Diagnose typealiases of non-public types in inlinable functions (#23929)

...as a warning, since we didn't check this correctly in Swift 5.
For parseable interfaces, these typealiases won't work at all.

(cherry picked from commit a15dec8)

Author: jrose-apple

lib/AST/DeclContext.cpp

@@ -309,7 +309,8 @@ bool DeclContext::isGenericContext() const {
 /// domains, this ensures that only sufficiently-conservative access patterns
 /// are used.
 ResilienceExpansion DeclContext::getResilienceExpansion() const {
-  for (const auto *dc = this; dc->isLocalContext(); dc = dc->getParent()) {
+  for (const auto *dc = getLocalContext(); dc && dc->isLocalContext();
+       dc = dc->getParent()) {
     // Default argument initializer contexts have their resilience expansion
     // set when they're type checked.
     if (isa<DefaultArgumentInitializer>(dc)) {

lib/Sema/ResilienceDiagnostics.cpp

@@ -25,7 +25,8 @@ using FragileFunctionKind = TypeChecker::FragileFunctionKind;
 
 std::pair<FragileFunctionKind, bool>
 TypeChecker::getFragileFunctionKind(const DeclContext *DC) {
-  for (; DC->isLocalContext(); DC = DC->getParent()) {
+  for (DC = DC->getLocalContext(); DC && DC->isLocalContext();
+       DC = DC->getParent()) {
     if (isa<DefaultArgumentInitializer>(DC)) {
       // Default argument generators of public functions cannot reference
       // @usableFromInline declarations; all other fragile function kinds
@@ -176,6 +177,11 @@ bool TypeChecker::diagnoseInlinableDeclRefAccess(SourceLoc loc,
     diagName = accessor->getStorage()->getFullName();
   }
 
+  // Swift 5.0 did not check the underlying types of local typealiases.
+  // FIXME: Conditionalize this once we have a new language mode.
+  if (isa<TypeAliasDecl>(DC))
+    downgradeToWarning = DowngradeToWarning::Yes;
+
   auto diagID = diag::resilience_decl_unavailable;
   if (downgradeToWarning == DowngradeToWarning::Yes)
     diagID = diag::resilience_decl_unavailable_warn;

test/attr/attr_inlinable_typealias.swift

@@ -5,6 +5,7 @@ private typealias PrivateAlias = Int
 
 internal typealias InternalAlias = Int
 // expected-note@-1 {{type alias 'InternalAlias' is not '@usableFromInline' or public}}
+// expected-note@-2 * {{type alias 'InternalAlias' is not '@usableFromInline' or public}}
 
 @usableFromInline typealias UsableFromInlineAlias = Int
 
@@ -21,3 +22,8 @@ public typealias PublicAlias = Int
 
   _ = PublicAlias.self
 }
+
+@inlinable public func localTypealiases() {
+  typealias LocalAlias = InternalAlias // expected-warning {{type alias 'InternalAlias' is internal and should not be referenced from an '@inlinable' function}}
+  typealias GenericAlias<T> = (T, InternalAlias) // expected-warning {{type alias 'InternalAlias' is internal and should not be referenced from an '@inlinable' function}}
+}