[gcc r16-3120] c++: Fix structured binding redeclaration error recovery [PR121442]

[Jakub Jelinek via Gcc-cvs]

https://gcc.gnu.org/g:af31db6461110dfb81efaf2a287327226ae5a7a8

commit r16-3120-gaf31db6461110dfb81efaf2a287327226ae5a7a8
Author: Jakub Jelinek <ja...@redhat.com>
Date:   Mon Aug 11 08:53:26 2025 +0200

    c++: Fix structured binding redeclaration error recovery [PR121442]
    
    My C++26 P2686R4 PR117784 caused ICE on the following testcase.
    While the earlier conditions guarantee decl2 is not error_mark_node,
    decl can be (that is used when something erroneous has been seen earlier
    and the whole structured bindings will be ignored after parsing).
    
    So, the following patch avoids the copying of constexpr/constinit flags
    if decl is error_mark_node.
    
    2025-08-11  Jakub Jelinek  <ja...@redhat.com>
    
            PR c++/121442
            * parser.cc (cp_parser_decomposition_declaration): Don't copy
            DECL_DECLARED_CONST{EXPR,INIT}_P bits from decl to decl2 if
            decl is error_mark_node.
    
            * g++.dg/cpp1z/decomp65.C: New test.

Diff:
---
 gcc/cp/parser.cc                      |  9 +++++++--
 gcc/testsuite/g++.dg/cpp1z/decomp65.C | 13 +++++++++++++
 2 files changed, 20 insertions(+), 2 deletions(-)

diff --git a/gcc/cp/parser.cc b/gcc/cp/parser.cc
index 743fd7498dec..1dbe35b6c986 100644
--- a/gcc/cp/parser.cc
+++ b/gcc/cp/parser.cc
@@ -17002,8 +17002,13 @@ cp_parser_decomposition_declaration (cp_parser *parser,
       else
        {
          prev = decl2;
-         DECL_DECLARED_CONSTEXPR_P (decl2) = DECL_DECLARED_CONSTEXPR_P (decl);
-         DECL_DECLARED_CONSTINIT_P (decl2) = DECL_DECLARED_CONSTINIT_P (decl);
+         if (decl != error_mark_node)
+           {
+             DECL_DECLARED_CONSTEXPR_P (decl2)
+               = DECL_DECLARED_CONSTEXPR_P (decl);
+             DECL_DECLARED_CONSTINIT_P (decl2)
+               = DECL_DECLARED_CONSTINIT_P (decl);
+           }
          if (j == (unsigned) pack)
            {
              tree dtype = cxx_make_type (DECLTYPE_TYPE);
diff --git a/gcc/testsuite/g++.dg/cpp1z/decomp65.C 
b/gcc/testsuite/g++.dg/cpp1z/decomp65.C
new file mode 100644
index 000000000000..5dd8effcdff2
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1z/decomp65.C
@@ -0,0 +1,13 @@
+// PR c++/121442
+// { dg-do compile { target c++11 } }
+// { dg-options "" }
+
+struct S { int a, b, c, d, e; };
+
+void
+foo ()
+{
+  auto [a, b, b, b, c ] = S {};                // { dg-warning "structured 
bindings only available with" "" { target c++14_down } }
+                                       // { dg-error "redeclaration of 'auto 
b'" "" { target *-*-* } .-1 }
+                                       // { dg-message "'auto b' previously 
declared here" "" { target *-*-* } .-2 }
+}