[clang] [Clang] Ensure child classes export inherited constructors from base classes (PR #182706)

[Hans Wennborg via cfe-commits]

================
@@ -6604,13 +6617,52 @@ void Sema::checkClassLevelDLLAttribute(CXXRecordDecl *Class) {
       if (MD->isDeleted())
         continue;
 
+      // Don't export inherited constructors whose parameters prevent ABI-
+      // compatible forwarding. When canEmitDelegateCallArgs (in CodeGen)
+      // returns false, Clang inlines the constructor body instead of
+      // emitting a forwarding thunk, producing code that is not ABI-
+      // compatible with MSVC. Suppress the export and warn so the user
+      // gets a linker error rather than a silent runtime mismatch.
+      if (ClassExported) {
+        if (auto *CD = dyn_cast<CXXConstructorDecl>(MD)) {
+          if (CD->getInheritedConstructor()) {
+            if (CD->isVariadic()) {
+              Diag(CD->getLocation(),
+                   diag::warn_dllexport_inherited_ctor_unsupported)
+                  << /*variadic=*/0;
+              continue;
+            }
+            if (Context.getTargetInfo()
+                    .getCXXABI()
+                    .areArgsDestroyedLeftToRightInCallee()) {
+              bool HasCalleeCleanupParam = false;
+              for (const auto *P : CD->parameters())
+                if (P->needsDestruction(Context)) {
+                  HasCalleeCleanupParam = true;
+                  break;
+                }
+              if (HasCalleeCleanupParam) {
+                Diag(CD->getLocation(),
+                     diag::warn_dllexport_inherited_ctor_unsupported)
+                    << /*callee-cleanup=*/1;
+                continue;
+              }
+            }
+          }
+        }
+      }
+
       if (MD->isInlined()) {
         // MinGW does not import or export inline methods. But do it for
-        // template instantiations.
+        // template instantiations and inherited constructors (which are
+        // marked inline but must be exported to match MSVC behavior).
----------------
zmodem wrote:

We don't need to match MSVC behavior when targeting MinGW. I think what matters is really what GCC does. @mstorsjo 

https://github.com/llvm/llvm-project/pull/182706