[Type checker] Delay the diagnosis of missing witnesses further.

The handling of "global" missing witnesses would diagnose a missing witness
(showing "almost" matches) and then, later, provide the Fix-It to add
the declaration if the user wants it. Delay the diagnostic part until later,
when we add the missing witness.

While here, correct an existing issue with memory corruption that
occurs because we would diagnose some of these witnesses, then clear a
"global" witness list, while still iterating through parts of that
list.

Author: DougGregor

lib/Sema/TypeCheckProtocol.cpp

@@ -1607,7 +1607,8 @@ void MultiConformanceChecker::checkAllConformances() {
   for (auto It = AllUsedCheckers.rbegin(); It != AllUsedCheckers.rend();
        ++It) {
     if (!It->getLocalMissingWitness().empty()) {
-      It->diagnoseMissingWitnesses(MissingWitnessDiagnosisKind::FixItOnly);
+      if (It->diagnoseMissingWitnesses(MissingWitnessDiagnosisKind::FixItOnly))
+        break;
     }
   }
 }
@@ -3099,13 +3100,30 @@ filterProtocolRequirements(
   return Filtered;
 }
 
-void ConformanceChecker::
+bool ConformanceChecker::
 diagnoseMissingWitnesses(MissingWitnessDiagnosisKind Kind) {
   auto LocalMissing = getLocalMissingWitness();
 
   // If this conformance has nothing to complain, return.
   if (LocalMissing.empty())
-    return;
+    return false;
+
+  // Diagnose the missing witnesses.
+  for (auto &Missing : LocalMissing) {
+    auto requirement = Missing.requirement;
+    auto matches = Missing.matches;
+    auto nominal = Adoptee->getAnyNominal();
+    diagnoseOrDefer(requirement, true,
+      [requirement, matches, nominal](NormalProtocolConformance *conformance) {
+        auto dc = conformance->getDeclContext();
+        auto *protocol = conformance->getProtocol();
+        // Possibly diagnose reason for automatic derivation failure
+        DerivedConformance::tryDiagnoseFailedDerivation(dc, nominal, protocol);
+        // Diagnose each of the matches.
+        for (const auto &match : matches)
+          diagnoseMatch(dc->getParentModule(), conformance, requirement, match);
+      });
+  }
 
   const auto InsertFixit = [](
       NormalProtocolConformance *Conf, SourceLoc ComplainLoc, bool EditorMode,
@@ -3226,19 +3244,19 @@ diagnoseMissingWitnesses(MissingWitnessDiagnosisKind Kind) {
           });
     }
     clearGlobalMissingWitnesses();
-    return;
+    return true;
   }
   case MissingWitnessDiagnosisKind::ErrorOnly: {
     diagnoseOrDefer(
         LocalMissing[0].requirement, true, [](NormalProtocolConformance *) {});
-    return;
+    return true;
   }
   case MissingWitnessDiagnosisKind::FixItOnly:
     InsertFixit(Conformance, Loc, IsEditorMode,
                 filterProtocolRequirements(GlobalMissingWitnesses.getArrayRef(),
                                            Adoptee));
     clearGlobalMissingWitnesses();
-    return;
+    return true;
   }
 }
 
@@ -3744,17 +3762,7 @@ ConformanceChecker::resolveWitnessViaLookup(ValueDecl *requirement) {
   if (!numViable) {
     // Save the missing requirement for later diagnosis.
     GlobalMissingWitnesses.insert({requirement, matches});
-    diagnoseOrDefer(requirement, true,
-      [requirement, matches, nominal](NormalProtocolConformance *conformance) {
-        auto dc = conformance->getDeclContext();
-        auto *protocol = conformance->getProtocol();
-        // Possibly diagnose reason for automatic derivation failure
-        DerivedConformance::tryDiagnoseFailedDerivation(dc, nominal, protocol);
-        // Diagnose each of the matches.
-        for (const auto &match : matches)
-          diagnoseMatch(dc->getParentModule(), conformance, requirement, match);
-      });
-    return ResolveWitnessResult::ExplicitFailed;
+    return ResolveWitnessResult::Missing;
   }
 
   diagnoseOrDefer(requirement, true,

lib/Sema/TypeCheckProtocol.h

@@ -789,7 +789,10 @@ class ConformanceChecker : public WitnessChecker {
 
 public:
   /// Call this to diagnose currently known missing witnesses.
-  void diagnoseMissingWitnesses(MissingWitnessDiagnosisKind Kind);
+  ///
+  /// \returns true if any witnesses were diagnosed.
+  bool diagnoseMissingWitnesses(MissingWitnessDiagnosisKind Kind);
+
   /// Emit any diagnostics that have been delayed.
   void emitDelayedDiags();