SIL: Fix assertion failure after deleting instructions that contain unresolved local archetypes

We maintained a counter of the number of pending local archetypes
that had not yet been defined. However, if an instruction that
references a pending local archetype was deleted before the
local archetype was defined, the counter would never decrement.

Before reading the counter value, garbage collect any inserted
placeholders that have no uses. These correspond to pending
local archetypes that are no longer in use and will never be
defined.

Author: slavapestov

include/swift/SIL/SILModule.h

@@ -477,6 +477,11 @@ class SILModule {
   /// This should only be the case during parsing or deserialization.
   bool hasUnresolvedLocalArchetypeDefinitions();
 
+  /// If we added any instructions that reference unresolved local archetypes
+  /// and then deleted those instructions without resolving those archetypes,
+  /// we must reclaim those unresolved local archetypes.
+  void reclaimUnresolvedLocalArchetypeDefinitions();
+
   /// Get a unique index for a struct or class field in layout order.
   ///
   /// Precondition: \p decl must be a non-resilient struct or class.

lib/SIL/IR/SILModule.cpp

@@ -687,6 +687,28 @@ SILValue SILModule::getRootLocalArchetypeDef(CanLocalArchetypeType archetype,
   return def;
 }
 
+void SILModule::reclaimUnresolvedLocalArchetypeDefinitions() {
+  llvm::DenseMap<LocalArchetypeKey, SILValue> newLocalArchetypeDefs;
+
+  for (auto pair : RootLocalArchetypeDefs) {
+    if (auto *placeholder = dyn_cast<PlaceholderValue>(pair.second)) {
+      // If a placeholder has no uses, the instruction that introduced it
+      // was deleted before the local archetype was resolved. Reclaim the
+      // placeholder so that we don't complain.
+      if (placeholder->use_empty()) {
+        assert(numUnresolvedLocalArchetypes > 0);
+        --numUnresolvedLocalArchetypes;
+        ::delete placeholder;
+        continue;
+      }
+    }
+
+    newLocalArchetypeDefs.insert(pair);
+  }
+
+  std::swap(newLocalArchetypeDefs, RootLocalArchetypeDefs);
+}
+
 bool SILModule::hasUnresolvedLocalArchetypeDefinitions() {
   return numUnresolvedLocalArchetypes != 0;
 }
@@ -753,6 +775,8 @@ void SILModule::notifyAddedInstruction(SILInstruction *inst) {
       auto *placeholder = cast<PlaceholderValue>(val);
       placeholder->replaceAllUsesWith(dependency);
       ::delete placeholder;
+
+      assert(numUnresolvedLocalArchetypes > 0);
       numUnresolvedLocalArchetypes--;
     }
     val = dependency;