diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 20d075cdfc07f..5af10d2b76808 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -19734,7 +19734,7 @@ namespace ts {
                         // with another type parameter unless one extends the other. (Remember: comparability is mostly bidirectional!)
                         let constraint = getConstraintOfTypeParameter(source);
                         if (constraint && hasNonCircularBaseConstraint(source)) {
-                            while (constraint && constraint.flags & TypeFlags.TypeParameter) {
+                            while (constraint && someType(constraint, c => !!(c.flags & TypeFlags.TypeParameter))) {
                                 if (result = isRelatedTo(constraint, target, RecursionFlags.Source, /*reportErrors*/ false)) {
                                     return result;
                                 }
diff --git a/tests/baselines/reference/comparabilityTypeParametersRelatedByUnion.js b/tests/baselines/reference/comparabilityTypeParametersRelatedByUnion.js
new file mode 100644
index 0000000000000..a0e56a3dd66de
--- /dev/null
+++ b/tests/baselines/reference/comparabilityTypeParametersRelatedByUnion.js
@@ -0,0 +1,27 @@
+//// [comparabilityTypeParametersRelatedByUnion.ts]
+class C<T> {
+  constructor(readonly x: T) {}
+
+  good<U extends T>(y: U) {
+      if (y === this.x) {}
+  }
+
+  bad<U extends T | string>(y: U) {
+      if (y === this.x) {}
+  }
+}
+
+
+//// [comparabilityTypeParametersRelatedByUnion.js]
+var C = /** @class */ (function () {
+    function C(x) {
+        this.x = x;
+    }
+    C.prototype.good = function (y) {
+        if (y === this.x) { }
+    };
+    C.prototype.bad = function (y) {
+        if (y === this.x) { }
+    };
+    return C;
+}());
diff --git a/tests/baselines/reference/comparabilityTypeParametersRelatedByUnion.symbols b/tests/baselines/reference/comparabilityTypeParametersRelatedByUnion.symbols
new file mode 100644
index 0000000000000..482900c650c8b
--- /dev/null
+++ b/tests/baselines/reference/comparabilityTypeParametersRelatedByUnion.symbols
@@ -0,0 +1,38 @@
+=== tests/cases/compiler/comparabilityTypeParametersRelatedByUnion.ts ===
+class C<T> {
+>C : Symbol(C, Decl(comparabilityTypeParametersRelatedByUnion.ts, 0, 0))
+>T : Symbol(T, Decl(comparabilityTypeParametersRelatedByUnion.ts, 0, 8))
+
+  constructor(readonly x: T) {}
+>x : Symbol(C.x, Decl(comparabilityTypeParametersRelatedByUnion.ts, 1, 14))
+>T : Symbol(T, Decl(comparabilityTypeParametersRelatedByUnion.ts, 0, 8))
+
+  good<U extends T>(y: U) {
+>good : Symbol(C.good, Decl(comparabilityTypeParametersRelatedByUnion.ts, 1, 31))
+>U : Symbol(U, Decl(comparabilityTypeParametersRelatedByUnion.ts, 3, 7))
+>T : Symbol(T, Decl(comparabilityTypeParametersRelatedByUnion.ts, 0, 8))
+>y : Symbol(y, Decl(comparabilityTypeParametersRelatedByUnion.ts, 3, 20))
+>U : Symbol(U, Decl(comparabilityTypeParametersRelatedByUnion.ts, 3, 7))
+
+      if (y === this.x) {}
+>y : Symbol(y, Decl(comparabilityTypeParametersRelatedByUnion.ts, 3, 20))
+>this.x : Symbol(C.x, Decl(comparabilityTypeParametersRelatedByUnion.ts, 1, 14))
+>this : Symbol(C, Decl(comparabilityTypeParametersRelatedByUnion.ts, 0, 0))
+>x : Symbol(C.x, Decl(comparabilityTypeParametersRelatedByUnion.ts, 1, 14))
+  }
+
+  bad<U extends T | string>(y: U) {
+>bad : Symbol(C.bad, Decl(comparabilityTypeParametersRelatedByUnion.ts, 5, 3))
+>U : Symbol(U, Decl(comparabilityTypeParametersRelatedByUnion.ts, 7, 6))
+>T : Symbol(T, Decl(comparabilityTypeParametersRelatedByUnion.ts, 0, 8))
+>y : Symbol(y, Decl(comparabilityTypeParametersRelatedByUnion.ts, 7, 28))
+>U : Symbol(U, Decl(comparabilityTypeParametersRelatedByUnion.ts, 7, 6))
+
+      if (y === this.x) {}
+>y : Symbol(y, Decl(comparabilityTypeParametersRelatedByUnion.ts, 7, 28))
+>this.x : Symbol(C.x, Decl(comparabilityTypeParametersRelatedByUnion.ts, 1, 14))
+>this : Symbol(C, Decl(comparabilityTypeParametersRelatedByUnion.ts, 0, 0))
+>x : Symbol(C.x, Decl(comparabilityTypeParametersRelatedByUnion.ts, 1, 14))
+  }
+}
+
diff --git a/tests/baselines/reference/comparabilityTypeParametersRelatedByUnion.types b/tests/baselines/reference/comparabilityTypeParametersRelatedByUnion.types
new file mode 100644
index 0000000000000..f2d380552d801
--- /dev/null
+++ b/tests/baselines/reference/comparabilityTypeParametersRelatedByUnion.types
@@ -0,0 +1,32 @@
+=== tests/cases/compiler/comparabilityTypeParametersRelatedByUnion.ts ===
+class C<T> {
+>C : C<T>
+
+  constructor(readonly x: T) {}
+>x : T
+
+  good<U extends T>(y: U) {
+>good : <U extends T>(y: U) => void
+>y : U
+
+      if (y === this.x) {}
+>y === this.x : boolean
+>y : U
+>this.x : T
+>this : this
+>x : T
+  }
+
+  bad<U extends T | string>(y: U) {
+>bad : <U extends string | T>(y: U) => void
+>y : U
+
+      if (y === this.x) {}
+>y === this.x : boolean
+>y : U
+>this.x : T
+>this : this
+>x : T
+  }
+}
+
diff --git a/tests/cases/compiler/comparabilityTypeParametersRelatedByUnion.ts b/tests/cases/compiler/comparabilityTypeParametersRelatedByUnion.ts
new file mode 100644
index 0000000000000..1e9f0eb6e227d
--- /dev/null
+++ b/tests/cases/compiler/comparabilityTypeParametersRelatedByUnion.ts
@@ -0,0 +1,11 @@
+class C<T> {
+  constructor(readonly x: T) {}
+
+  good<U extends T>(y: U) {
+      if (y === this.x) {}
+  }
+
+  bad<U extends T | string>(y: U) {
+      if (y === this.x) {}
+  }
+}