' after the guard
+if (order1 && order1.type === "a") {
+    order1.type; // Should be OK
+}
+
+interface B {
+    subType: "b";
+    type: "a";
+}
+
+declare let order2:
+    | { type: "1" }
+    | { type: "2" }
+    | { type: "3" }
+    | B
+    | undefined;
+
+// Should NOT error: 'order2' is possibly 'undefined' after the guard
+if (order2 && order2.type === "a") {
+    order2.type; // Should be OK
+}
+
+// Also test with !. type assertion
+if (order1 && order1.type === "a") {
+    order1.type; // Should be OK
+}
+if (order2 && order2.type === "a") {
+    order2.type; // Should be OK
+}
+
+//// [discriminantOrderIndependence.js]
+// Should NOT error: 'order1' is possibly 'undefined' after the guard
+if (order1 && order1.type === "a") {
+    order1.type; // Should be OK
+}
+// Should NOT error: 'order2' is possibly 'undefined' after the guard
+if (order2 && order2.type === "a") {
+    order2.type; // Should be OK
+}
+// Also test with !. type assertion
+if (order1 && order1.type === "a") {
+    order1.type; // Should be OK
+}
+if (order2 && order2.type === "a") {
+    order2.type; // Should be OK
+}
diff --git a/tests/baselines/reference/discriminantOrderIndependence.symbols b/tests/baselines/reference/discriminantOrderIndependence.symbols
new file mode 100644
index 0000000000000..e470f892e5268
--- /dev/null
+++ b/tests/baselines/reference/discriminantOrderIndependence.symbols
@@ -0,0 +1,106 @@
+//// [tests/cases/compiler/discriminantOrderIndependence.ts] ////
+
+=== discriminantOrderIndependence.ts ===
+interface A {
+>A : Symbol(A, Decl(discriminantOrderIndependence.ts, 0, 0))
+
+    subType: "b";
+>subType : Symbol(A.subType, Decl(discriminantOrderIndependence.ts, 0, 13))
+
+    type: "a";
+>type : Symbol(A.type, Decl(discriminantOrderIndependence.ts, 1, 17))
+}
+
+declare let order1:
+>order1 : Symbol(order1, Decl(discriminantOrderIndependence.ts, 5, 11))
+
+    | { type: "1" }
+>type : Symbol(type, Decl(discriminantOrderIndependence.ts, 6, 7))
+
+    | A
+>A : Symbol(A, Decl(discriminantOrderIndependence.ts, 0, 0))
+
+    | { type: "2" }
+>type : Symbol(type, Decl(discriminantOrderIndependence.ts, 8, 7))
+
+    | { type: "3" }
+>type : Symbol(type, Decl(discriminantOrderIndependence.ts, 9, 7))
+
+    | undefined;
+
+// Should NOT error: 'order1' is possibly 'undefined' after the guard
+if (order1 && order1.type === "a") {
+>order1 : Symbol(order1, Decl(discriminantOrderIndependence.ts, 5, 11))
+>order1.type : Symbol(type, Decl(discriminantOrderIndependence.ts, 1, 17), Decl(discriminantOrderIndependence.ts, 6, 7), Decl(discriminantOrderIndependence.ts, 8, 7), Decl(discriminantOrderIndependence.ts, 9, 7))
+>order1 : Symbol(order1, Decl(discriminantOrderIndependence.ts, 5, 11))
+>type : Symbol(type, Decl(discriminantOrderIndependence.ts, 1, 17), Decl(discriminantOrderIndependence.ts, 6, 7), Decl(discriminantOrderIndependence.ts, 8, 7), Decl(discriminantOrderIndependence.ts, 9, 7))
+
+    order1.type; // Should be OK
+>order1.type : Symbol(A.type, Decl(discriminantOrderIndependence.ts, 1, 17))
