[C++20][Modules] Implement P2615R1 revised export diagnostics.

It has been reported to that the current clang  errors for, specifically,
static_assert in export contexts are a serious blocker to adoption of
modules in some cases.

There is also implementation divergence with GCC and MSVC allowing the
constructs mentioned below where clang currently rejects them with an
error.

The category of errors [for declarations in an exported context] is:
(unnamed, static_assert, empty and asm decls). These are now permitted
after P2615R1 which was approved by WG21 as a DR (and thus should be
applied to C++20 as well).

This patch removes these diagnostics and amends the testsuite accordingly.

Differential Revision: https://reviews.llvm.org/D152946

1 files changed, 29 insertions, 89 deletions

diff --git a/clang/lib/Sema/SemaModule.cpp b/clang/lib/Sema/SemaModule.cpp
index 5ce5330..9b2982e 100644
--- a/clang/lib/Sema/SemaModule.cpp
+++ b/clang/lib/Sema/SemaModule.cpp
@@ -814,76 +814,22 @@ Decl *Sema::ActOnStartExportDecl(Scope *S, SourceLocation ExportLoc,
   return D;
 }
 
-static bool checkExportedDeclContext(Sema &S, DeclContext *DC,
-                                     SourceLocation BlockStart);
-
-namespace {
-enum class UnnamedDeclKind {
-  Empty,
-  StaticAssert,
-  Asm,
-  UsingDirective,
-  Namespace,
-  Context
-};
-}
-
-static std::optional<UnnamedDeclKind> getUnnamedDeclKind(Decl *D) {
-  if (isa<EmptyDecl>(D))
-    return UnnamedDeclKind::Empty;
-  if (isa<StaticAssertDecl>(D))
-    return UnnamedDeclKind::StaticAssert;
-  if (isa<FileScopeAsmDecl>(D))
-    return UnnamedDeclKind::Asm;
-  if (isa<UsingDirectiveDecl>(D))
-    return UnnamedDeclKind::UsingDirective;
-  // Everything else either introduces one or more names or is ill-formed.
-  return std::nullopt;
-}
-
-unsigned getUnnamedDeclDiag(UnnamedDeclKind UDK, bool InBlock) {
-  switch (UDK) {
-  case UnnamedDeclKind::Empty:
-  case UnnamedDeclKind::StaticAssert:
-    // Allow empty-declarations and static_asserts in an export block as an
-    // extension.
-    return InBlock ? diag::ext_export_no_name_block : diag::err_export_no_name;
-
-  case UnnamedDeclKind::UsingDirective:
-    // Allow exporting using-directives as an extension.
-    return diag::ext_export_using_directive;
-
-  case UnnamedDeclKind::Namespace:
-    // Anonymous namespace with no content.
-    return diag::introduces_no_names;
-
-  case UnnamedDeclKind::Context:
-    // Allow exporting DeclContexts that transitively contain no declarations
-    // as an extension.
-    return diag::ext_export_no_names;
-
-  case UnnamedDeclKind::Asm:
-    return diag::err_export_no_name;
-  }
-  llvm_unreachable("unknown kind");
-}
+static bool checkExportedDecl(Sema &, Decl *, SourceLocation);
 
-static void diagExportedUnnamedDecl(Sema &S, UnnamedDeclKind UDK, Decl *D,
-                                    SourceLocation BlockStart) {
-  S.Diag(D->getLocation(), getUnnamedDeclDiag(UDK, BlockStart.isValid()))
-      << (unsigned)UDK;
-  if (BlockStart.isValid())
-    S.Diag(BlockStart, diag::note_export);
+/// Check that it's valid to export all the declarations in \p DC.
+static bool checkExportedDeclContext(Sema &S, DeclContext *DC,
+                                     SourceLocation BlockStart) {
+  bool AllUnnamed = true;
+  for (auto *D : DC->decls())
+    AllUnnamed &= checkExportedDecl(S, D, BlockStart);
+  return AllUnnamed;
 }
 
