From 692d828e2c30019202a25ff85b1ff99902a798a5 Mon Sep 17 00:00:00 2001
From: kntkymt <kntkc0de@gmail.com>
Date: Sun, 22 Jun 2025 15:48:28 +0900
Subject: [PATCH 1/3] Add getNumCurryLevels() to AnyFunctionType

---
 include/swift/AST/Types.h | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/include/swift/AST/Types.h b/include/swift/AST/Types.h
index e8dc2b3d1216e..89154c097e8df 100644
--- a/include/swift/AST/Types.h
+++ b/include/swift/AST/Types.h
@@ -3884,6 +3884,17 @@ class AnyFunctionType : public TypeBase {
     return containsPackExpansionType(getParams());
   }
 
+  /// Returns the number of curry levels of the function type.
+  unsigned getNumCurryLevels() const {
+    unsigned num = 1;
+    auto fn = this;
+    while (auto resultFn = fn->getResult()->getAs<AnyFunctionType>()) {
+      num++;
+      fn = resultFn;
+    }
+    return num;
+  }
+
   static bool containsPackExpansionType(ArrayRef<Param> params);
 
   static void printParams(ArrayRef<Param> Params, raw_ostream &OS,

From e898bda66b70ed7b20886b9a22db7c4488e1e43e Mon Sep 17 00:00:00 2001
From: kntkymt <kntkc0de@gmail.com>
Date: Sun, 22 Jun 2025 15:53:08 +0900
Subject: [PATCH 2/3] check curryLevels at repairByInsertingExplicitCall not to
 repair conversion of functions with same curryLevels

---
 lib/Sema/CSSimplify.cpp | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/lib/Sema/CSSimplify.cpp b/lib/Sema/CSSimplify.cpp
index 7266ac632853a..b9d9b4dd385ef 100644
--- a/lib/Sema/CSSimplify.cpp
+++ b/lib/Sema/CSSimplify.cpp
@@ -5347,10 +5347,12 @@ bool ConstraintSystem::repairFailures(
     // this by forming an explicit call.
     auto convertTo = dstType->lookThroughAllOptionalTypes();
 
-    // If the RHS is a function type, the source must be a function-returning
-    // function.
-    if (convertTo->is<FunctionType>() && !resultType->is<FunctionType>())
-      return false;
+    // If the RHS is a function type,
+    // the source must have more curry levels.
+    if (auto convertToFnType = convertTo->getAs<FunctionType>()) {
+      if (fnType->getNumCurryLevels() <= convertToFnType->getNumCurryLevels())
+        return false;
+    }
 
     // Right-hand side can't be a type variable or dependent member, or `Any`
     // (if function conversion to `Any` didn't succeed there is something else

From 0f98c1b7317a1ac52fd5fc6d961ed7d4f1c1bbed Mon Sep 17 00:00:00 2001
From: kntkymt <kntkc0de@gmail.com>
Date: Sun, 22 Jun 2025 16:04:17 +0900
Subject: [PATCH 3/3] Add test for function-returning function conversion

---
 test/Constraints/closures.swift | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/test/Constraints/closures.swift b/test/Constraints/closures.swift
index 26547edae4053..8e76342f255a3 100644
--- a/test/Constraints/closures.swift
+++ b/test/Constraints/closures.swift
@@ -1366,3 +1366,14 @@ do {
     try $0.missing // expected-error {{value of type 'Int' has no member 'missing'}}
   }
 }
+
+do {
+  let a: () -> () -> Void = {{}}
+  let _: (() -> () -> Void)? = a
+  let _: (() -> () -> Void)?? = a
+
+  class Super {}
+  class Sub: Super {}
+  let b: () -> () -> Sub = {{ return Sub() }}
+  let _: (() -> () -> Super)? = b
+}