[AST] OpaqueDecl: Simplify storage of conditionally available alternatives

Instead of using a special type with a set of trailing objects, let's
just re-allocate array of substitutions into a permanent storage directly.

Author: xedin

include/swift/AST/Decl.h

@@ -2793,34 +2793,6 @@ class OpaqueTypeDecl final :
   class ConditionallyAvailableSubstitutions;
 
 private:
-  /// A container to keep a set of conditional available underlying type
-  /// substitutions with their availability conditions.
-  class ConditionalAlternatives final
-      : private llvm::TrailingObjects<ConditionalAlternatives,
-                                      ConditionallyAvailableSubstitutions *> {
-    friend TrailingObjects;
-
-    unsigned NumAlternatives;
-
-    ConditionalAlternatives(
-        ArrayRef<ConditionallyAvailableSubstitutions *> underlyingTypes)
-        : NumAlternatives(underlyingTypes.size()) {
-      std::uninitialized_copy(
-          underlyingTypes.begin(), underlyingTypes.end(),
-          getTrailingObjects<ConditionallyAvailableSubstitutions *>());
-    }
-
-  public:
-    ArrayRef<ConditionallyAvailableSubstitutions *> getAlternatives() const {
-      return {getTrailingObjects<ConditionallyAvailableSubstitutions *>(),
-              NumAlternatives};
-    }
-
-    static ConditionalAlternatives *
-    get(ASTContext &ctx,
-        ArrayRef<ConditionallyAvailableSubstitutions *> underlyingTypes);
-  };
-
   /// The original declaration that "names" the opaque type. Although a specific
   /// opaque type cannot be explicitly named, oapque types can propagate
   /// arbitrarily through expressions, so we need to know *which* opaque type is
@@ -2845,7 +2817,11 @@ class OpaqueTypeDecl final :
   /// A set of substitutions which are used based on the availability
   /// checks performed at runtime. This set of only populated if there
   /// is no single unique underlying type for this opaque type declaration.
-  ConditionalAlternatives *ConditionallyAvailableTypes = nullptr;
+  ///
+  /// It always contains one or more conditionally available substitutions
+  /// followed by a universally available type used as a fallback.
+  Optional<MutableArrayRef<ConditionallyAvailableSubstitutions *>>
+      ConditionallyAvailableTypes = None;
 
   mutable Identifier OpaqueReturnTypeIdentifier;
 
@@ -2932,14 +2908,15 @@ class OpaqueTypeDecl final :
     UniqueUnderlyingType = subs;
   }
 
-  void setConditionallyAvailableSubstitutions(
-      ArrayRef<ConditionallyAvailableSubstitutions *> substitutions) {
-    assert(!ConditionallyAvailableTypes &&
-           "resetting conditional substitutions?!");
-    ConditionallyAvailableTypes =
-        ConditionalAlternatives::get(getASTContext(), substitutions);
+  ArrayRef<ConditionallyAvailableSubstitutions *>
+  getConditionallyAvailableSubstitutions() const {
+    assert(ConditionallyAvailableTypes);
+    return ConditionallyAvailableTypes.getValue();
   }
 
+  void setConditionallyAvailableSubstitutions(
+      ArrayRef<ConditionallyAvailableSubstitutions *> substitutions);
+
   // Opaque type decls are currently always implicit
   SourceRange getSourceRange() const { return SourceRange(); }
 

lib/AST/Decl.cpp

@@ -8173,14 +8173,11 @@ Identifier OpaqueTypeDecl::getOpaqueReturnTypeIdentifier() const {
   return OpaqueReturnTypeIdentifier;
 }
 
-OpaqueTypeDecl::ConditionalAlternatives *
-OpaqueTypeDecl::ConditionalAlternatives::get(
-    ASTContext &ctx,
-    ArrayRef<ConditionallyAvailableSubstitutions *> underlyingTypes) {
-  auto size = totalSizeToAlloc<ConditionallyAvailableSubstitutions *>(
-      underlyingTypes.size());
-  auto mem = ctx.Allocate(size, alignof(ConditionalAlternatives));
-  return new (mem) ConditionalAlternatives(underlyingTypes);
+void OpaqueTypeDecl::setConditionallyAvailableSubstitutions(
+    ArrayRef<ConditionallyAvailableSubstitutions *> substitutions) {
+  assert(!ConditionallyAvailableTypes &&
+         "resetting conditionally available substitutions?!");
+  ConditionallyAvailableTypes = getASTContext().AllocateCopy(substitutions);
 }
 
 OpaqueTypeDecl::ConditionallyAvailableSubstitutions *