[gcc r15-348] c++: lambda capturing structured bindings [PR85889]

[gcc r15-348] c++: lambda capturing structured bindings [PR85889]

[Marek Polacek]

https://gcc.gnu.org/g:46bafd9a6b9b776142e0b1424a6ac02e3a2fd300

commit r15-348-g46bafd9a6b9b776142e0b1424a6ac02e3a2fd300
Author: Marek Polacek <polacek@redhat.com>
Date:   Fri Mar 1 17:13:02 2024 -0500

    c++: lambda capturing structured bindings [PR85889]
    
    <https://wg21.link/p1381r1> clarifies that it's OK to capture structured
    bindings.
    
    [expr.prim.lambda.capture]/4 says "The identifier in a simple-capture shall
    denote a local entity" and [basic.pre]/3: "An entity is a [...] structured
    binding".
    
    It doesn't appear that this was made a DR, so, strictly speaking, we
    should have a -Wc++20-extensions warning, like clang++.
    
            PR c++/85889
    
    gcc/cp/ChangeLog:
    
            * lambda.cc (add_capture): Add a pedwarn for capturing structured
            bindings.
    
    gcc/testsuite/ChangeLog:
    
            * g++.dg/cpp2a/decomp3.C: Use -Wno-c++20-extensions.
            * g++.dg/cpp1z/decomp60.C: New test.

Diff:
---
 gcc/cp/lambda.cc                      | 10 ++++++++++
 gcc/testsuite/g++.dg/cpp1z/decomp60.C | 12 ++++++++++++
 gcc/testsuite/g++.dg/cpp2a/decomp3.C  |  2 +-
 3 files changed, 23 insertions(+), 1 deletion(-)

diff --git a/gcc/cp/lambda.cc b/gcc/cp/lambda.cc
index 4b1f9391fee1..630cc4eade14 100644
--- a/gcc/cp/lambda.cc
+++ b/gcc/cp/lambda.cc
@@ -607,6 +607,16 @@ add_capture (tree lambda, tree id, tree orig_init, bool by_reference_p,
 					 TCTX_CAPTURE_BY_COPY, type))
 	    return error_mark_node;
 	}
+
+      if (cxx_dialect < cxx20)
+	{
+	  auto_diagnostic_group d;
+	  tree stripped_init = tree_strip_any_location_wrapper (initializer);
+	  if (DECL_DECOMPOSITION_P (stripped_init)
+	      && pedwarn (input_location, OPT_Wc__20_extensions,
+			  "captured structured bindings are a C++20 extension"))
+	    inform (DECL_SOURCE_LOCATION (stripped_init), "declared here");
+	}
     }
 
   /* Add __ to the beginning of the field name so that user code
diff --git a/gcc/testsuite/g++.dg/cpp1z/decomp60.C b/gcc/testsuite/g++.dg/cpp1z/decomp60.C
new file mode 100644
index 000000000000..b6117f3af583
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1z/decomp60.C
@@ -0,0 +1,12 @@
+// PR c++/85889
+// { dg-do compile { target c++17 } }
+// { dg-options "" }
+
+struct X { int i, j; };
+void f() {
+  X x{};
+  auto [i, j] = x;
+  [&i]() { }; // { dg-warning "captured structured bindings" "" { target c++17_only } }
+  [i]() { }; // { dg-warning "captured structured bindings" "" { target c++17_only } }
+}
+
diff --git a/gcc/testsuite/g++.dg/cpp2a/decomp3.C b/gcc/testsuite/g++.dg/cpp2a/decomp3.C
index 5e77973d41f7..25ab83199757 100644
--- a/gcc/testsuite/g++.dg/cpp2a/decomp3.C
+++ b/gcc/testsuite/g++.dg/cpp2a/decomp3.C
@@ -1,6 +1,6 @@
 // P1381R1
 // { dg-do compile { target c++11 } }
-// { dg-options "" }
+// { dg-options "-Wno-c++20-extensions" }
 
 struct Foo { int a : 1; int b; };