c++: Fix up handling of name independent structured binding packs [PR117783]

I've realized I haven't added testsuite coverage for name independent
structured binding packs.  And the
  auto [i, ..._, j] = T {};
  auto [k, ..._, l] = T {};
case shows a problem with that.  The elements of the structured binding
pack have #i appended to their names, so for the _ case e.g. _#0, _#1
etc. (to print something useful in diagnostics, perhaps debug info later
on).  The above is valid though as long as one doesn't use _ (which is
ambiguous), but we were emitting errors on redeclaration of _#0, _#1
etc.

The following patch uses DECL_NAME (decl) = NULL_TREE; for the
name independent decl case so that the false positive redeclaration
errors aren't emitted.

2025-08-11  Jakub Jelinek  <jakub@redhat.com>

	PR c++/117783
	* decl.cc (set_sb_pack_name): For name independent decls
	just clear DECL_NAME instead of appending #i to it.

	* g++.dg/cpp26/name-independent-decl11.C: New test.

2 files changed, 146 insertions, 7 deletions

diff --git a/gcc/cp/decl.cc b/gcc/cp/decl.cc
index 693cf65..5aa8203 100644
--- a/gcc/cp/decl.cc
+++ b/gcc/cp/decl.cc
@@ -9870,17 +9870,27 @@ cp_maybe_mangle_decomp (tree decl, cp_decomp *decomp)
     }
 }
 
-/* Append #i to DECL_NAME (decl).  */
+/* Append #i to DECL_NAME (decl) or for name independent decls
+   clear DECL_NAME (decl).  */
 
 static void
 set_sb_pack_name (tree decl, unsigned HOST_WIDE_INT i)
 {
-  tree name = DECL_NAME (decl);
-  size_t len = IDENTIFIER_LENGTH (name) + 22;
-  char *n = XALLOCAVEC (char, len);
-  snprintf (n, len, "%s#" HOST_WIDE_INT_PRINT_UNSIGNED,
-	    IDENTIFIER_POINTER (name), i);
-  DECL_NAME (decl) = get_identifier (n);
+  if (name_independent_decl_p (decl))
+    /* Only "_" names are treated as name independent, "_#0" etc. is not and
+       because we pushdecl the individual decl elements of structured binding
+       pack, we could get redeclaration errors if there are 2 or more name
+       independent structured binding packs in the same scope.  */
+    DECL_NAME (decl) = NULL_TREE;
+  else
+    {
+      tree name = DECL_NAME (decl);
+      size_t len = IDENTIFIER_LENGTH (name) + 22;
+      char *n = XALLOCAVEC (char, len);
+      snprintf (n, len, "%s#" HOST_WIDE_INT_PRINT_UNSIGNED,
+		IDENTIFIER_POINTER (name), i);
+      DECL_NAME (decl) = get_identifier (n);
+    }
 }
 
