From 843ce585a3b6ea93e7494de6866780a136272016 Mon Sep 17 00:00:00 2001
From: Greg Titus <toon@omnigroup.com>
Date: Sun, 9 Jun 2024 16:11:19 -0700
Subject: [PATCH] Set fix impact for non-settable overload choices higher, so
 we get diagnoses from better choices.

---
 lib/Sema/CSSimplify.cpp      |  7 ++++++-
 test/Sema/immutability.swift | 13 +++++++++++++
 2 files changed, 19 insertions(+), 1 deletion(-)

diff --git a/lib/Sema/CSSimplify.cpp b/lib/Sema/CSSimplify.cpp
index 9a9b2cdb365a4..a6c5234afda80 100644
--- a/lib/Sema/CSSimplify.cpp
+++ b/lib/Sema/CSSimplify.cpp
@@ -15201,7 +15201,12 @@ ConstraintSystem::SolutionKind ConstraintSystem::simplifyFixConstraint(
                locator.endsWith<LocatorPathElt::SubscriptMember>())
                   ? 1
                   : 0;
-
+    // An overload choice that isn't settable is least interesting for diagnosis.
+    if (auto overload = findSelectedOverloadFor(getCalleeLocator(fix->getLocator()))) {
+      if (auto *var = dyn_cast_or_null<VarDecl>(overload->choice.getDeclOrNull())) {
+         impact += !var->isSettableInSwift(DC) ? 1 : 0;
+      }
+    }
     return recordFix(fix, impact) ? SolutionKind::Error : SolutionKind::Solved;
   }
 
diff --git a/test/Sema/immutability.swift b/test/Sema/immutability.swift
index 00026f169983a..7669a9275b74d 100644
--- a/test/Sema/immutability.swift
+++ b/test/Sema/immutability.swift
@@ -762,3 +762,16 @@ struct S2<T> {
     // expected-note@-1 {{add explicit 'S2<T>.' to refer to mutable static property of 'S2<T>'}} {{5-5=S2<T>.}}
   }
 }
+
+// SR-3680, https://github.com/apple/swift/issues/46265
+protocol HasFoo {
+  var foo: String { get }
+}
+protocol CanSetFoo {
+  var foo: String { get set }
+}
+extension HasFoo where Self: CanSetFoo {
+  func bar() { // expected-note {{mark method 'mutating' to make 'self' mutable}}{{3-3=mutating }}
+    self.foo = "bar" // expected-error {{cannot assign to property: 'self' is immutable}}
+  }
+}