[ASTPrinter] Put desugaring constraints for existential types behind a

PrintOptions flag that is only enabled for interface printing.

Author: hborla

include/swift/AST/PrintOptions.h

@@ -285,6 +285,9 @@ struct PrintOptions {
   /// types.
   bool PrintExplicitAny = false;
 
+  /// Whether to desugar the constraint for an existential type.
+  bool DesugarExistentialConstraint = false;
+
   /// Whether to skip keywords with a prefix of underscore such as __consuming.
   bool SkipUnderscoredKeywords = false;
 

lib/AST/ASTPrinter.cpp

@@ -146,6 +146,7 @@ PrintOptions PrintOptions::printSwiftInterfaceFile(ModuleDecl *ModuleToPrint,
   result.AlwaysTryPrintParameterLabels = true;
   result.PrintSPIs = printSPIs;
   result.PrintExplicitAny = true;
+  result.DesugarExistentialConstraint = true;
 
   // We should print __consuming, __owned, etc for the module interface file.
   result.SkipUnderscoredKeywords = false;
@@ -6200,7 +6201,11 @@ class TypePrinter : public TypeVisitor<TypePrinter> {
     // interfaces. Verifying that the underlying type of a
     // protocol typealias is a constriant type is fundamentally
     // circular, so the desugared type should be written in source.
-    visit(T->getConstraintType()->getDesugaredType());
+    if (Options.DesugarExistentialConstraint) {
+      visit(T->getConstraintType()->getDesugaredType());
+    } else {
+      visit(T->getConstraintType());
+    }
   }
 
   void visitLValueType(LValueType *T) {