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  <[email protected]>

	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.

Author: jakubjelinek

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;
     }

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 ()
+{
+}