[CSSimplify] Increase score if conformance constraint finds missing conformances

Although the solver is allowed to use types with missing synthesizable
conformances we need to note their presence in the score in order
to rank overloads without such types higher.

Author: xedin

lib/Sema/CSSimplify.cpp

@@ -8412,6 +8412,16 @@ ConstraintSystem::SolutionKind ConstraintSystem::simplifyConformsToConstraint(
     if (isConformanceUnavailable(conformance, loc))
       increaseScore(SK_Unavailable, locator);
 
+    unsigned numMissing = 0;
+    conformance.forEachMissingConformance(DC->getParentModule(),
+                                          [&numMissing](auto *missing) {
+                                            ++numMissing;
+                                            return false;
+                                          });
+
+    if (numMissing > 0)
+      increaseScore(SK_MissingSynthesizableConformance, locator, numMissing);
+
     // This conformance may be conditional, in which case we need to consider
     // those requirements as constraints too.
     if (conformance.isConcrete()) {

test/Concurrency/sendable_disambiguation.swift

@@ -0,0 +1,30 @@
+// RUN: %target-swift-frontend %s -emit-sil -o /dev/null -verify -strict-concurrency=complete
+
+// REQUIRES: concurrency
+// REQUIRES: asserts
+// REQUIRES: OS=macosx
+
+do {
+  struct Test {
+    init<T: Sendable>(value: T) {}
+    // expected-note@-1 {{found this candidate}}
+    init<T>(value: T) {}
+    // expected-note@-1 {{found this candidate}}
+  }
+
+  struct SendableOnly {
+    init<T: Sendable>(value: T) {}
+  }
+
+  func testNonSendable<T>(v: T) { // expected-note {{consider making generic parameter 'T' conform to the 'Sendable' protocol}}
+    _ = Test(value: v) // Ok (non-Sendable overload)
+    _ = SendableOnly(value: v)
+    // expected-warning@-1 {{type 'T' does not conform to the 'Sendable' protocol}}
+  }
+
+  func testSendable<T: Sendable>(v: T) {
+    _ = Test(value: v)
+    // expected-error@-1 {{ambiguous use of 'init(value:)'}}
+    _ = SendableOnly(value: v) // Ok
+  }
+}