[AST] Remove requiresSubstitution invariant from Witness

Author: xedin

include/swift/AST/Witness.h

@@ -150,9 +150,6 @@ class Witness {
   /// Determines whether there is a witness at all.
   explicit operator bool() const { return !storage.isNull(); }
 
-  /// Determine whether this witness requires any substitutions.
-  bool requiresSubstitution() const { return storage.is<StoredWitness *>(); }
-
   /// Retrieve the substitutions required to use this witness from the
   /// synthetic environment.
   ///
@@ -164,15 +161,17 @@ class Witness {
 
   /// Retrieve the synthetic generic environment.
   GenericEnvironment *getSyntheticEnvironment() const {
-    assert(requiresSubstitution() && "No substitutions required for witness");
-    return storage.get<StoredWitness *>()->syntheticEnvironment;
+    if (auto *storedWitness = storage.dyn_cast<StoredWitness *>())
+      return storedWitness->syntheticEnvironment;
+    return nullptr;
   }
 
   /// Retrieve the substitution map that maps the interface types of the
   /// requirement to the interface types of the synthetic environment.
   SubstitutionList getRequirementToSyntheticSubs() const {
-    assert(requiresSubstitution() && "No substitutions required for witness");
-    return storage.get<StoredWitness *>()->reqToSyntheticEnvSubs;
+    if (auto *storedWitness = storage.dyn_cast<StoredWitness *>())
+      return storedWitness->reqToSyntheticEnvSubs;
+    return {};
   }
 
   LLVM_ATTRIBUTE_DEPRECATED(

lib/AST/ASTVerifier.cpp

@@ -2507,13 +2507,18 @@ class Verifier : public ASTWalker {
           // Check the witness substitutions.
           const auto &witness = normal->getWitness(req, nullptr);
 
-          if (witness.requiresSubstitution()) {
-            GenericEnv.push_back({witness.getSyntheticEnvironment()});
-            for (const auto &sub : witness.getSubstitutions()) {
-              verifyChecked(sub.getReplacement());
-            }
+          if (auto *genericEnv = witness.getSyntheticEnvironment())
+            GenericEnv.push_back({genericEnv});
+
+          for (const auto &sub : witness.getRequirementToSyntheticSubs())
+            verifyChecked(sub.getReplacement());
+
+          for (const auto &sub : witness.getSubstitutions())
+            verifyChecked(sub.getReplacement());
+
+          if (auto *genericEnv = witness.getSyntheticEnvironment()) {
             assert(GenericEnv.back().storage.dyn_cast<GenericEnvironment *>()
-                     == witness.getSyntheticEnvironment());
+                     == genericEnv);
             GenericEnv.pop_back();
           }
 

lib/Serialization/Serialization.cpp

@@ -1573,9 +1573,7 @@ void Serializer::writeNormalConformance(
    // Write the witness substitutions.
    writeSubstitutions(witness.getSubstitutions(),
                       DeclTypeAbbrCodes,
-                      witness.requiresSubstitution()
-                        ? witness.getSyntheticEnvironment()
-                        : nullptr);
+                      witness.getSyntheticEnvironment());
   });
 }