diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 813e09f97ca..82ab87d3160 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -12923,7 +12923,7 @@ namespace ts {
                 const discriminantType = getUnionType(clauseTypes);
                 const caseType =
                     discriminantType.flags & TypeFlags.Never ? neverType :
-                        replacePrimitivesWithLiterals(filterType(type, t => isTypeComparableTo(discriminantType, t)), discriminantType);
+                        replacePrimitivesWithLiterals(filterType(type, t => areTypesComparable(discriminantType, t)), discriminantType);
                 if (!hasDefaultClause) {
                     return caseType;
                 }
diff --git a/tests/baselines/reference/switchWithConstrainedTypeVariable.js b/tests/baselines/reference/switchWithConstrainedTypeVariable.js
new file mode 100644
index 00000000000..8e6bbe07fe1
--- /dev/null
+++ b/tests/baselines/reference/switchWithConstrainedTypeVariable.js
@@ -0,0 +1,28 @@
+//// [switchWithConstrainedTypeVariable.ts]
+// Repro from #20840
+
+function function1<T extends 'a' | 'b'>(key: T) {
+  switch (key) {
+    case 'a':
+      key.toLowerCase();
+      break;
+    default:
+      key.toLowerCase();
+      break;
+  }
+}
+
+
+//// [switchWithConstrainedTypeVariable.js]
+"use strict";
+// Repro from #20840
+function function1(key) {
+    switch (key) {
+        case 'a':
+            key.toLowerCase();
+            break;
+        default:
+            key.toLowerCase();
+            break;
+    }
+}
diff --git a/tests/baselines/reference/switchWithConstrainedTypeVariable.symbols b/tests/baselines/reference/switchWithConstrainedTypeVariable.symbols
new file mode 100644
index 00000000000..76a3e1c818f
--- /dev/null
+++ b/tests/baselines/reference/switchWithConstrainedTypeVariable.symbols
@@ -0,0 +1,29 @@
+=== tests/cases/conformance/controlFlow/switchWithConstrainedTypeVariable.ts ===
+// Repro from #20840
+
+function function1<T extends 'a' | 'b'>(key: T) {
+>function1 : Symbol(function1, Decl(switchWithConstrainedTypeVariable.ts, 0, 0))
+>T : Symbol(T, Decl(switchWithConstrainedTypeVariable.ts, 2, 19))
+>key : Symbol(key, Decl(switchWithConstrainedTypeVariable.ts, 2, 40))
+>T : Symbol(T, Decl(switchWithConstrainedTypeVariable.ts, 2, 19))
+
+  switch (key) {
+>key : Symbol(key, Decl(switchWithConstrainedTypeVariable.ts, 2, 40))
+
+    case 'a':
+      key.toLowerCase();
+>key.toLowerCase : Symbol(String.toLowerCase, Decl(lib.d.ts, --, --))
+>key : Symbol(key, Decl(switchWithConstrainedTypeVariable.ts, 2, 40))
+>toLowerCase : Symbol(String.toLowerCase, Decl(lib.d.ts, --, --))
+
+      break;
+    default:
+      key.toLowerCase();
+>key.toLowerCase : Symbol(String.toLowerCase, Decl(lib.d.ts, --, --))
+>key : Symbol(key, Decl(switchWithConstrainedTypeVariable.ts, 2, 40))
+>toLowerCase : Symbol(String.toLowerCase, Decl(lib.d.ts, --, --))
+
+      break;
+  }
+}
+
diff --git a/tests/baselines/reference/switchWithConstrainedTypeVariable.types b/tests/baselines/reference/switchWithConstrainedTypeVariable.types
new file mode 100644
index 00000000000..be7cc59f348
--- /dev/null
+++ b/tests/baselines/reference/switchWithConstrainedTypeVariable.types
@@ -0,0 +1,33 @@
+=== tests/cases/conformance/controlFlow/switchWithConstrainedTypeVariable.ts ===
+// Repro from #20840
+
+function function1<T extends 'a' | 'b'>(key: T) {
+>function1 : <T extends "a" | "b">(key: T) => void
+>T : T
+>key : T
+>T : T
+
+  switch (key) {
+>key : T
+
+    case 'a':
+>'a' : "a"
+
+      key.toLowerCase();
+>key.toLowerCase() : string
+>key.toLowerCase : () => string
+>key : T
+>toLowerCase : () => string
+
+      break;
+    default:
+      key.toLowerCase();
+>key.toLowerCase() : string
+>key.toLowerCase : () => string
+>key : T
+>toLowerCase : () => string
+
+      break;
+  }
+}
+
diff --git a/tests/cases/conformance/controlFlow/switchWithConstrainedTypeVariable.ts b/tests/cases/conformance/controlFlow/switchWithConstrainedTypeVariable.ts
new file mode 100644
index 00000000000..f6efd36811e
--- /dev/null
+++ b/tests/cases/conformance/controlFlow/switchWithConstrainedTypeVariable.ts
@@ -0,0 +1,14 @@
+// @strict: true
+
+// Repro from #20840
+
+function function1<T extends 'a' | 'b'>(key: T) {
+  switch (key) {
+    case 'a':
+      key.toLowerCase();
+      break;
+    default:
+      key.toLowerCase();
+      break;
+  }
+}