Add isolation info to ExtInfo for SubscriptDecls

Subscripts with `isolated` arguments currently do not pass a compiler
assertion about the consistency of the function type w.r.t. isolation.
This is because the `ExtInfo` for `SubscriptDecl`s doesn't currently
have isolation information applied (though this works in practice);
annotating the `ExtInfo` with isolation allows this assertion to pass.

Author: itaiferber

lib/Sema/TypeOfReference.cpp

@@ -22,6 +22,7 @@
 #include "TypeCheckType.h"
 #include "TypeChecker.h"
 #include "swift/AST/ConformanceLookup.h"
+#include "swift/AST/ExtInfo.h"
 #include "swift/AST/GenericEnvironment.h"
 #include "swift/AST/Effects.h"
 #include "swift/AST/MacroDefinition.h"
@@ -1628,6 +1629,13 @@ DeclReferenceType ConstraintSystem::getTypeOfMemberReference(
         thrownErrorType = Type();
       }
 
+      // Mark the isolation if any parameter is isolated.
+      if (llvm::any_of(indices, [&](AnyFunctionType::Param p) {
+            return p.isIsolated();
+          })) {
+        info = info.withIsolation(FunctionTypeIsolation::forParameter());
+      }
+
       refType = FunctionType::get(indices, elementTy, info);
     } else {
       // Delay the adjustment for preconcurrency until after we've formed