diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 3b1771d209664..948be393e006d 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -8144,7 +8144,7 @@ namespace ts {
             // types with type parameters mapped to the wildcard type, the most permissive instantiations
             // possible (the wildcard type is assignable to and from all types). If those are not related,
             // then no instatiations will be and we can just return the false branch type.
-            if (!isTypeAssignableTo(getWildcardInstantiation(checkType), getWildcardInstantiation(extendsType))) {
+            if (!typeMaybeAssignableTo(getWildcardInstantiation(checkType), getWildcardInstantiation(extendsType))) {
                 return instantiateType(baseFalseType, mapper);
             }
             // The check could be true for some instantiation
diff --git a/tests/baselines/reference/conditionalTypes1.errors.txt b/tests/baselines/reference/conditionalTypes1.errors.txt
index a7717d182631c..4f515876b1ef7 100644
--- a/tests/baselines/reference/conditionalTypes1.errors.txt
+++ b/tests/baselines/reference/conditionalTypes1.errors.txt
@@ -344,4 +344,24 @@ tests/cases/conformance/types/conditional/conditionalTypes1.ts(157,5): error TS2
     type T50 = IsNever<never>;  // true
     type T51 = IsNever<number>;  // false
     type T52 = IsNever<any>;  // false
+    
+    // Repros from #21664
+    
+    type Eq<T, U> = T extends U ? U extends T ? true : false : false;
+    type T60 = Eq<true, true>;  // true
+    type T61 = Eq<true, false>;  // false
+    type T62 = Eq<false, true>;  // false
+    type T63 = Eq<false, false>;  // true
+    
+    type Eq1<T, U> = Eq<T, U> extends false ? false : true;
+    type T70 = Eq1<true, true>;  // true
+    type T71 = Eq1<true, false>;  // false
+    type T72 = Eq1<false, true>;  // false
+    type T73 = Eq1<false, false>;  // true
+    
+    type Eq2<T, U> = Eq<T, U> extends true ? true : false;
+    type T80 = Eq2<true, true>;  // true
+    type T81 = Eq2<true, false>;  // false
+    type T82 = Eq2<false, true>;  // false
+    type T83 = Eq2<false, false>;  // true
     
\ No newline at end of file
diff --git a/tests/baselines/reference/conditionalTypes1.js b/tests/baselines/reference/conditionalTypes1.js
index 0da7794a76214..3ca5966ce8eb9 100644
--- a/tests/baselines/reference/conditionalTypes1.js
+++ b/tests/baselines/reference/conditionalTypes1.js
@@ -204,6 +204,26 @@ type IsNever<T> = T extends never ? true : false;
 type T50 = IsNever<never>;  // true
 type T51 = IsNever<number>;  // false
 type T52 = IsNever<any>;  // false
+
+// Repros from #21664
+
+type Eq<T, U> = T extends U ? U extends T ? true : false : false;
+type T60 = Eq<true, true>;  // true
+type T61 = Eq<true, false>;  // false
+type T62 = Eq<false, true>;  // false
+type T63 = Eq<false, false>;  // true
+
+type Eq1<T, U> = Eq<T, U> extends false ? false : true;
+type T70 = Eq1<true, true>;  // true
+type T71 = Eq1<true, false>;  // false
+type T72 = Eq1<false, true>;  // false
+type T73 = Eq1<false, false>;  // true
+
+type Eq2<T, U> = Eq<T, U> extends true ? true : false;
+type T80 = Eq2<true, true>;  // true
+type T81 = Eq2<true, false>;  // false
+type T82 = Eq2<false, true>;  // false
+type T83 = Eq2<false, false>;  // true
 
 
 //// [conditionalTypes1.js]
@@ -415,3 +435,18 @@ declare type IsNever<T> = T extends never ? true : false;
 declare type T50 = IsNever<never>;
 declare type T51 = IsNever<number>;
 declare type T52 = IsNever<any>;
+declare type Eq<T, U> = T extends U ? U extends T ? true : false : false;
+declare type T60 = Eq<true, true>;
+declare type T61 = Eq<true, false>;
