diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index bbd7def0d784f..4e1698451632c 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -18545,8 +18545,9 @@ namespace ts {
                         // Two conditional types 'T1 extends U1 ? X1 : Y1' and 'T2 extends U2 ? X2 : Y2' are related if
                         // one of T1 and T2 is related to the other, U1 and U2 are identical types, X1 is related to X2,
                         // and Y1 is related to Y2.
+
                         const sourceParams = (source as ConditionalType).root.inferTypeParameters;
-                        let sourceExtends = (<ConditionalType>source).extendsType;
+                        let sourceExtends = instantiateType((<ConditionalType>source).extendsType, makeFunctionTypeMapper(reportUnmeasurableMarkers));
                         let mapper: TypeMapper | undefined;
                         if (sourceParams) {
                             // If the source has infer type parameters, we instantiate them in the context of the target
diff --git a/tests/baselines/reference/variance.js b/tests/baselines/reference/variance.js
index df84404b31beb..e17b9a71194ea 100644
--- a/tests/baselines/reference/variance.js
+++ b/tests/baselines/reference/variance.js
@@ -24,11 +24,48 @@ class Bar<T extends string> {
     Bar.instance.push(this);
   }
 }
+
+
+
+interface Set<T> {
+    add(value: T): this;
+}
+
+declare const Set: new <T>() => Set<T>;
+
+// Repro from #31251 (removed getter)
+
+export abstract class Supervisor<N extends string, P = unknown, R = unknown> {
+    private static instances_: Set<Supervisor<string, unknown, unknown>>;
+    private static instances(): typeof Supervisor.instances_ {
+        return this.hasOwnProperty('instances_')
+            ? this.instances_
+            : this.instances_ = new Set();
+    }
+    constructor() {
+        void (this.constructor as typeof Supervisor).instances().add(this);
+    }
+    public abstract call(name: N | ('' extends N ? undefined : never), param: P, timeout?: number): Promise<R>;
+}
+
+
+// Minimal repro for catching variance probing in then extends type.
+
+interface A<T> {
+    x: number extends T ? 1 : 1;
+}
+
+declare let a: A<number>;
+declare let b: A<3>;
+
+a = b; // error
+b = a; // error
 
 
 //// [variance.js]
 "use strict";
 // Test cases for parameter variances affected by conditional types.
+exports.__esModule = true;
 var foo = { prop: true };
 var x = foo;
 var y = foo;
@@ -43,3 +80,18 @@ var Bar = /** @class */ (function () {
     };
     return Bar;
 }());
+// Repro from #31251 (removed getter)
+var Supervisor = /** @class */ (function () {
+    function Supervisor() {
+        void this.constructor.instances().add(this);
+    }
+    Supervisor.instances = function () {
+        return this.hasOwnProperty('instances_')
+            ? this.instances_
+            : this.instances_ = new Set();
+    };
+    return Supervisor;
+}());
+exports.Supervisor = Supervisor;
+a = b; // error
+b = a; // error
diff --git a/tests/baselines/reference/variance.symbols b/tests/baselines/reference/variance.symbols
index 0a6ec56d725b6..4d22305f55776 100644
--- a/tests/baselines/reference/variance.symbols
+++ b/tests/baselines/reference/variance.symbols
@@ -60,3 +60,108 @@ class Bar<T extends string> {
   }
 }
 
+
+
+interface Set<T> {
+>Set : Symbol(Set, Decl(variance.ts, 24, 1), Decl(variance.ts, 32, 13))
+>T : Symbol(T, Decl(variance.ts, 28, 14))
+
+    add(value: T): this;
+>add : Symbol(Set.add, Decl(variance.ts, 28, 18))
+>value : Symbol(value, Decl(variance.ts, 29, 8))
+>T : Symbol(T, Decl(variance.ts, 28, 14))
+}
+
+declare const Set: new <T>() => Set<T>;
+>Set : Symbol(Set, Decl(variance.ts, 24, 1), Decl(variance.ts, 32, 13))
+>T : Symbol(T, Decl(variance.ts, 32, 24))
+>Set : Symbol(Set, Decl(variance.ts, 24, 1), Decl(variance.ts, 32, 13))
+>T : Symbol(T, Decl(variance.ts, 32, 24))
+
+// Repro from #31251 (removed getter)
+
+export abstract class Supervisor<N extends string, P = unknown, R = unknown> {
+>Supervisor : Symbol(Supervisor, Decl(variance.ts, 32, 39))
+>N : Symbol(N, Decl(variance.ts, 36, 33))
+>P : Symbol(P, Decl(variance.ts, 36, 50))
+>R : Symbol(R, Decl(variance.ts, 36, 63))
+
+    private static instances_: Set<Supervisor<string, unknown, unknown>>;