 /* Finish a decomposition declaration.  DECL is the underlying declaration
diff --git a/gcc/testsuite/g++.dg/cpp26/name-independent-decl11.C b/gcc/testsuite/g++.dg/cpp26/name-independent-decl11.C
new file mode 100644
index 0000000..643b1db
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp26/name-independent-decl11.C
@@ -0,0 +1,129 @@
+// P2169R4 - A nice placeholder with no name
+// { dg-do compile { target c++11 } }
+// { dg-options "" }
+
+template <typename T>
+void
+foo ()
+{
+  {
+    auto [i, ..._, k] = T {};	// { dg-warning "structured bindings only available with" "" { target c++14_down } }
+				// { dg-warning "structured binding packs only available with" "" { target { c++17 && c++23_down } } .-1 }
+    ++_...[0];			// { dg-warning "pack indexing only available with" "" { target c++23_down } }
+  }
+  {
+    auto [_, ..._, k] = T {};	// { dg-warning "structured bindings only available with" "" { target c++14_down } }
+				// { dg-warning "name-independent declarations only available with" "" { target c++23_down } .-1 }
+				// { dg-warning "structured binding packs only available with" "" { target { c++17 && c++23_down } } .-2 }
+    ++_;			// { dg-error "reference to '_' is ambiguous" }
+  }
+  {
+    auto [i, ..._, _] = T {};	// { dg-warning "structured bindings only available with" "" { target c++14_down } }
+				// { dg-warning "name-independent declarations only available with" "" { target c++23_down } .-1 }
+				// { dg-warning "structured binding packs only available with" "" { target { c++17 && c++23_down } } .-2 }
+    ++_;			// { dg-error "reference to '_' is ambiguous" }
+  }
+  {
+    auto [i, ..._, j] = T {};	// { dg-warning "structured bindings only available with" "" { target c++14_down } }
+				// { dg-warning "structured binding packs only available with" "" { target { c++17 && c++23_down } } .-1 }
+    auto [k, ..._, l] = T {};	// { dg-warning "structured bindings only available with" "" { target c++14_down } }
+				// { dg-warning "name-independent declarations only available with" "" { target c++23_down } .-1 }
+				// { dg-warning "structured binding packs only available with" "" { target { c++17 && c++23_down } } .-2 }
+    ++_...[0];			// { dg-error "reference to '_' is ambiguous" }
+  }
+  {
+    static auto [i, ..._, j] = T {};// { dg-warning "structured bindings only available with" "" { target c++14_down } }
+				// { dg-warning "structured binding packs only available with" "" { target { c++17 && c++23_down } } .-1 }
+				// { dg-warning "structured binding declaration can be 'static' only in" "" { target c++17_down } .-2 }
+    ++_...[0];			// { dg-warning "pack indexing only available with" "" { target c++23_down } }
+  }
+  {
+    static auto [_, ..._, j] = T {};// { dg-error "redeclaration of 'auto _'" }
+				// { dg-warning "structured bindings only available with" "" { target c++14_down } .-1 }
+				// { dg-warning "structured binding packs only available with" "" { target { c++17 && c++23_down } } .-2 }
+				// { dg-warning "structured binding declaration can be 'static' only in" "" { target c++17_down } .-3 }
+  }
+  {
+    static auto [i, ..._, _] = T {};// { dg-error "conflicting declaration 'auto _'" }
+				// { dg-warning "structured bindings only available with" "" { target c++14_down } .-1 }
+				// { dg-warning "structured binding packs only available with" "" { target { c++17 && c++23_down } } .-2 }
+				// { dg-warning "structured binding declaration can be 'static' only in" "" { target c++17_down } .-3 }
+  }
+}
+
+template <typename T>
+void
+bar ()
+{
+  T s[4] = {};
+  for (auto [..._, _, a] : s)	// { dg-warning "name-independent declarations only available with" "" { target c++23_down } }
+				// { dg-warning "structured bindings only available with" "" { target c++14_down } .-1 }
+				// { dg-warning "structured binding packs only available with" "" { target { c++17 && c++23_down } } .-2 }
+    ++_;			// { dg-error "reference to '_' is ambiguous" }
+  for (auto [a, ..._, b] : s)	// { dg-warning "structured bindings only available with" "" { target c++14_down } }
+				// { dg-warning "structured binding packs only available with" "" { target { c++17 && c++23_down } } .-1 }
+    ++_...[0];			// { dg-warning "pack indexing only available with" "" { target c++23_down } }
+  for (auto [_, ..._, a] : s)	// { dg-warning "name-independent declarations only available with" "" { target c++23_down } }
+    {				// { dg-warning "structured bindings only available with" "" { target c++14_down } .-1 }
+				// { dg-warning "structured binding packs only available with" "" { target { c++17 && c++23_down } } .-2 }
+      ++_;			// { dg-error "reference to '_' is ambiguous" }
+    }
+  for (auto [a, ..._, b] : s)	// { dg-warning "structured bindings only available with" "" { target c++14_down } }
+    {				// { dg-warning "structured binding packs only available with" "" { target { c++17 && c++23_down } } .-1 }
+      ++_...[0];		// { dg-warning "pack indexing only available with" "" { target c++23_down } }
+      int _ = ++b;		// { dg-warning "name-independent declarations only available with" "" { target c++23_down } }
+      ++_;			// { dg-error "reference to '_' is ambiguous" }
+    }
+}
+
+template <typename T>
+void
+baz ()
+{
+  T s[4] = {};
+  for (auto [a, ..._, _] : s)	// { dg-warning "name-independent declarations only available with" "" { target c++23_down } }
+				// { dg-warning "structured bindings only available with" "" { target c++14_down } .-1 }
+				// { dg-warning "structured binding packs only available with" "" { target { c++17 && c++23_down } } .-2 }
+    ++_;			// { dg-error "reference to '_' is ambiguous" }
+  for (auto [a, b, ..._] : s)	// { dg-warning "structured bindings only available with" "" { target c++14_down } }
+				// { dg-warning "structured binding packs only available with" "" { target { c++17 && c++23_down } } .-1 }
+    ++_...[0];			// { dg-warning "pack indexing only available with" "" { target c++23_down } }
+  for (auto [a, ..._, _] : s)	// { dg-warning "name-independent declarations only available with" "" { target c++23_down } }
+    {				// { dg-warning "structured bindings only available with" "" { target c++14_down } .-1 }
+				// { dg-warning "structured binding packs only available with" "" { target { c++17 && c++23_down } } .-2 }
+      ++_;			// { dg-error "reference to '_' is ambiguous" }
+      ++a;
+    }
+  for (auto [a, b, ..._] : s)	// { dg-warning "structured bindings only available with" "" { target c++14_down } }
+    {				// { dg-warning "structured binding packs only available with" "" { target { c++17 && c++23_down } } .-1 }
+      ++_...[0];		// { dg-warning "pack indexing only available with" "" { target c++23_down } }
+      int _ = a + b;		// { dg-warning "name-independent declarations only available with" "" { target c++23_down } }
+      ++_;			// { dg-error "reference to '_' is ambiguous" }
+    }
+}
+
+struct A { int a, b, c, d, e; };
+
+namespace std {
+  template<typename T> struct tuple_size;
+  template<int, typename> struct tuple_element;
+}
+
+struct B {
+  int a[5];
+  template <int I> int &get () { return a[I]; }
+};
+
+template<> struct std::tuple_size<B> { static const int value = 5; };
+template<int I> struct std::tuple_element<I,B> { using type = int; };
+
+void
+qux ()
+{
+  foo <A> ();
+  bar <A> ();
+  baz <A> ();
+  foo <B> ();
+  bar <B> ();
+  baz <B> ();
+}