[gcc r13-5265] c++: Fix up handling of references to anon union members in initializers [PR53932]

[gcc r13-5265] c++: Fix up handling of references to anon union members in initializers [PR53932]

[Jakub Jelinek]

https://gcc.gnu.org/g:9b9a989adc042b304572fd6d4ade46b47be6ccb8

commit r13-5265-g9b9a989adc042b304572fd6d4ade46b47be6ccb8
Author: Jakub Jelinek <jakub@redhat.com>
Date:   Thu Jan 19 23:27:34 2023 +0100

    c++: Fix up handling of references to anon union members in initializers [PR53932]
    
    For anonymous union members we create artificial VAR_DECLs which
    have DECL_VALUE_EXPR for the actual COMPONENT_REF.  That works
    just fine inside of functions (including global dynamic constructors),
    because during gimplification such VAR_DECLs are gimplified as
    their DECL_VALUE_EXPR.  This is also done during regimplification.
    
    But references to these artificial vars in DECL_INITIAL expressions
    aren't ever replaced by the DECL_VALUE_EXPRs, so we end up either
    with link failures like on the testcase below, or worse ICEs with
    LTO.
    
    The following patch fixes those during cp_fully_fold_init where we
    already walk all the trees (!data->genericize means that
    function rather than cp_fold_function).
    
    2023-01-19  Jakub Jelinek  <jakub@redhat.com>
    
            PR c++/53932
            * cp-gimplify.cc (cp_fold_r): During cp_fully_fold_init replace
            DECL_ANON_UNION_VAR_P VAR_DECLs with their corresponding
            DECL_VALUE_EXPR.
    
            * g++.dg/init/pr53932.C: New test.

Diff:
---
 gcc/cp/cp-gimplify.cc               | 10 ++++++++++
 gcc/testsuite/g++.dg/init/pr53932.C | 25 +++++++++++++++++++++++++
 2 files changed, 35 insertions(+)

diff --git a/gcc/cp/cp-gimplify.cc b/gcc/cp/cp-gimplify.cc
index a28215655e7..340b4641046 100644
--- a/gcc/cp/cp-gimplify.cc
+++ b/gcc/cp/cp-gimplify.cc
@@ -1010,6 +1010,16 @@ cp_fold_r (tree *stmt_p, int *walk_subtrees, void *data_)
 	}
       break;
 
+    case VAR_DECL:
+      /* In initializers replace anon union artificial VAR_DECLs
+	 with their DECL_VALUE_EXPRs, as nothing will do it later.  */
+      if (DECL_ANON_UNION_VAR_P (stmt) && !data->genericize)
+	{
+	  *stmt_p = stmt = unshare_expr (DECL_VALUE_EXPR (stmt));
+	  break;
+	}
+      break;
+
     default:
       break;
     }
diff --git a/gcc/testsuite/g++.dg/init/pr53932.C b/gcc/testsuite/g++.dg/init/pr53932.C
new file mode 100644
index 00000000000..3b129e7d68a
--- /dev/null
+++ b/gcc/testsuite/g++.dg/init/pr53932.C
@@ -0,0 +1,25 @@
+// PR c++/53932
+// { dg-do link }
+
+static union { int i; };
+int &r = i;
+int s = i;
+int *t = &i;
+
+void
+foo (int **p, int *q)
+{
+  static int &u = i;
+  static int v = i;
+  static int *w = &i;
+  int &x = i;
+  int y = i;
+  int *z = &i;
+  *p = &i;
+  *q = i;
+}
+
+int
+main ()
+{
+}