Re: [PATCH] c++/modules: Check linkage of structured binding decls

[Jason Merrill]

On 1/19/25 4:39 AM, Nathaniel Shead wrote:

Bootstrapped and regtested on x86_64-pc-linux-gnu, OK for trunk?

OK.

-- >8 --

When looking at PR c++/118513 I noticed that we don't currently check
the linkage of structured binding declarations in modules.  This patch
adds those checks, and corrects decl_linkage to properly recognise
structured binding declarations as potentially having linkage.

gcc/cp/ChangeLog:

        * parser.cc (cp_parser_decomposition_declaration): Check linkage
        of structured bindings in modules.
        * tree.cc (decl_linkage): Structured bindings don't necessarily
        have no linkage.

gcc/testsuite/ChangeLog:

        * g++.dg/modules/export-6.C: Add structured binding tests.
        * g++.dg/modules/hdr-2.H: Likewise.

Signed-off-by: Nathaniel Shead <nathanielosh...@gmail.com>
---
  gcc/cp/parser.cc                        | 1 +
  gcc/cp/tree.cc                          | 2 ++
  gcc/testsuite/g++.dg/modules/export-6.C | 6 ++++++
  gcc/testsuite/g++.dg/modules/hdr-2.H    | 9 +++++++++
  4 files changed, 18 insertions(+)

diff --git a/gcc/cp/parser.cc b/gcc/cp/parser.cc
index ff58a8ec98e..a8ac8af0955 100644
--- a/gcc/cp/parser.cc
+++ b/gcc/cp/parser.cc
@@ -16728,6 +16728,7 @@ cp_parser_decomposition_declaration (cp_parser *parser,
          cp_finish_decl (decl, initializer, non_constant_p, NULL_TREE,
                          (is_direct_init ? LOOKUP_NORMAL : LOOKUP_IMPLICIT),
                          &decomp);
+         check_module_decl_linkage (decl);
        }
      }
    else if (decl != error_mark_node)
diff --git a/gcc/cp/tree.cc b/gcc/cp/tree.cc
index ed01ca42eaf..36581865a17 100644
--- a/gcc/cp/tree.cc
+++ b/gcc/cp/tree.cc
@@ -6003,6 +6003,8 @@ decl_linkage (tree decl)
      {
        if (TREE_CODE (decl) == TYPE_DECL && !TYPE_ANON_P (TREE_TYPE (decl)))
        /* This entity has a typedef name for linkage purposes.  */;
+      else if (DECL_DECOMPOSITION_P (decl) && DECL_DECOMP_IS_BASE (decl))
+       /* Namespace-scope structured bindings can have linkage.  */;
        else if (TREE_CODE (decl) == NAMESPACE_DECL && cxx_dialect >= cxx11)
        /* An anonymous namespace has internal linkage since C++11.  */
        return lk_internal;
diff --git a/gcc/testsuite/g++.dg/modules/export-6.C 
b/gcc/testsuite/g++.dg/modules/export-6.C
index c59944aa688..460cdf08ea9 100644
--- a/gcc/testsuite/g++.dg/modules/export-6.C
+++ b/gcc/testsuite/g++.dg/modules/export-6.C
@@ -3,6 +3,7 @@

export module bad;

  namespace global {}
+struct S { int x; };

export static int x = 123; // { dg-error "internal linkage" }

  export static void f();  // { dg-error "internal linkage" }
@@ -10,12 +11,17 @@ export static void g() {}  // { dg-error "internal linkage" 
}
  export template <typename T> static void t();  // { dg-error "internal 
linkage" }
  export template <typename T> static void u() {}  // { dg-error "internal 
linkage" }

+#if __cplusplus >= 202002L

+export static auto [d] = S{};  // { dg-error "internal linkage" "" { target 
c++20 } }
+#endif
+
  namespace {
    export int y = 456;  // { dg-error "internal linkage" }
    export void h();  // { dg-error "internal linkage" }
    export void i() {}  // { dg-error "internal linkage" }
    export template <typename T> void v(); // { dg-error "internal linkage" }
    export template <typename T> void w() {} // { dg-error "internal linkage" }
+  export auto [e] = S{};  // { dg-error "internal linkage" }

export namespace ns {} // { dg-error "internal linkage" }

    export namespace alias = global;  // { dg-error "internal linkage" }
diff --git a/gcc/testsuite/g++.dg/modules/hdr-2.H 
b/gcc/testsuite/g++.dg/modules/hdr-2.H
index 097546d5667..834c68241b8 100644
--- a/gcc/testsuite/g++.dg/modules/hdr-2.H
+++ b/gcc/testsuite/g++.dg/modules/hdr-2.H
@@ -3,8 +3,11 @@
  // external linkage variables or functions in header units must
  // not have non-inline definitions

+struct S { int x; };

+
  int x_err;  // { dg-error "external linkage definition" }
  int y_err = 123;  // { dg-error "external linkage definition" }
+auto [d_err] = S{};  // { dg-error "external linkage definition" }
  void f_err() {}  // { dg-error "external linkage definition" }

struct Err {

@@ -59,6 +62,7 @@ struct Inl {
  // Internal linkage decls are OK
  static int x_internal;
  static int y_internal = 123;
+namespace { auto [d_internal] = S{}; }
  static void f_internal() {}

namespace {

@@ -81,6 +85,11 @@ inline void f_static() {
    thread_local int x_thread_local;
    thread_local int y_thread_local = 123;

+#if __cplusplus >= 202002L

+  static auto [d_static] = S{};
+  thread_local auto [d_thread_local] = S{};
+#endif
+
    x_static = y_static;
    x_thread_local = y_thread_local;
  }