[clang] [clang][Modules] Complete the implementation of P2615: Meaningful exports (PR #194201)

[Corentin Jabot via cfe-commits]

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@@ -440,6 +443,87 @@ Decl *Parser::ParseExportDeclaration() {
                                        T.getCloseLocation());
 }
 
+void Parser::CheckUnbracedLinkageOrExportDeclaration(
+    Decl *LinkageOrExportDecl) {
+  const auto *DC = cast<DeclContext>(LinkageOrExportDecl);
+  if (DC->decls_empty())
+    return;
+
+  const Decl *D = *DC->decls_begin();
+
+  // Nested export declarations are diagnosed elsewhere.
+  if (isa<LinkageSpecDecl>(LinkageOrExportDecl) && isa<ExportDecl>(D)) {
+    Diag(LinkageOrExportDecl->getLocation(),
+         diag::err_invalid_decl_in_linkage_spec)
+        << 2;
+    return;
+  }
+
+  // [module.interface]/1 says:
+  //
+  //     The name-declaration of an export-declaration shall not declare a
+  //     partial specialization.
+  //
+  // There's no equivalent wording for linkage-specification.
+  if (isa<ClassTemplatePartialSpecializationDecl,
+          VarTemplatePartialSpecializationDecl>(D) &&
+      isa<LinkageSpecDecl>(LinkageOrExportDecl))
+    return;
+
+  TemplateSpecializationKind TSK = [&] {
+    if (const auto *EID = dyn_cast<ExplicitInstantiationDecl>(D))
+      return EID->getTemplateSpecializationKind();
+    if (const auto *CTSD = dyn_cast<ClassTemplateSpecializationDecl>(D))
+      return CTSD->getTemplateSpecializationKind();
+    if (const auto *VTSD = dyn_cast<VarTemplateSpecializationDecl>(D))
+      return VTSD->getTemplateSpecializationKind();
+    if (const auto *FD = dyn_cast<FunctionDecl>(D))
+      return FD->getTemplateSpecializationKind();
+    return TSK_Undeclared;
+  }();
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cor3ntin wrote:

There is similar code in `isTemplateSpecializationKind`, maybe in other places, we should try to clean that up

https://github.com/llvm/llvm-project/pull/194201