From ad4542e79eabadd25e0add133dadbe90e0f0cfcc Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Mateusz=20Burzy=C5=84ski?= <mateuszburzynski@gmail.com>
Date: Thu, 5 Jun 2025 20:00:27 +0200
Subject: [PATCH] Port "Check if switch statements are exhaustive when their
 expressions is generic with a literal type constraint"

---
 internal/checker/flow.go                      |  2 +-
 ...exhaustiveSwitchStatementsGeneric1.symbols | 48 +++++++++++++++++++
 .../exhaustiveSwitchStatementsGeneric1.types  | 44 +++++++++++++++++
 .../exhaustiveSwitchStatementsGeneric1.ts     | 26 ++++++++++
 4 files changed, 119 insertions(+), 1 deletion(-)
 create mode 100644 testdata/baselines/reference/compiler/exhaustiveSwitchStatementsGeneric1.symbols
 create mode 100644 testdata/baselines/reference/compiler/exhaustiveSwitchStatementsGeneric1.types
 create mode 100644 testdata/tests/cases/compiler/exhaustiveSwitchStatementsGeneric1.ts

diff --git a/internal/checker/flow.go b/internal/checker/flow.go
index d43a0ddccc..b8e1841171 100644
--- a/internal/checker/flow.go
+++ b/internal/checker/flow.go
@@ -1929,7 +1929,7 @@ func (c *Checker) computeExhaustiveSwitchStatement(node *ast.Node) bool {
 			return c.getTypeFacts(t, notEqualFacts) == notEqualFacts
 		})
 	}
-	t := c.checkExpressionCached(node.Expression())
+	t := c.getBaseConstraintOrType(c.checkExpressionCached(node.Expression()))
 	if !isLiteralType(t) {
 		return false
 	}
diff --git a/testdata/baselines/reference/compiler/exhaustiveSwitchStatementsGeneric1.symbols b/testdata/baselines/reference/compiler/exhaustiveSwitchStatementsGeneric1.symbols
new file mode 100644
index 0000000000..136cbc0e93
--- /dev/null
+++ b/testdata/baselines/reference/compiler/exhaustiveSwitchStatementsGeneric1.symbols
@@ -0,0 +1,48 @@
+//// [tests/cases/compiler/exhaustiveSwitchStatementsGeneric1.ts] ////
+
+=== exhaustiveSwitchStatementsGeneric1.ts ===
+// https://github.com/microsoft/typescript-go/issues/986
+
+interface A {
+>A : Symbol(A, Decl(exhaustiveSwitchStatementsGeneric1.ts, 0, 0))
+
+  type: "a";
+>type : Symbol(type, Decl(exhaustiveSwitchStatementsGeneric1.ts, 2, 13))
+}
+
+interface B {
+>B : Symbol(B, Decl(exhaustiveSwitchStatementsGeneric1.ts, 4, 1))
+
+  type: "b";
+>type : Symbol(type, Decl(exhaustiveSwitchStatementsGeneric1.ts, 6, 13))
+}
+
+interface Types {
+>Types : Symbol(Types, Decl(exhaustiveSwitchStatementsGeneric1.ts, 8, 1))
+
+  a: A;
+>a : Symbol(a, Decl(exhaustiveSwitchStatementsGeneric1.ts, 10, 17))
+>A : Symbol(A, Decl(exhaustiveSwitchStatementsGeneric1.ts, 0, 0))
+
+  b: B;
+>b : Symbol(b, Decl(exhaustiveSwitchStatementsGeneric1.ts, 11, 7))
+>B : Symbol(B, Decl(exhaustiveSwitchStatementsGeneric1.ts, 4, 1))
+}
+
+export function exhaustiveSwitch<T extends keyof Types>(type: T): boolean {
+>exhaustiveSwitch : Symbol(exhaustiveSwitch, Decl(exhaustiveSwitchStatementsGeneric1.ts, 13, 1))
+>T : Symbol(T, Decl(exhaustiveSwitchStatementsGeneric1.ts, 15, 33))
+>Types : Symbol(Types, Decl(exhaustiveSwitchStatementsGeneric1.ts, 8, 1))
+>type : Symbol(type, Decl(exhaustiveSwitchStatementsGeneric1.ts, 15, 56))
+>T : Symbol(T, Decl(exhaustiveSwitchStatementsGeneric1.ts, 15, 33))
+
+  switch (type) {
+>type : Symbol(type, Decl(exhaustiveSwitchStatementsGeneric1.ts, 15, 56))
+
+    case "a":
+      return true;
+    case "b":
+      return true;
+  }
+}
+
diff --git a/testdata/baselines/reference/compiler/exhaustiveSwitchStatementsGeneric1.types b/testdata/baselines/reference/compiler/exhaustiveSwitchStatementsGeneric1.types
new file mode 100644
index 0000000000..5cce40cc85
--- /dev/null
+++ b/testdata/baselines/reference/compiler/exhaustiveSwitchStatementsGeneric1.types
@@ -0,0 +1,44 @@
+//// [tests/cases/compiler/exhaustiveSwitchStatementsGeneric1.ts] ////
+
+=== exhaustiveSwitchStatementsGeneric1.ts ===
+// https://github.com/microsoft/typescript-go/issues/986
+
+interface A {
+  type: "a";
+>type : "a"
+}
+
+interface B {
+  type: "b";
+>type : "b"
+}
+
+interface Types {
+  a: A;
+>a : A
+
+  b: B;
+>b : B
+}
+
+export function exhaustiveSwitch<T extends keyof Types>(type: T): boolean {
+>exhaustiveSwitch : <T extends keyof Types>(type: T) => boolean
+>type : T
+
+  switch (type) {
+>type : T
+
+    case "a":
+>"a" : "a"
+
+      return true;
+>true : true
+
+    case "b":
+>"b" : "b"
+
+      return true;
+>true : true
+  }
+}
+
diff --git a/testdata/tests/cases/compiler/exhaustiveSwitchStatementsGeneric1.ts b/testdata/tests/cases/compiler/exhaustiveSwitchStatementsGeneric1.ts
new file mode 100644
index 0000000000..0b5dcb762f
--- /dev/null
+++ b/testdata/tests/cases/compiler/exhaustiveSwitchStatementsGeneric1.ts
@@ -0,0 +1,26 @@
+// @strict: true
+// @noEmit: true
+
+// https://github.com/microsoft/typescript-go/issues/986
+
+interface A {
+  type: "a";
+}
+
+interface B {
+  type: "b";
+}
+
+interface Types {
+  a: A;
+  b: B;
+}
+
+export function exhaustiveSwitch<T extends keyof Types>(type: T): boolean {
+  switch (type) {
+    case "a":
+      return true;
+    case "b":
+      return true;
+  }
+}