Demangler: edge case in existential demangling

We now may have constrained existentials that have no primary associated
types, so there won't be any arguments for the parameterized protocol
type. Because the "constrained" part is that there's an inverse.

resolves rdar://128695929

Author: kavon

lib/AST/ASTDemangler.cpp

@@ -783,8 +783,18 @@ Type ASTBuilder::createConstrainedExistentialType(
     }
     args.push_back(argTy->getSecond());
   }
-  Type constrainedBase =
-      ParameterizedProtocolType::get(base->getASTContext(), baseTy, args);
+
+  Type constrainedBase;
+
+  // We may not have any arguments because the constrained existential is a
+  // plain protocol with an inverse requirement.
+  if (args.empty()) {
+    constrainedBase =
+        ProtocolType::get(baseDecl, baseTy, base->getASTContext());
+  } else {
+    constrainedBase =
+        ParameterizedProtocolType::get(base->getASTContext(), baseTy, args);
+  }
 
   // Handle inverse requirements.
   if (!inverseRequirements.empty()) {

test/Interpreter/moveonly_existentials.swift

@@ -1,5 +1,5 @@
 // RUN: %empty-directory(%t)
-// RUN: %target-build-swift  %s -o %t/bin
+// RUN: %target-build-swift -g %s -o %t/bin
 // RUN: %target-codesign %t/bin
 // RUN: %target-run %t/bin | %FileCheck %s