Revert "[ConformanceLookup] Just kidding, the compiler needs to prefer available"

This reverts commit b139770.

Author: hborla

lib/AST/ConformanceLookupTable.cpp

@@ -70,18 +70,9 @@ void ConformanceLookupTable::ConformanceEntry::markSupersededBy(
   SupersededBy = entry;
 
   if (diagnose) {
-    // If an unavailable Sendable conformance is superseded by a
-    // retroactive one in the client, we need to record this error
-    // at the client decl context.
-    auto *dc = getDeclContext();
-    if (getProtocol()->isMarkerProtocol() && isFixed() &&
-        !entry->isFixed()) {
-      dc = entry->getDeclContext();
-    }
-
     // Record the problem in the conformance table. We'll
     // diagnose these in semantic analysis.
-    table.AllSupersededDiagnostics[dc].push_back(this);
+    table.AllSupersededDiagnostics[getDeclContext()].push_back(this);
   }
 }
 
@@ -607,22 +598,21 @@ ConformanceLookupTable::Ordering ConformanceLookupTable::compareConformances(
     }
   }
 
-  // If only one of the conformances is unconditionally available on the
-  // current deployment target, pick that one.
-  //
-  // FIXME: Conformance lookup should really depend on source location for
-  // this to be 100% correct.
-  // FIXME: When a class and an extension with the same availability declare the
-  // same conformance, this silently takes the class and drops the extension.
-  if (lhs->getDeclContext()->isAlwaysAvailableConformanceContext() !=
-      rhs->getDeclContext()->isAlwaysAvailableConformanceContext()) {
-    // Diagnose conflicting marker protocol conformances that differ in
-    // un-availability.
-    diagnoseSuperseded = lhs->getProtocol()->isMarkerProtocol();
-
-    return (lhs->getDeclContext()->isAlwaysAvailableConformanceContext()
-            ? Ordering::Before
-            : Ordering::After);
+  // Unavailable Sendable conformances cannot be replaced by available ones.
+  if (!lhs->getProtocol()->isMarkerProtocol()) {
+    // If only one of the conformances is unconditionally available on the
+    // current deployment target, pick that one.
+    //
+    // FIXME: Conformance lookup should really depend on source location for
+    // this to be 100% correct.
+    // FIXME: When a class and an extension with the same availability declare the
+    // same conformance, this silently takes the class and drops the extension.
+    if (lhs->getDeclContext()->isAlwaysAvailableConformanceContext() !=
+        rhs->getDeclContext()->isAlwaysAvailableConformanceContext()) {
+      return (lhs->getDeclContext()->isAlwaysAvailableConformanceContext()
+              ? Ordering::Before
+              : Ordering::After);
+    }
   }
 
   // If one entry is fixed and the other is not, we have our answer.
@@ -1138,17 +1128,9 @@ void ConformanceLookupTable::lookupConformances(
   if (diagnostics) {
     auto knownDiags = AllSupersededDiagnostics.find(dc);
     if (knownDiags != AllSupersededDiagnostics.end()) {
-      for (auto *entry : knownDiags->second) {
+      for (const auto *entry : knownDiags->second) {
         ConformanceEntry *supersededBy = entry->getSupersededBy();
 
-        // Diagnose the client conformance as superseded.
-        auto *definingModule = nominal->getParentModule();
-        if (entry->getDeclContext()->getParentModule() == definingModule &&
-            supersededBy->getDeclContext()->getParentModule() != definingModule) {
-          supersededBy = entry;
-          entry = entry->getSupersededBy();
-        }
-
         diagnostics->push_back({entry->getProtocol(), 
                                 entry->getDeclaredLoc(),
                                 entry->getKind(),

test/Concurrency/Inputs/SendableConformances.swift

@@ -5,5 +5,3 @@ public class NonSendableClass {}
 extension NonSendableClass: @unchecked Sendable {}
 
 public struct SendableStruct: Sendable {}
-
-public struct AnotherSendableStruct: Sendable {}

test/Concurrency/redundant_sendable_conformance.swift

@@ -13,12 +13,7 @@ extension NonSendableClass: @retroactive @unchecked Sendable {}
 // expected-warning@-1 {{conformance of 'NonSendableClass' to protocol 'Sendable' was already stated in the type's module 'SendableConformances'}}
 
 typealias CheckNonSendableClass = RequireSendable<NonSendableClass>
+// expected-error@-1 {{conformance of 'NonSendableClass' to 'Sendable' is unavailable}}
 
 extension SendableStruct: @retroactive @unchecked Sendable {}
 // expected-warning@-1 {{conformance of 'SendableStruct' to protocol 'Sendable' was already stated in the type's module 'SendableConformances'}}
-
-@available(*, unavailable)
-extension AnotherSendableStruct: @retroactive @unchecked Sendable {}
-// expected-warning@-1 {{conformance of 'AnotherSendableStruct' to protocol 'Sendable' was already stated in the type's module 'SendableConformances'}}
-
-typealias CheckAnotherSendableStruct = RequireSendable<AnotherSendableStruct>