+>order1 : Symbol(order1, Decl(discriminantOrderIndependence.ts, 5, 11))
+>type : Symbol(A.type, Decl(discriminantOrderIndependence.ts, 1, 17))
+}
+
+interface B {
+>B : Symbol(B, Decl(discriminantOrderIndependence.ts, 15, 1))
+
+    subType: "b";
+>subType : Symbol(B.subType, Decl(discriminantOrderIndependence.ts, 17, 13))
+
+    type: "a";
+>type : Symbol(B.type, Decl(discriminantOrderIndependence.ts, 18, 17))
+}
+
+declare let order2:
+>order2 : Symbol(order2, Decl(discriminantOrderIndependence.ts, 22, 11))
+
+    | { type: "1" }
+>type : Symbol(type, Decl(discriminantOrderIndependence.ts, 23, 7))
+
+    | { type: "2" }
+>type : Symbol(type, Decl(discriminantOrderIndependence.ts, 24, 7))
+
+    | { type: "3" }
+>type : Symbol(type, Decl(discriminantOrderIndependence.ts, 25, 7))
+
+    | B
+>B : Symbol(B, Decl(discriminantOrderIndependence.ts, 15, 1))
+
+    | undefined;
+
+// Should NOT error: 'order2' is possibly 'undefined' after the guard
+if (order2 && order2.type === "a") {
+>order2 : Symbol(order2, Decl(discriminantOrderIndependence.ts, 22, 11))
+>order2.type : Symbol(type, Decl(discriminantOrderIndependence.ts, 18, 17), Decl(discriminantOrderIndependence.ts, 23, 7), Decl(discriminantOrderIndependence.ts, 24, 7), Decl(discriminantOrderIndependence.ts, 25, 7))
+>order2 : Symbol(order2, Decl(discriminantOrderIndependence.ts, 22, 11))
+>type : Symbol(type, Decl(discriminantOrderIndependence.ts, 18, 17), Decl(discriminantOrderIndependence.ts, 23, 7), Decl(discriminantOrderIndependence.ts, 24, 7), Decl(discriminantOrderIndependence.ts, 25, 7))
+
+    order2.type; // Should be OK
+>order2.type : Symbol(B.type, Decl(discriminantOrderIndependence.ts, 18, 17))
+>order2 : Symbol(order2, Decl(discriminantOrderIndependence.ts, 22, 11))
+>type : Symbol(B.type, Decl(discriminantOrderIndependence.ts, 18, 17))
+}
+
+// Also test with !. type assertion
+if (order1 && order1.type === "a") {
+>order1 : Symbol(order1, Decl(discriminantOrderIndependence.ts, 5, 11))
+>order1.type : Symbol(type, Decl(discriminantOrderIndependence.ts, 1, 17), Decl(discriminantOrderIndependence.ts, 6, 7), Decl(discriminantOrderIndependence.ts, 8, 7), Decl(discriminantOrderIndependence.ts, 9, 7))
+>order1 : Symbol(order1, Decl(discriminantOrderIndependence.ts, 5, 11))
+>type : Symbol(type, Decl(discriminantOrderIndependence.ts, 1, 17), Decl(discriminantOrderIndependence.ts, 6, 7), Decl(discriminantOrderIndependence.ts, 8, 7), Decl(discriminantOrderIndependence.ts, 9, 7))
+
+    order1.type; // Should be OK
+>order1.type : Symbol(A.type, Decl(discriminantOrderIndependence.ts, 1, 17))
+>order1 : Symbol(order1, Decl(discriminantOrderIndependence.ts, 5, 11))
+>type : Symbol(A.type, Decl(discriminantOrderIndependence.ts, 1, 17))
+}
+if (order2 && order2.type === "a") {
+>order2 : Symbol(order2, Decl(discriminantOrderIndependence.ts, 22, 11))
+>order2.type : Symbol(type, Decl(discriminantOrderIndependence.ts, 18, 17), Decl(discriminantOrderIndependence.ts, 23, 7), Decl(discriminantOrderIndependence.ts, 24, 7), Decl(discriminantOrderIndependence.ts, 25, 7))
