c++: Fix up name independent decl in structured binding handling in range for [PR115586]

cp_parser_range_for temporarily reverts IDENTIFIER_BINDING changes
to hide the decls from the structured bindings from lookup during
parsing of the expression after :
If there are 2 or more name independent decls, we undo IDENTIFIER_BINDING
for the same name multiple times, even when just one has been added
(with a TREE_LIST inside of it as decl).

The following patch fixes it by handling the _ name at most once, the
later loop will DTRT then and just reinstall the temporarily hidden
binding with the TREE_LIST in there.

2025-02-07  Jakub Jelinek  <jakub@redhat.com>

	PR c++/115586
	* parser.cc (cp_parser_range_for): For name independent decls in
	structured bindings, only push the name/binding once per
	structured binding.

	* g++.dg/cpp26/name-independent-decl9.C: New test.
	* g++.dg/cpp26/name-independent-decl10.C: New test.

3 files changed, 122 insertions, 0 deletions

diff --git a/gcc/cp/parser.cc b/gcc/cp/parser.cc
index 9b8be08..bd43f88 100644
--- a/gcc/cp/parser.cc
+++ b/gcc/cp/parser.cc
@@ -14570,9 +14570,19 @@ cp_parser_range_for (cp_parser *parser, tree scope, tree init, tree range_decl,
 	      decomp = &decomp_d;
 	      decomp->count = tree_to_uhwi (TREE_OPERAND (v, 1)) + 1;
 	      decomp->decl = d;
+	      bool seen_name_independent_decl = false;
 	      for (unsigned int i = 0; i < decomp->count;
 		   i++, d = DECL_CHAIN (d))
 		{
+		  if (name_independent_decl_p (d))
+		    {
+		      /* If there is more than one _ decl in
+			 the structured binding, just push and move it
+			 away once.  */
+		      if (seen_name_independent_decl)
+			continue;
+		      seen_name_independent_decl = true;
+		    }
 		  tree name = DECL_NAME (d);
 		  names.safe_push (name);
 		  bindings.safe_push (IDENTIFIER_BINDING (name));
diff --git a/gcc/testsuite/g++.dg/cpp26/name-independent-decl10.C b/gcc/testsuite/g++.dg/cpp26/name-independent-decl10.C
new file mode 100644
index 0000000..792c2bd
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp26/name-independent-decl10.C
@@ -0,0 +1,63 @@
+// PR c++/115586
+// { dg-do compile { target c++11 } }
+// { dg-options "" }
+
+struct S { int a, b, c; };
+
+void
+foo ()
+{
+  S 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-error "reference to '_' is ambiguous" "" { target *-*-* } .-1 }
+  for (auto _ : s)
+    ++_.b;
+  for (auto [a, _, b] : s)	// { dg-warning "structured bindings only available with" "" { target c++14_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-error "reference to '_' is ambiguous" }
+    }
+  for (auto _ : s)
+    {
+      ++_.b;
+      int _ = 2;		// { dg-warning "name-independent declarations only available with" "" { target c++23_down } }
+      ++_;			// { dg-error "reference to '_' is ambiguous" }
+    }
+  for (auto [a, _, b] : s)	// { dg-warning "structured bindings only available with" "" { target c++14_down } }
+    {
+      ++_;
+      int _ = ++b;		// { dg-warning "name-independent declarations only available with" "" { target c++23_down } }
+      ++_;			// { dg-error "reference to '_' is ambiguous" }
+    }
+}
+
+void
+bar ()
+{
+  S s[4] = {};
+  auto [_, c, _] = s[0];	// { 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-error "reference to '_' is ambiguous" "" { target *-*-* } .-1 }
+  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-error "reference to '_' is ambiguous" "" { target *-*-* } .-1 }
+  for (auto _ : s)
+    ++_.c;
+  for (auto [a, b, _] : s)	// { dg-warning "structured bindings only available with" "" { target c++14_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-error "reference to '_' is ambiguous" }
+      ++a;
+    }
+  for (auto _ : s)
+    int _ = 5;			// { dg-warning "name-independent declarations only available with" "" { target c++23_down } }
+  for (auto [a, b, _] : s)	// { dg-warning "structured bindings only available with" "" { target c++14_down } }
+    {
+      ++_;
+      int _ = a + b;		// { dg-warning "name-independent declarations only available with" "" { target c++23_down } }
+      ++_;			// { dg-error "reference to '_' is ambiguous" }
+    }
+}
diff --git a/gcc/testsuite/g++.dg/cpp26/name-independent-decl9.C b/gcc/testsuite/g++.dg/cpp26/name-independent-decl9.C
new file mode 100644
index 0000000..0a8ef04
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp26/name-independent-decl9.C
@@ -0,0 +1,49 @@
+// PR c++/115586
+// { dg-do compile { target c++11 } }
+// { dg-options "-Wunused-variable -Wunused-but-set-variable -Wunused-parameter -Wshadow" }
+
+struct S { int a, b, c; };
+
+void
+foo ()
+{
+  S s[4] = {};
+  for (auto [_, _, a] : s)	// { dg-warning "name-independent declarations only available with" "" { target c++23_down } }
+    ++a;			// { dg-warning "structured bindings only available with" "" { target c++14_down } .-1 }
+  for (auto _ : s)
+    ++_.b;
+  for (auto [a, _, b] : s)	// { dg-warning "structured bindings only available with" "" { target c++14_down } }
+    ++a;
+  for (auto [_, _, a] : s)	// { dg-warning "name-independent declarations only available with" "" { target c++23_down } }
+    int _ = ++a;		// { dg-warning "structured bindings only available with" "" { target c++14_down } .-1 }
+  for (auto _ : s)
+    {
+      int _ = 2;		// { dg-warning "name-independent declarations only available with" "" { target c++23_down } }
+    }
+  for (auto [a, _, b] : s)	// { dg-warning "structured bindings only available with" "" { target c++14_down } }
+    int _ = ++b;		// { dg-warning "name-independent declarations only available with" "" { target c++23_down } }
+}
+
+void
+bar ()
+{
+  S s[4] = {};
+  auto [_, c, _] = s[0];	// { dg-warning "name-independent declarations only available with" "" { target c++23_down } }
+  ++c;				// { dg-warning "structured bindings only available with" "" { target c++14_down } .-1 }
+  for (auto [a, _, _] : s)	// { dg-warning "name-independent declarations only available with" "" { target c++23_down } }
+    ++a;			// { dg-warning "structured bindings only available with" "" { target c++14_down } .-1 }
+  for (auto _ : s)
+    ++_.c;
+  for (auto [a, b, _] : s)	// { dg-warning "structured bindings only available with" "" { target c++14_down } }
+    ++b;
+  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 }
+      int _ = ++a;
+    }
+  for (auto _ : s)
+    int _ = 5;			// { dg-warning "name-independent declarations only available with" "" { target c++23_down } }
+  for (auto [a, b, _] : s)	// { dg-warning "structured bindings only available with" "" { target c++14_down } }
+    {
+      int _ = a + b;		// { dg-warning "name-independent declarations only available with" "" { target c++23_down } }
+    }
+}