[clang][Parser] Allow private type aliases in out-of-line member function return types

When parsing qualified type names (e.g., `io_context::impl_type`) at file scope in
clang-repl, suppress access checks during type annotation. This allows private member
type aliases to be used in return types of out-of-line member function definitions,
matching the C++ standard's scoping rules for such declarations.

Fixes: Parsing errors in clang-repl when including headers with out-of-line member
functions that return private nested types (e.g., ASIO's io_context::impl_type).

Author: pfeodrippe

clang/lib/Parse/Parser.cpp

@@ -2020,6 +2020,27 @@ bool Parser::TryAnnotateTypeOrScopeTokenAfterScopeSpec(
     CXXScopeSpec &SS, bool IsNewScope,
     ImplicitTypenameContext AllowImplicitTypename) {
   if (Tok.is(tok::identifier)) {
+    // In incremental/clang-repl mode, suppress access checks for qualified
+    // type lookups when no delayed diagnostic pool is active. This handles
+    // the case where we're parsing input like "A::B *A::foo()" where the type
+    // "A::B" might be a private member type, but if this turns out to be an
+    // out-of-line member function definition, access should be allowed.
+    //
+    // When the declaration is actually parsed (via ParseDeclarationOrFunctionDefinition),
+    // the ParsingDeclSpec will set up proper delayed diagnostics to handle
+    // access checking in the correct context.
+    //
+    // We only do this in incremental mode because this is where the issue
+    // manifests - disambiguation happens before ParsingDeclSpec is created.
+    // In normal compilation, these access checks would be re-triggered during
+    // actual parsing with delayed diagnostics active.
+    bool SuppressAccess = getLangOpts().IncrementalExtensions &&
+                          SS.isNotEmpty() &&
+                          !Actions.DelayedDiagnostics.shouldDelayDiagnostics();
+    std::optional<SuppressAccessChecks> SAC;
+    if (SuppressAccess)
+      SAC.emplace(*this, true);
+
     // Determine whether the identifier is a type name.
     if (ParsedType Ty = Actions.getTypeName(
             *Tok.getIdentifierInfo(), Tok.getLocation(), getCurScope(), &SS,

clang/test/Interpreter/private-member-access.cpp

@@ -0,0 +1,31 @@
+// RUN: cat %s | clang-repl | FileCheck %s
+
+extern "C" int printf(const char*, ...);
+
+struct scheduler { };
+class io_context { using impl_type = scheduler; public: impl_type *get_impl(); };
+io_context::impl_type *io_context::get_impl() { return nullptr; }
+printf("Private type alias: passed\n");
+// CHECK: Private type alias: passed
+
+class Container { struct Node { int data; }; public: Node* create(); };
+Container::Node* Container::create() { return new Node{456}; }
+printf("Private nested struct: %d\n", Container().create()->data);
+// CHECK: Private nested struct: 456
+
+class Status { enum Code { OK = 0 }; public: Code get(); };
+Status::Code Status::get() { return OK; }
+printf("Private enum: %d\n", Status().get());
+// CHECK: Private enum: 0
+
+template<typename T> class Handler { using ptr = T*; public: ptr get(); };
+template<typename T> typename Handler<T>::ptr Handler<T>::get() { return nullptr; }
+printf("Template with private type: passed\n");
+// CHECK: Template with private type: passed
+
+namespace ns { class C { using val_t = double; public: val_t compute(); }; }
+ns::C::val_t ns::C::compute() { return 3.14; }
+printf("Namespace qualified: %.2f\n", ns::C().compute());
+// CHECK: Namespace qualified: 3.14
+
+%quit