diff --git a/tests/baselines/reference/conditionalTypeRelaxingConstraintAssignability.js b/tests/baselines/reference/conditionalTypeRelaxingConstraintAssignability.js
index f066b18d5c6..5f142116bab 100644
--- a/tests/baselines/reference/conditionalTypeRelaxingConstraintAssignability.js
+++ b/tests/baselines/reference/conditionalTypeRelaxingConstraintAssignability.js
@@ -21,7 +21,21 @@ export class Elem<
 new Elem(undefined as ElChildren.Void);
 new Elem('' as ElChildren.Text);
 new Elem('' as ElChildren.Void | ElChildren.Text); // error
-new Elem('' as ElChildren); // error
+new Elem('' as ElChildren); // error
+
+// Repro from #31766
+
+interface I { a: string }
+
+type DeepPartial<T> =
+    T extends object ? {[K in keyof T]?: DeepPartial<T[K]>} : T;
+
+declare function f<T>(t: T, partial: DeepPartial<T>): T;
+
+function g(p1: I, p2: Partial<I>): I {
+  return f(p1, p2);
+}
+
 
 //// [conditionalTypeRelaxingConstraintAssignability.js]
 "use strict";
@@ -37,3 +51,6 @@ new Elem(undefined);
 new Elem('');
 new Elem(''); // error
 new Elem(''); // error
+function g(p1, p2) {
+    return f(p1, p2);
+}
diff --git a/tests/baselines/reference/conditionalTypeRelaxingConstraintAssignability.symbols b/tests/baselines/reference/conditionalTypeRelaxingConstraintAssignability.symbols
index 7ce85b0fbfa..cd526f9d4bc 100644
--- a/tests/baselines/reference/conditionalTypeRelaxingConstraintAssignability.symbols
+++ b/tests/baselines/reference/conditionalTypeRelaxingConstraintAssignability.symbols
@@ -71,3 +71,47 @@ new Elem('' as ElChildren); // error
 >Elem : Symbol(Elem, Decl(conditionalTypeRelaxingConstraintAssignability.ts, 8, 83))
 >ElChildren : Symbol(ElChildren, Decl(conditionalTypeRelaxingConstraintAssignability.ts, 0, 0), Decl(conditionalTypeRelaxingConstraintAssignability.ts, 2, 20))
 
