[AST] Don't consider 'ExportDecl' when calculating DeclContext 'Encloses'

[ChuanqiXu]

Summary

This mimics the style of https://github.com/llvm/llvm-project/commit/90010c2e1d60c6a9a4a0b30a113d4dae2b7214eb.

Since ExportDecl and LinkageSpec are transparent DeclContext, they share some similarity. I've checked the test case linked in the revision would fail before this patch.

clang/lib/AST/DeclBase.cpp

bool DeclContext::isExternCXXContext() const {
   return isLinkageSpecContext(this, LinkageSpecDecl::lang_cxx);
 }
 
 bool DeclContext::Encloses(const DeclContext *DC) const {
   if (getPrimaryContext() != this)
     return getPrimaryContext()->Encloses(DC);
 
   for (; DC; DC = DC->getParent())
-    if (!isa<LinkageSpecDecl>(DC) && DC->getPrimaryContext() == this)
+    if (!isa<LinkageSpecDecl>(DC) && !isa<ExportDecl>(DC) &&
+        DC->getPrimaryContext() == this)
       return true;
   return false;
 }
 
 DeclContext *DeclContext::getNonTransparentContext() {
   DeclContext *DC = this;
   while (DC->isTransparentContext()) {
     DC = DC->getParent();

clang/test/SemaCXX/lookup-through-export.cpp

+// RUN: %clang_cc1 -std=c++20 %s -verify
+
+// expected-no-diagnostics
+export module X;
+export {
+  namespace A {
+  namespace B {
+  int bar;
+  }
+  } // namespace A
+  namespace C {
+  void foo() {
+    using namespace A;
+    (void)B::bar;
+  }
+  } // namespace C
+}
+
+export {
+  namespace D {
+  namespace E {
+  int bar;
+  }
+  } // namespace D
+  namespace F {
+  void foo() {
+    using namespace D;
+    (void)E::bar;
+  }
+  } // namespace F
+}