+>order2 : Symbol(order2, Decl(discriminantOrderIndependence.ts, 22, 11))
+>type : Symbol(type, Decl(discriminantOrderIndependence.ts, 18, 17), Decl(discriminantOrderIndependence.ts, 23, 7), Decl(discriminantOrderIndependence.ts, 24, 7), Decl(discriminantOrderIndependence.ts, 25, 7))
+
+    order2.type; // Should be OK
+>order2.type : Symbol(B.type, Decl(discriminantOrderIndependence.ts, 18, 17))
+>order2 : Symbol(order2, Decl(discriminantOrderIndependence.ts, 22, 11))
+>type : Symbol(B.type, Decl(discriminantOrderIndependence.ts, 18, 17))
+}
diff --git a/tests/baselines/reference/discriminantOrderIndependence.types b/tests/baselines/reference/discriminantOrderIndependence.types
new file mode 100644
index 0000000000000..d561f4cbefc21
--- /dev/null
+++ b/tests/baselines/reference/discriminantOrderIndependence.types
@@ -0,0 +1,162 @@
+//// [tests/cases/compiler/discriminantOrderIndependence.ts] ////
+
+=== discriminantOrderIndependence.ts ===
+interface A {
+    subType: "b";
+>subType : "b"
+>        : ^^^
+
+    type: "a";
+>type : "a"
+>     : ^^^
+}
+
+declare let order1:
+>order1 : A | { type: "1"; } | { type: "2"; } | { type: "3"; }
+>       : ^^^^^^^^^^^^   ^^^^^^^^^^^^^^   ^^^^^^^^^^^^^^   ^^^
+
+    | { type: "1" }
+>type : "1"
+>     : ^^^
+
+    | A
+    | { type: "2" }
+>type : "2"
+>     : ^^^
+
+    | { type: "3" }
+>type : "3"
+>     : ^^^
+
+    | undefined;
+
+// Should NOT error: 'order1' is possibly 'undefined' after the guard
+if (order1 && order1.type === "a") {
+>order1 && order1.type === "a" : boolean
+>                              : ^^^^^^^
+>order1 : A | { type: "1"; } | { type: "2"; } | { type: "3"; }
+>       : ^^^^^^^^^^^^   ^^^^^^^^^^^^^^   ^^^^^^^^^^^^^^   ^^^
+>order1.type === "a" : boolean
+>                    : ^^^^^^^
+>order1.type : "a" | "1" | "2" | "3"
+>            : ^^^^^^^^^^^^^^^^^^^^^
+>order1 : A | { type: "1"; } | { type: "2"; } | { type: "3"; }
+>       : ^^^^^^^^^^^^   ^^^^^^^^^^^^^^   ^^^^^^^^^^^^^^   ^^^
+>type : "a" | "1" | "2" | "3"
+>     : ^^^^^^^^^^^^^^^^^^^^^
+>"a" : "a"
+>    : ^^^
+
+    order1.type; // Should be OK
+>order1.type : "a"
+>            : ^^^
+>order1 : A
+>       : ^
+>type : "a"
+>     : ^^^
+}
+
+interface B {
+    subType: "b";
+>subType : "b"
+>        : ^^^
+
+    type: "a";
+>type : "a"
+>     : ^^^
+}
+
+declare let order2:
+>order2 : B | { type: "1"; } | { type: "2"; } | { type: "3"; }
+>       : ^^^^^^^^^^^^   ^^^^^^^^^^^^^^   ^^^^^^^^^^^^^^   ^^^
+
+    | { type: "1" }
+>type : "1"
+>     : ^^^
+
+    | { type: "2" }
+>type : "2"
+>     : ^^^
+
+    | { type: "3" }
+>type : "3"
+>     : ^^^
+
+    | B
+    | undefined;
+
+// Should NOT error: 'order2' is possibly 'undefined' after the guard
+if (order2 && order2.type === "a") {
+>order2 && order2.type === "a" : boolean
+>                              : ^^^^^^^
+>order2 : B | { type: "1"; } | { type: "2"; } | { type: "3"; }
+>       : ^^^^^^^^^^^^   ^^^^^^^^^^^^^^   ^^^^^^^^^^^^^^   ^^^
+>order2.type === "a" : boolean
+>                    : ^^^^^^^
