Diagnose if lifetime dependence specifiers are not in the function result position

Author: meg-gupta

include/swift/AST/DiagnosticsSema.def

@@ -7782,6 +7782,10 @@ ERROR(pack_iteration_where_clause_not_supported, none,
  ERROR(lifetime_dependence_cannot_use_kind, none,
        "invalid use of %0 lifetime dependence for %1 ownership",
        (StringRef, StringRef))
+ ERROR(lifetime_dependence_only_on_function_method_init_result, none,
+       "lifetime dependence specifiers may only be used on result of "
+       "functions, methods, initializers",
+       ())
 
 #define UNDEFINE_DIAGNOSTIC_MACROS
 #include "DefineDiagnosticMacros.h"

lib/Sema/TypeCheckType.cpp

@@ -2148,6 +2148,8 @@ namespace {
                                                   TypeResolutionOptions options);
     NeverNullType resolveResultDependsOnTypeRepr(ResultDependsOnTypeRepr *repr,
                                                  TypeResolutionOptions options);
+    NeverNullType resolveLifetimeDependentReturnTypeRepr(
+        LifetimeDependentReturnTypeRepr *repr, TypeResolutionOptions options);
     NeverNullType resolveArrayType(ArrayTypeRepr *repr,
                                    TypeResolutionOptions options);
     NeverNullType resolveDictionaryType(DictionaryTypeRepr *repr,
@@ -2638,10 +2640,9 @@ NeverNullType TypeResolver::resolveType(TypeRepr *repr,
     return resolveResultDependsOnTypeRepr(cast<ResultDependsOnTypeRepr>(repr),
                                           options);
 
-  case TypeReprKind::LifetimeDependentReturn: {
-    auto lifetimeDependenceRepr = cast<LifetimeDependentReturnTypeRepr>(repr);
-    return resolveType(lifetimeDependenceRepr->getBase(), options);
-  }
+  case TypeReprKind::LifetimeDependentReturn:
+    return resolveLifetimeDependentReturnTypeRepr(
+        cast<LifetimeDependentReturnTypeRepr>(repr), options);
   }
   llvm_unreachable("all cases should be handled");
 }
@@ -4605,6 +4606,18 @@ TypeResolver::resolveResultDependsOnTypeRepr(ResultDependsOnTypeRepr *repr,
   return resolveType(repr->getBase(), options);
 }
 
+NeverNullType TypeResolver::resolveLifetimeDependentReturnTypeRepr(
+    LifetimeDependentReturnTypeRepr *repr, TypeResolutionOptions options) {
+
+  if (!options.is(TypeResolverContext::FunctionResult)) {
+    diagnoseInvalid(
+        repr, repr->getSpecifierLoc(),
+        diag::lifetime_dependence_only_on_function_method_init_result);
+    return ErrorType::get(getASTContext());
+  }
+  return resolveType(repr->getBase(), options);
+}
+
 NeverNullType
 TypeResolver::resolveDictionaryType(DictionaryTypeRepr *repr,
                                     TypeResolutionOptions options) {

test/Sema/explicit_lifetime_dependence_specifiers.swift

@@ -112,3 +112,11 @@ struct Wrapper : ~Escapable {
     return view
   } 
 }
+
+struct ArrayOfBufferView : ~Escapable {
+    let arr: [BufferView]
+    subscript(index: Int) -> _borrow(self) BufferView {
+        return arr[index]
+    }
+}
+