+>instances_ : Symbol(Supervisor.instances_, Decl(variance.ts, 36, 78))
+>Set : Symbol(Set, Decl(variance.ts, 24, 1), Decl(variance.ts, 32, 13))
+>Supervisor : Symbol(Supervisor, Decl(variance.ts, 32, 39))
+
+    private static instances(): typeof Supervisor.instances_ {
+>instances : Symbol(Supervisor.instances, Decl(variance.ts, 37, 73))
+>Supervisor.instances_ : Symbol(Supervisor.instances_, Decl(variance.ts, 36, 78))
+>Supervisor : Symbol(Supervisor, Decl(variance.ts, 32, 39))
+>instances_ : Symbol(Supervisor.instances_, Decl(variance.ts, 36, 78))
+
+        return this.hasOwnProperty('instances_')
+>this.hasOwnProperty : Symbol(Object.hasOwnProperty, Decl(lib.es5.d.ts, --, --))
+>this : Symbol(Supervisor, Decl(variance.ts, 32, 39))
+>hasOwnProperty : Symbol(Object.hasOwnProperty, Decl(lib.es5.d.ts, --, --))
+
+            ? this.instances_
+>this.instances_ : Symbol(Supervisor.instances_, Decl(variance.ts, 36, 78))
+>this : Symbol(Supervisor, Decl(variance.ts, 32, 39))
+>instances_ : Symbol(Supervisor.instances_, Decl(variance.ts, 36, 78))
+
+            : this.instances_ = new Set();
+>this.instances_ : Symbol(Supervisor.instances_, Decl(variance.ts, 36, 78))
+>this : Symbol(Supervisor, Decl(variance.ts, 32, 39))
+>instances_ : Symbol(Supervisor.instances_, Decl(variance.ts, 36, 78))
+>Set : Symbol(Set, Decl(variance.ts, 24, 1), Decl(variance.ts, 32, 13))
+    }
+    constructor() {
+        void (this.constructor as typeof Supervisor).instances().add(this);
+>(this.constructor as typeof Supervisor).instances().add : Symbol(Set.add, Decl(variance.ts, 28, 18))
+>(this.constructor as typeof Supervisor).instances : Symbol(Supervisor.instances, Decl(variance.ts, 37, 73))
+>this.constructor : Symbol(Object.constructor, Decl(lib.es5.d.ts, --, --))
+>this : Symbol(Supervisor, Decl(variance.ts, 32, 39))
+>constructor : Symbol(Object.constructor, Decl(lib.es5.d.ts, --, --))
+>Supervisor : Symbol(Supervisor, Decl(variance.ts, 32, 39))
+>instances : Symbol(Supervisor.instances, Decl(variance.ts, 37, 73))
+>add : Symbol(Set.add, Decl(variance.ts, 28, 18))
+>this : Symbol(Supervisor, Decl(variance.ts, 32, 39))
+    }
+    public abstract call(name: N | ('' extends N ? undefined : never), param: P, timeout?: number): Promise<R>;
+>call : Symbol(Supervisor.call, Decl(variance.ts, 45, 5))
+>name : Symbol(name, Decl(variance.ts, 46, 25))
+>N : Symbol(N, Decl(variance.ts, 36, 33))
+>N : Symbol(N, Decl(variance.ts, 36, 33))
+>param : Symbol(param, Decl(variance.ts, 46, 70))
+>P : Symbol(P, Decl(variance.ts, 36, 50))
+>timeout : Symbol(timeout, Decl(variance.ts, 46, 80))
+>Promise : Symbol(Promise, Decl(lib.es5.d.ts, --, --))
+>R : Symbol(R, Decl(variance.ts, 36, 63))
+}
+
+
+// Minimal repro for catching variance probing in then extends type.
+
+interface A<T> {
+>A : Symbol(A, Decl(variance.ts, 47, 1))
+>T : Symbol(T, Decl(variance.ts, 52, 12))
+
+    x: number extends T ? 1 : 1;
+>x : Symbol(A.x, Decl(variance.ts, 52, 16))
+>T : Symbol(T, Decl(variance.ts, 52, 12))
+}
+
+declare let a: A<number>;
+>a : Symbol(a, Decl(variance.ts, 56, 11))
+>A : Symbol(A, Decl(variance.ts, 47, 1))
+
+declare let b: A<3>;
+>b : Symbol(b, Decl(variance.ts, 57, 11))
+>A : Symbol(A, Decl(variance.ts, 47, 1))
+
+a = b; // error
+>a : Symbol(a, Decl(variance.ts, 56, 11))
+>b : Symbol(b, Decl(variance.ts, 57, 11))
+
+b = a; // error
+>b : Symbol(b, Decl(variance.ts, 57, 11))
+>a : Symbol(a, Decl(variance.ts, 56, 11))
+
diff --git a/tests/baselines/reference/variance.types b/tests/baselines/reference/variance.types
index 08ee1053b3dba..8c80c8e82255c 100644
--- a/tests/baselines/reference/variance.types
+++ b/tests/baselines/reference/variance.types
@@ -56,3 +56,97 @@ class Bar<T extends string> {
   }
 }
 
+
+
+interface Set<T> {
+    add(value: T): this;
+>add : (value: T) => this
+>value : T
+}
+
+declare const Set: new <T>() => Set<T>;
+>Set : new <T>() => Set<T>
+
+// Repro from #31251 (removed getter)
+
+export abstract class Supervisor<N extends string, P = unknown, R = unknown> {
+>Supervisor : Supervisor<N, P, R>
+
+    private static instances_: Set<Supervisor<string, unknown, unknown>>;
+>instances_ : Set<Supervisor<string, unknown, unknown>>
+