+>order2.type : "a" | "1" | "2" | "3"
+>            : ^^^^^^^^^^^^^^^^^^^^^
+>order2 : B | { type: "1"; } | { type: "2"; } | { type: "3"; }
+>       : ^^^^^^^^^^^^   ^^^^^^^^^^^^^^   ^^^^^^^^^^^^^^   ^^^
+>type : "a" | "1" | "2" | "3"
+>     : ^^^^^^^^^^^^^^^^^^^^^
+>"a" : "a"
+>    : ^^^
+
+    order2.type; // Should be OK
+>order2.type : "a"
+>            : ^^^
+>order2 : B
+>       : ^
+>type : "a"
+>     : ^^^
+}
+
+// Also test with !. type assertion
+if (order1 && order1.type === "a") {
+>order1 && order1.type === "a" : boolean
+>                              : ^^^^^^^
+>order1 : A | { type: "1"; } | { type: "2"; } | { type: "3"; }
+>       : ^^^^^^^^^^^^   ^^^^^^^^^^^^^^   ^^^^^^^^^^^^^^   ^^^
+>order1.type === "a" : boolean
+>                    : ^^^^^^^
+>order1.type : "a" | "1" | "2" | "3"
+>            : ^^^^^^^^^^^^^^^^^^^^^
+>order1 : A | { type: "1"; } | { type: "2"; } | { type: "3"; }
+>       : ^^^^^^^^^^^^   ^^^^^^^^^^^^^^   ^^^^^^^^^^^^^^   ^^^
+>type : "a" | "1" | "2" | "3"
+>     : ^^^^^^^^^^^^^^^^^^^^^
+>"a" : "a"
+>    : ^^^
+
+    order1.type; // Should be OK
+>order1.type : "a"
+>            : ^^^
+>order1 : A
+>       : ^
+>type : "a"
+>     : ^^^
+}
+if (order2 && order2.type === "a") {
+>order2 && order2.type === "a" : boolean
+>                              : ^^^^^^^
+>order2 : B | { type: "1"; } | { type: "2"; } | { type: "3"; }
+>       : ^^^^^^^^^^^^   ^^^^^^^^^^^^^^   ^^^^^^^^^^^^^^   ^^^
+>order2.type === "a" : boolean
+>                    : ^^^^^^^
+>order2.type : "a" | "1" | "2" | "3"
+>            : ^^^^^^^^^^^^^^^^^^^^^
+>order2 : B | { type: "1"; } | { type: "2"; } | { type: "3"; }
+>       : ^^^^^^^^^^^^   ^^^^^^^^^^^^^^   ^^^^^^^^^^^^^^   ^^^
+>type : "a" | "1" | "2" | "3"
+>     : ^^^^^^^^^^^^^^^^^^^^^
+>"a" : "a"
+>    : ^^^
+
+    order2.type; // Should be OK
+>order2.type : "a"
+>            : ^^^
+>order2 : B
+>       : ^
+>type : "a"
+>     : ^^^
+}
diff --git a/tests/cases/compiler/discriminantOrderIndependence.ts b/tests/cases/compiler/discriminantOrderIndependence.ts
new file mode 100644
index 0000000000000..f43924efdf0b8
--- /dev/null
+++ b/tests/cases/compiler/discriminantOrderIndependence.ts
@@ -0,0 +1,41 @@
+interface A {
+    subType: "b";
+    type: "a";
+}
+
+declare let order1:
+    | { type: "1" }
+    | A
+    | { type: "2" }
+    | { type: "3" }
+    | undefined;
+
+// Should NOT error: 'order1' is possibly 'undefined' after the guard
+if (order1 && order1.type === "a") {
+    order1.type; // Should be OK
+}
+
+interface B {
+    subType: "b";
+    type: "a";
+}
+
+declare let order2:
+    | { type: "1" }
+    | { type: "2" }
+    | { type: "3" }
+    | B
+    | undefined;
+
+// Should NOT error: 'order2' is possibly 'undefined' after the guard
+if (order2 && order2.type === "a") {
+    order2.type; // Should be OK
+}
+
+// Also test with !. type assertion
+if (order1 && order1.type === "a") {
+    order1.type; // Should be OK
+}
+if (order2 && order2.type === "a") {
+    order2.type; // Should be OK
+}
\ No newline at end of file