+declare type T62 = Eq<false, true>;
+declare type T63 = Eq<false, false>;
+declare type Eq1<T, U> = Eq<T, U> extends false ? false : true;
+declare type T70 = Eq1<true, true>;
+declare type T71 = Eq1<true, false>;
+declare type T72 = Eq1<false, true>;
+declare type T73 = Eq1<false, false>;
+declare type Eq2<T, U> = Eq<T, U> extends true ? true : false;
+declare type T80 = Eq2<true, true>;
+declare type T81 = Eq2<true, false>;
+declare type T82 = Eq2<false, true>;
+declare type T83 = Eq2<false, false>;
diff --git a/tests/baselines/reference/conditionalTypes1.symbols b/tests/baselines/reference/conditionalTypes1.symbols
index 9d3b79d5c30a0..276c1af740a62 100644
--- a/tests/baselines/reference/conditionalTypes1.symbols
+++ b/tests/baselines/reference/conditionalTypes1.symbols
@@ -777,3 +777,78 @@ type T52 = IsNever<any>;  // false
 >T52 : Symbol(T52, Decl(conditionalTypes1.ts, 203, 27))
 >IsNever : Symbol(IsNever, Decl(conditionalTypes1.ts, 198, 47))
 
+// Repros from #21664
+
+type Eq<T, U> = T extends U ? U extends T ? true : false : false;
+>Eq : Symbol(Eq, Decl(conditionalTypes1.ts, 204, 24))
+>T : Symbol(T, Decl(conditionalTypes1.ts, 208, 8))
+>U : Symbol(U, Decl(conditionalTypes1.ts, 208, 10))
+>T : Symbol(T, Decl(conditionalTypes1.ts, 208, 8))
+>U : Symbol(U, Decl(conditionalTypes1.ts, 208, 10))
+>U : Symbol(U, Decl(conditionalTypes1.ts, 208, 10))
+>T : Symbol(T, Decl(conditionalTypes1.ts, 208, 8))
+
+type T60 = Eq<true, true>;  // true
+>T60 : Symbol(T60, Decl(conditionalTypes1.ts, 208, 65))
+>Eq : Symbol(Eq, Decl(conditionalTypes1.ts, 204, 24))
+
+type T61 = Eq<true, false>;  // false
+>T61 : Symbol(T61, Decl(conditionalTypes1.ts, 209, 26))
+>Eq : Symbol(Eq, Decl(conditionalTypes1.ts, 204, 24))
+
+type T62 = Eq<false, true>;  // false
+>T62 : Symbol(T62, Decl(conditionalTypes1.ts, 210, 27))
+>Eq : Symbol(Eq, Decl(conditionalTypes1.ts, 204, 24))
+
+type T63 = Eq<false, false>;  // true
+>T63 : Symbol(T63, Decl(conditionalTypes1.ts, 211, 27))
+>Eq : Symbol(Eq, Decl(conditionalTypes1.ts, 204, 24))
+
+type Eq1<T, U> = Eq<T, U> extends false ? false : true;
+>Eq1 : Symbol(Eq1, Decl(conditionalTypes1.ts, 212, 28))
+>T : Symbol(T, Decl(conditionalTypes1.ts, 214, 9))
+>U : Symbol(U, Decl(conditionalTypes1.ts, 214, 11))
+>Eq : Symbol(Eq, Decl(conditionalTypes1.ts, 204, 24))
+>T : Symbol(T, Decl(conditionalTypes1.ts, 214, 9))
+>U : Symbol(U, Decl(conditionalTypes1.ts, 214, 11))
+
+type T70 = Eq1<true, true>;  // true
+>T70 : Symbol(T70, Decl(conditionalTypes1.ts, 214, 55))
+>Eq1 : Symbol(Eq1, Decl(conditionalTypes1.ts, 212, 28))
+
+type T71 = Eq1<true, false>;  // false
+>T71 : Symbol(T71, Decl(conditionalTypes1.ts, 215, 27))
+>Eq1 : Symbol(Eq1, Decl(conditionalTypes1.ts, 212, 28))
+
+type T72 = Eq1<false, true>;  // false
+>T72 : Symbol(T72, Decl(conditionalTypes1.ts, 216, 28))
+>Eq1 : Symbol(Eq1, Decl(conditionalTypes1.ts, 212, 28))
+
+type T73 = Eq1<false, false>;  // true
+>T73 : Symbol(T73, Decl(conditionalTypes1.ts, 217, 28))
+>Eq1 : Symbol(Eq1, Decl(conditionalTypes1.ts, 212, 28))
+
+type Eq2<T, U> = Eq<T, U> extends true ? true : false;
+>Eq2 : Symbol(Eq2, Decl(conditionalTypes1.ts, 218, 29))
+>T : Symbol(T, Decl(conditionalTypes1.ts, 220, 9))
+>U : Symbol(U, Decl(conditionalTypes1.ts, 220, 11))
+>Eq : Symbol(Eq, Decl(conditionalTypes1.ts, 204, 24))
+>T : Symbol(T, Decl(conditionalTypes1.ts, 220, 9))
+>U : Symbol(U, Decl(conditionalTypes1.ts, 220, 11))
+
+type T80 = Eq2<true, true>;  // true
+>T80 : Symbol(T80, Decl(conditionalTypes1.ts, 220, 54))
+>Eq2 : Symbol(Eq2, Decl(conditionalTypes1.ts, 218, 29))
+
+type T81 = Eq2<true, false>;  // false
+>T81 : Symbol(T81, Decl(conditionalTypes1.ts, 221, 27))
