[C++20] [Modules] [Reduced BMI] Don't write specializations with local args

Close #119947

As discussed in the above thread, we shouldn't write specializations
with local args in reduced BMI. Since users can't find such
specializations any way.

Author: ChuanqiXu9

clang/lib/Serialization/ASTWriterDecl.cpp

@@ -221,6 +221,48 @@ namespace clang {
         Record.AddDeclRef(F.second);
     }
 
+    template <typename T> bool shouldSkipWritingSpecializations(T *Spec) {
+      // Now we will only avoid writing specializations if we're generating
+      // reduced BMI.
+      if (!GeneratingReducedBMI)
+        return false;
+
+      assert((isa<FunctionDecl, ClassTemplateSpecializationDecl,
+                  VarTemplateSpecializationDecl>(Spec)));
+
+      ArrayRef<TemplateArgument> Args;
+      if (auto *CTSD = dyn_cast<ClassTemplateSpecializationDecl>(Spec))
+        Args = CTSD->getTemplateArgs().asArray();
+      else if (auto *VTSD = dyn_cast<VarTemplateSpecializationDecl>(Spec))
+        Args = VTSD->getTemplateArgs().asArray();
+      else
+        Args = cast<FunctionDecl>(Spec)
+                   ->getTemplateSpecializationArgs()
+                   ->asArray();
+
+      // If there is any template argument is TULocal, we can avoid writing the
+      // specialization since the consumers of reduced BMI won't get the
+      // specialization anyway.
+      for (const TemplateArgument &TA : Args) {
+        switch (TA.getKind()) {
+        case TemplateArgument::Type: {
+          Linkage L = TA.getAsType()->getLinkage();
+          if (!isExternallyVisible(L))
+            return true;
+          break;
+        }
+        case TemplateArgument::Declaration:
+          if (!TA.getAsDecl()->isExternallyVisible())
+            return true;
+          break;
+        default:
+          break;
+        }
+      }
+
+      return false;
+    }
+
     /// Add to the record the first template specialization from each module
     /// file that provides a declaration of D. We store the DeclId and an
     /// ODRHash of the template arguments of D which should provide enough
@@ -235,6 +277,9 @@ namespace clang {
       CollectFirstDeclFromEachModule(D, /*IncludeLocal*/ true, Firsts);
 
       for (const auto &F : Firsts) {
+        if (shouldSkipWritingSpecializations(F.second))
+          continue;
+
         if (isa<ClassTemplatePartialSpecializationDecl,
                 VarTemplatePartialSpecializationDecl>(F.second))
           PartialSpecsInMap.push_back(F.second);

clang/test/Modules/pr119947.cppm

@@ -0,0 +1,54 @@
+// RUN: rm -rf %t
+// RUN: mkdir -p %t
+// RUN: split-file %s %t
+//
+// RUN: %clang_cc1 -std=c++20 %t/a.cppm -emit-reduced-module-interface -o %t/a.pcm
+// RUN: %clang_cc1 -std=c++20 %t/use.cpp -fprebuilt-module-path=%t -emit-llvm -o -
+
+
+//--- a.cppm
+export module a;
+
+struct a_inner {
+	~a_inner() {
+	}
+	void f(auto) {
+	}
+};
+
+export template<typename T>
+struct a {
+	a() {
+		struct local {};
+		inner.f(local());
+	}
+private:
+	a_inner inner;
+};
+
+
+namespace {
+
+struct s {
+};
+
+} // namespace
+
+void f() {
+	a<s> x;
+}
+
+//--- use.cpp
+import a;
+
+namespace {
+
+struct s {
+};
+
+} // namespace
+
+void g() {
+	a<s> x;
+}
+