Set fix impact for non-settable overload choices higher, so we get diagnoses from better choices.

Author: Greg Titus

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;
   }
 

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}}
+  }
+}