[Sema] NFC: Add accessor to determine whether a type variable represents an opaque type

Author: xedin

include/swift/Sema/ConstraintSystem.h

@@ -487,6 +487,9 @@ class TypeVariableType::Implementation {
   /// expression.
   bool isCodeCompletionToken() const;
 
+  /// Determine whether this type variable represents an opened opaque type.
+  bool isOpaqueType() const;
+
   /// Retrieve the representative of the equivalence class to which this
   /// type variable belongs.
   ///

lib/Sema/TypeCheckConstraints.cpp

@@ -154,6 +154,22 @@ bool TypeVariableType::Implementation::isCodeCompletionToken() const {
   return locator && locator->directlyAt<CodeCompletionExpr>();
 }
 
+bool TypeVariableType::Implementation::isOpaqueType() const {
+  if (!locator)
+    return false;
+
+  auto GP = locator->getLastElementAs<LocatorPathElt::GenericParameter>();
+  if (!GP)
+    return false;
+
+  if (auto *GPT = GP->getType()->getAs<GenericTypeParamType>()) {
+    auto *decl = GPT->getDecl();
+    return decl && decl->isOpaqueType();
+  }
+
+  return false;
+}
+
 void *operator new(size_t bytes, ConstraintSystem& cs,
                    size_t alignment) {
   return cs.getAllocator().Allocate(bytes, alignment);