SIL: Serialize default arguments of inlinable local functions

Fixes <https://bugs.swift.org/browse/SR-12404>.

Author: slavapestov

lib/SIL/IR/SILDeclRef.cpp

@@ -442,12 +442,9 @@ bool SILDeclRef::isTransparent() const {
 
 /// True if the function should have its body serialized.
 IsSerialized_t SILDeclRef::isSerialized() const {
-  DeclContext *dc;
   if (auto closure = getAbstractClosureExpr()) {
-    dc = closure->getLocalContext();
-
-    // Otherwise, ask the AST if we're inside an @inlinable context.
-    if (dc->getResilienceExpansion() == ResilienceExpansion::Minimal) {
+    // Ask the AST if we're inside an @inlinable context.
+    if (closure->getResilienceExpansion() == ResilienceExpansion::Minimal) {
       if (isForeign)
         return IsSerializable;
 
@@ -465,6 +462,13 @@ IsSerialized_t SILDeclRef::isSerialized() const {
   // Default argument generators are serialized if the containing
   // declaration is public.
   if (isDefaultArgGenerator()) {
+    // Ask the AST if we're inside an @inlinable context.
+    if (d->getDeclContext()->getResilienceExpansion()
+          == ResilienceExpansion::Minimal) {
+      return IsSerialized;
+    }
+
+    // Otherwise, check if the owning declaration is public.
     auto scope =
       d->getFormalAccessScope(/*useDC=*/nullptr,
                               /*treatUsableFromInlineAsPublic=*/true);
@@ -490,7 +494,7 @@ IsSerialized_t SILDeclRef::isSerialized() const {
 
   // Note: if 'd' is a function, then 'dc' is the function itself, not
   // its parent context.
-  dc = d->getInnermostDeclContext();
+  auto *dc = d->getInnermostDeclContext();
 
   // Local functions are serializable if their parent function is
   // serializable.

test/SILGen/default_arguments_local.swift

@@ -57,3 +57,16 @@ class ArtClass<T> {
     return 0
   }
 }
+
+// Default arguments of local functions inside @inlinable contexts should be serialized.
+// https://bugs.swift.org/browse/SR-12404
+
+// CHECK-LABEL: sil [serialized] [ossa] @$s23default_arguments_local5outeryyF : $@convention(thin) () -> () {
+@inlinable public func outer() {
+  // CHECK-LABEL: sil shared [serialized] [ossa] @$s23default_arguments_local5outeryyF5innerL_1xySi_tFfA_ : $@convention(thin) () -> Int {
+
+  // CHECK-LABEL: sil shared [serializable] [ossa] @$s23default_arguments_local5outeryyF5innerL_1xySi_tF : $@convention(thin) (Int) -> () {
+  func inner(x: Int = 0) {}
+
+  inner()
+}