 /// Check that it's valid to export \p D.
 static bool checkExportedDecl(Sema &S, Decl *D, SourceLocation BlockStart) {
-  // C++2a [module.interface]p3:
-  //   An exported declaration shall declare at least one name
-  if (auto UDK = getUnnamedDeclKind(D))
-    diagExportedUnnamedDecl(S, *UDK, D, BlockStart);
 
-  //   [...] shall not declare a name with internal linkage.
+  //  C++20 [module.interface]p3:
+  //   [...] it shall not declare a name with internal linkage.
   bool HasName = false;
   if (auto *ND = dyn_cast<NamedDecl>(D)) {
     // Don't diagnose anonymous union objects; we'll diagnose their members
@@ -893,6 +839,7 @@ static bool checkExportedDecl(Sema &S, Decl *D, SourceLocation BlockStart) {
       S.Diag(ND->getLocation(), diag::err_export_internal) << ND;
       if (BlockStart.isValid())
         S.Diag(BlockStart, diag::note_export);
+      return false;
     }
   }
 
@@ -908,31 +855,29 @@ static bool checkExportedDecl(Sema &S, Decl *D, SourceLocation BlockStart) {
       S.Diag(Target->getLocation(), diag::note_using_decl_target);
       if (BlockStart.isValid())
         S.Diag(BlockStart, diag::note_export);
+      return false;
     }
   }
 
   // Recurse into namespace-scope DeclContexts. (Only namespace-scope
-  // declarations are exported.).
+  // declarations are exported).
   if (auto *DC = dyn_cast<DeclContext>(D)) {
-    if (isa<NamespaceDecl>(D) && DC->decls().empty()) {
-      if (!HasName)
-        // We don't allow an empty anonymous namespace (we don't allow decls
-        // in them either, but that's handled in the recursion).
-        diagExportedUnnamedDecl(S, UnnamedDeclKind::Namespace, D, BlockStart);
-      // We allow an empty named namespace decl.
-    } else if (DC->getRedeclContext()->isFileContext() && !isa<EnumDecl>(D))
-      return checkExportedDeclContext(S, DC, BlockStart);
+    if (!isa<NamespaceDecl>(D))
+      return true;
+
+    if (auto *ND = dyn_cast<NamedDecl>(D)) {
+      if (!ND->getDeclName()) {
+        S.Diag(ND->getLocation(), diag::err_export_anon_ns_internal);
+        if (BlockStart.isValid())
+          S.Diag(BlockStart, diag::note_export);
+        return false;
+      } else if (!DC->decls().empty() &&
+                 DC->getRedeclContext()->isFileContext()) {
+        return checkExportedDeclContext(S, DC, BlockStart);
+      }
+    }
   }
-  return false;
-}
-
-/// Check that it's valid to export all the declarations in \p DC.
-static bool checkExportedDeclContext(Sema &S, DeclContext *DC,
-                                     SourceLocation BlockStart) {
-  bool AllUnnamed = true;
-  for (auto *D : DC->decls())
-    AllUnnamed &= checkExportedDecl(S, D, BlockStart);
-  return AllUnnamed;
+  return true;
 }
 
 /// Complete the definition of an export declaration.
@@ -947,12 +892,7 @@ Decl *Sema::ActOnFinishExportDecl(Scope *S, Decl *D, SourceLocation RBraceLoc) {
     SourceLocation BlockStart =
         ED->hasBraces() ? ED->getBeginLoc() : SourceLocation();
     for (auto *Child : ED->decls()) {
-      if (checkExportedDecl(*this, Child, BlockStart)) {
-        // If a top-level child is a linkage-spec declaration, it might contain
-        // no declarations (transitively), in which case it's ill-formed.
-        diagExportedUnnamedDecl(*this, UnnamedDeclKind::Context, Child,
-                                BlockStart);
-      }
+      checkExportedDecl(*this, Child, BlockStart);
       if (auto *FD = dyn_cast<FunctionDecl>(Child)) {
         // [dcl.inline]/7
         // If an inline function or variable that is attached to a named module