+    private static instances(): typeof Supervisor.instances_ {
+>instances : () => Set<Supervisor<string, unknown, unknown>>
+>Supervisor.instances_ : Set<Supervisor<string, unknown, unknown>>
+>Supervisor : typeof Supervisor
+>instances_ : Set<Supervisor<string, unknown, unknown>>
+
+        return this.hasOwnProperty('instances_')
+>this.hasOwnProperty('instances_')            ? this.instances_            : this.instances_ = new Set() : Set<Supervisor<string, unknown, unknown>>
+>this.hasOwnProperty('instances_') : boolean
+>this.hasOwnProperty : (v: string | number | symbol) => boolean
+>this : typeof Supervisor
+>hasOwnProperty : (v: string | number | symbol) => boolean
+>'instances_' : "instances_"
+
+            ? this.instances_
+>this.instances_ : Set<Supervisor<string, unknown, unknown>>
+>this : typeof Supervisor
+>instances_ : Set<Supervisor<string, unknown, unknown>>
+
+            : this.instances_ = new Set();
+>this.instances_ = new Set() : Set<Supervisor<string, unknown, unknown>>
+>this.instances_ : Set<Supervisor<string, unknown, unknown>>
+>this : typeof Supervisor
+>instances_ : Set<Supervisor<string, unknown, unknown>>
+>new Set() : Set<Supervisor<string, unknown, unknown>>
+>Set : new <T>() => Set<T>
+    }
+    constructor() {
+        void (this.constructor as typeof Supervisor).instances().add(this);
+>void (this.constructor as typeof Supervisor).instances().add(this) : undefined
+>(this.constructor as typeof Supervisor).instances().add(this) : Set<Supervisor<string, unknown, unknown>>
+>(this.constructor as typeof Supervisor).instances().add : (value: Supervisor<string, unknown, unknown>) => Set<Supervisor<string, unknown, unknown>>
+>(this.constructor as typeof Supervisor).instances() : Set<Supervisor<string, unknown, unknown>>
+>(this.constructor as typeof Supervisor).instances : () => Set<Supervisor<string, unknown, unknown>>
+>(this.constructor as typeof Supervisor) : typeof Supervisor
+>this.constructor as typeof Supervisor : typeof Supervisor
+>this.constructor : Function
+>this : this
+>constructor : Function
+>Supervisor : typeof Supervisor
+>instances : () => Set<Supervisor<string, unknown, unknown>>
+>add : (value: Supervisor<string, unknown, unknown>) => Set<Supervisor<string, unknown, unknown>>
+>this : this
+    }
+    public abstract call(name: N | ('' extends N ? undefined : never), param: P, timeout?: number): Promise<R>;
+>call : (name: N | ("" extends N ? undefined : never), param: P, timeout?: number | undefined) => Promise<R>
+>name : N | ("" extends N ? undefined : never)
+>param : P
+>timeout : number | undefined
+}
+
+
+// Minimal repro for catching variance probing in then extends type.
+
+interface A<T> {
+    x: number extends T ? 1 : 1;
+>x : number extends T ? 1 : 1
+}
+
+declare let a: A<number>;
+>a : A<number>
+
+declare let b: A<3>;
+>b : A<3>
+
+a = b; // error
+>a = b : A<3>
+>a : A<number>
+>b : A<3>
+
+b = a; // error
+>b = a : A<number>
+>b : A<3>
+>a : A<number>
+
diff --git a/tests/cases/conformance/types/conditional/variance.ts b/tests/cases/conformance/types/conditional/variance.ts
index 1dd5db5c67e8f..761c460bef3a6 100644
--- a/tests/cases/conformance/types/conditional/variance.ts
+++ b/tests/cases/conformance/types/conditional/variance.ts
@@ -25,3 +25,39 @@ class Bar<T extends string> {
     Bar.instance.push(this);
   }
 }
+
+
+
+interface Set<T> {
+    add(value: T): this;
+}
+
+declare const Set: new <T>() => Set<T>;
+
+// Repro from #31251 (removed getter)
+
+export abstract class Supervisor<N extends string, P = unknown, R = unknown> {
+    private static instances_: Set<Supervisor<string, unknown, unknown>>;
+    private static instances(): typeof Supervisor.instances_ {
+        return this.hasOwnProperty('instances_')
+            ? this.instances_
+            : this.instances_ = new Set();
+    }
+    constructor() {
+        void (this.constructor as typeof Supervisor).instances().add(this);
+    }
+    public abstract call(name: N | ('' extends N ? undefined : never), param: P, timeout?: number): Promise<R>;
+}
+
+
+// Minimal repro for catching variance probing in then extends type.
+
+interface A<T> {
+    x: number extends T ? 1 : 1;
+}
+
+declare let a: A<number>;
+declare let b: A<3>;
+
+a = b; // error
+b = a; // error