+>Eq2 : Symbol(Eq2, Decl(conditionalTypes1.ts, 218, 29))
+
+type T82 = Eq2<false, true>;  // false
+>T82 : Symbol(T82, Decl(conditionalTypes1.ts, 222, 28))
+>Eq2 : Symbol(Eq2, Decl(conditionalTypes1.ts, 218, 29))
+
+type T83 = Eq2<false, false>;  // true
+>T83 : Symbol(T83, Decl(conditionalTypes1.ts, 223, 28))
+>Eq2 : Symbol(Eq2, Decl(conditionalTypes1.ts, 218, 29))
+
diff --git a/tests/baselines/reference/conditionalTypes1.types b/tests/baselines/reference/conditionalTypes1.types
index df825ecd40b7d..e8a50e60fcc4c 100644
--- a/tests/baselines/reference/conditionalTypes1.types
+++ b/tests/baselines/reference/conditionalTypes1.types
@@ -883,3 +883,111 @@ type T52 = IsNever<any>;  // false
 >T52 : false
 >IsNever : IsNever<T>
 
+// Repros from #21664
+
+type Eq<T, U> = T extends U ? U extends T ? true : false : false;
+>Eq : Eq<T, U>
+>T : T
+>U : U
+>T : T
+>U : U
+>U : U
+>T : T
+>true : true
+>false : false
+>false : false
+
+type T60 = Eq<true, true>;  // true
+>T60 : true
+>Eq : Eq<T, U>
+>true : true
+>true : true
+
+type T61 = Eq<true, false>;  // false
+>T61 : false
+>Eq : Eq<T, U>
+>true : true
+>false : false
+
+type T62 = Eq<false, true>;  // false
+>T62 : false
+>Eq : Eq<T, U>
+>false : false
+>true : true
+
+type T63 = Eq<false, false>;  // true
+>T63 : true
+>Eq : Eq<T, U>
+>false : false
+>false : false
+
+type Eq1<T, U> = Eq<T, U> extends false ? false : true;
+>Eq1 : Eq1<T, U>
+>T : T
+>U : U
+>Eq : Eq<T, U>
+>T : T
+>U : U
+>false : false
+>false : false
+>true : true
+
+type T70 = Eq1<true, true>;  // true
+>T70 : true
+>Eq1 : Eq1<T, U>
+>true : true
+>true : true
+
+type T71 = Eq1<true, false>;  // false
+>T71 : false
+>Eq1 : Eq1<T, U>
+>true : true
+>false : false
+
+type T72 = Eq1<false, true>;  // false
+>T72 : false
+>Eq1 : Eq1<T, U>
+>false : false
+>true : true
+
+type T73 = Eq1<false, false>;  // true
+>T73 : true
+>Eq1 : Eq1<T, U>
+>false : false
+>false : false
+
+type Eq2<T, U> = Eq<T, U> extends true ? true : false;
+>Eq2 : Eq2<T, U>
+>T : T
+>U : U
+>Eq : Eq<T, U>
+>T : T
+>U : U
+>true : true
+>true : true
+>false : false
+
+type T80 = Eq2<true, true>;  // true
+>T80 : true
+>Eq2 : Eq2<T, U>
+>true : true
+>true : true
+
+type T81 = Eq2<true, false>;  // false
+>T81 : false
+>Eq2 : Eq2<T, U>
+>true : true
+>false : false
+
+type T82 = Eq2<false, true>;  // false
+>T82 : false
+>Eq2 : Eq2<T, U>
+>false : false
+>true : true
+
+type T83 = Eq2<false, false>;  // true
+>T83 : true
+>Eq2 : Eq2<T, U>
+>false : false
+>false : false
+
diff --git a/tests/cases/conformance/types/conditional/conditionalTypes1.ts b/tests/cases/conformance/types/conditional/conditionalTypes1.ts
index a6169d428ff25..7ac6255b7d59c 100644
--- a/tests/cases/conformance/types/conditional/conditionalTypes1.ts
+++ b/tests/cases/conformance/types/conditional/conditionalTypes1.ts
@@ -206,3 +206,23 @@ type IsNever<T> = T extends never ? true : false;
 type T50 = IsNever<never>;  // true
 type T51 = IsNever<number>;  // false
 type T52 = IsNever<any>;  // false
+
+// Repros from #21664
+
+type Eq<T, U> = T extends U ? U extends T ? true : false : false;
+type T60 = Eq<true, true>;  // true
+type T61 = Eq<true, false>;  // false
+type T62 = Eq<false, true>;  // false
+type T63 = Eq<false, false>;  // true
+
+type Eq1<T, U> = Eq<T, U> extends false ? false : true;
+type T70 = Eq1<true, true>;  // true
+type T71 = Eq1<true, false>;  // false
+type T72 = Eq1<false, true>;  // false
+type T73 = Eq1<false, false>;  // true
+
+type Eq2<T, U> = Eq<T, U> extends true ? true : false;
+type T80 = Eq2<true, true>;  // true
+type T81 = Eq2<true, false>;  // false
+type T82 = Eq2<false, true>;  // false
+type T83 = Eq2<false, false>;  // true