+// Repro from #31766
+
+interface I { a: string }
+>I : Symbol(I, Decl(conditionalTypeRelaxingConstraintAssignability.ts, 22, 27))
+>a : Symbol(I.a, Decl(conditionalTypeRelaxingConstraintAssignability.ts, 26, 13))
+
+type DeepPartial<T> =
+>DeepPartial : Symbol(DeepPartial, Decl(conditionalTypeRelaxingConstraintAssignability.ts, 26, 25))
+>T : Symbol(T, Decl(conditionalTypeRelaxingConstraintAssignability.ts, 28, 17))
+
+    T extends object ? {[K in keyof T]?: DeepPartial<T[K]>} : T;
+>T : Symbol(T, Decl(conditionalTypeRelaxingConstraintAssignability.ts, 28, 17))
+>K : Symbol(K, Decl(conditionalTypeRelaxingConstraintAssignability.ts, 29, 25))
+>T : Symbol(T, Decl(conditionalTypeRelaxingConstraintAssignability.ts, 28, 17))
+>DeepPartial : Symbol(DeepPartial, Decl(conditionalTypeRelaxingConstraintAssignability.ts, 26, 25))
+>T : Symbol(T, Decl(conditionalTypeRelaxingConstraintAssignability.ts, 28, 17))
+>K : Symbol(K, Decl(conditionalTypeRelaxingConstraintAssignability.ts, 29, 25))
+>T : Symbol(T, Decl(conditionalTypeRelaxingConstraintAssignability.ts, 28, 17))
+
+declare function f<T>(t: T, partial: DeepPartial<T>): T;
+>f : Symbol(f, Decl(conditionalTypeRelaxingConstraintAssignability.ts, 29, 64))
+>T : Symbol(T, Decl(conditionalTypeRelaxingConstraintAssignability.ts, 31, 19))
+>t : Symbol(t, Decl(conditionalTypeRelaxingConstraintAssignability.ts, 31, 22))
+>T : Symbol(T, Decl(conditionalTypeRelaxingConstraintAssignability.ts, 31, 19))
+>partial : Symbol(partial, Decl(conditionalTypeRelaxingConstraintAssignability.ts, 31, 27))
+>DeepPartial : Symbol(DeepPartial, Decl(conditionalTypeRelaxingConstraintAssignability.ts, 26, 25))
+>T : Symbol(T, Decl(conditionalTypeRelaxingConstraintAssignability.ts, 31, 19))
+>T : Symbol(T, Decl(conditionalTypeRelaxingConstraintAssignability.ts, 31, 19))
+
+function g(p1: I, p2: Partial<I>): I {
+>g : Symbol(g, Decl(conditionalTypeRelaxingConstraintAssignability.ts, 31, 56))
+>p1 : Symbol(p1, Decl(conditionalTypeRelaxingConstraintAssignability.ts, 33, 11))
+>I : Symbol(I, Decl(conditionalTypeRelaxingConstraintAssignability.ts, 22, 27))
+>p2 : Symbol(p2, Decl(conditionalTypeRelaxingConstraintAssignability.ts, 33, 17))
+>Partial : Symbol(Partial, Decl(lib.es5.d.ts, --, --))
+>I : Symbol(I, Decl(conditionalTypeRelaxingConstraintAssignability.ts, 22, 27))
+>I : Symbol(I, Decl(conditionalTypeRelaxingConstraintAssignability.ts, 22, 27))
+
+  return f(p1, p2);
+>f : Symbol(f, Decl(conditionalTypeRelaxingConstraintAssignability.ts, 29, 64))
+>p1 : Symbol(p1, Decl(conditionalTypeRelaxingConstraintAssignability.ts, 33, 11))
+>p2 : Symbol(p2, Decl(conditionalTypeRelaxingConstraintAssignability.ts, 33, 17))
+}
+
diff --git a/tests/baselines/reference/conditionalTypeRelaxingConstraintAssignability.types b/tests/baselines/reference/conditionalTypeRelaxingConstraintAssignability.types
index c827e26bddb..2df682290f5 100644
--- a/tests/baselines/reference/conditionalTypeRelaxingConstraintAssignability.types
+++ b/tests/baselines/reference/conditionalTypeRelaxingConstraintAssignability.types
@@ -62,3 +62,30 @@ new Elem('' as ElChildren); // error
 >'' as ElChildren : ElChildren
 >'' : ""
 
+// Repro from #31766
+
+interface I { a: string }
+>a : string
+
+type DeepPartial<T> =
+>DeepPartial : DeepPartial<T>
+
+    T extends object ? {[K in keyof T]?: DeepPartial<T[K]>} : T;
+
+declare function f<T>(t: T, partial: DeepPartial<T>): T;
+>f : <T>(t: T, partial: DeepPartial<T>) => T
+>t : T
+>partial : DeepPartial<T>
+
+function g(p1: I, p2: Partial<I>): I {
+>g : (p1: I, p2: Partial<I>) => I
+>p1 : I
+>p2 : Partial<I>
+
+  return f(p1, p2);
+>f(p1, p2) : I
+>f : <T>(t: T, partial: DeepPartial<T>) => T
+>p1 : I
+>p2 : Partial<I>
+}
+
diff --git a/tests/cases/compiler/conditionalTypeRelaxingConstraintAssignability.ts b/tests/cases/compiler/conditionalTypeRelaxingConstraintAssignability.ts
index 2f18e464e3b..04eaee9b511 100644
--- a/tests/cases/compiler/conditionalTypeRelaxingConstraintAssignability.ts
+++ b/tests/cases/compiler/conditionalTypeRelaxingConstraintAssignability.ts
@@ -21,4 +21,17 @@ export class Elem<
 new Elem(undefined as ElChildren.Void);
 new Elem('' as ElChildren.Text);
 new Elem('' as ElChildren.Void | ElChildren.Text); // error
-new Elem('' as ElChildren); // error
\ No newline at end of file
+new Elem('' as ElChildren); // error
+
+// Repro from #31766
+
+interface I { a: string }
+
+type DeepPartial<T> =
+    T extends object ? {[K in keyof T]?: DeepPartial<T[K]>} : T;
+
+declare function f<T>(t: T, partial: DeepPartial<T>): T;
+
+function g(p1: I, p2: Partial<I>): I {
+  return f(p1, p2);
+}