c++: Don't clear TREE_READONLY for -fmerge-all-constants for non-aggregates [PR107558]

The following testcase ICEs, because OpenMP lowering for shared clause
on l variable with REFERENCE_TYPE creates POINTER_TYPE to REFERENCE_TYPE.
The reason is that the automatic variable has non-trivial construction
(reference to a lambda) and -fmerge-all-constants is on and so TREE_READONLY
isn't set - omp-low will handle automatic TREE_READONLY vars in shared
specially and only copy to the construct and not back, while !TREE_READONLY
are assumed to be changeable.
The PR91529 change rationale was that the gimplification can change
some non-addressable automatic variables to TREE_STATIC with
-fmerge-all-constants and therefore TREE_READONLY on them is undesirable.
But, the gimplifier does that only for aggregate variables:
  switch (TREE_CODE (type))
    {
    case RECORD_TYPE:
    case UNION_TYPE:
    case QUAL_UNION_TYPE:
    case ARRAY_TYPE:
and not for anything else.  So, I think clearing TREE_READONLY for
automatic integral or reference or pointer etc. vars for
-fmerge-all-constants only is unnecessary.

2023-03-10  Jakub Jelinek  <jakub@redhat.com>

	PR c++/107558
	* decl.cc (cp_finish_decl): Don't clear TREE_READONLY on
	automatic non-aggregate variables just because of
	-fmerge-all-constants.

	* g++.dg/gomp/pr107558.C: New test.

gcc/cp/decl.cc

@@ -8685,8 +8685,10 @@ cp_finish_decl (tree decl, tree init, bool init_const_expr_p,
 
       if (var_definition_p
 	  /* With -fmerge-all-constants, gimplify_init_constructor
-	     might add TREE_STATIC to the variable.  */
-	  && (TREE_STATIC (decl) || flag_merge_constants >= 2))
+	     might add TREE_STATIC to aggregate variables.  */
+	  && (TREE_STATIC (decl)
+	      || (flag_merge_constants >= 2
+		  && AGGREGATE_TYPE_P (type))))
 	{
 	  /* If a TREE_READONLY variable needs initialization
 	     at runtime, it is no longer readonly and we need to

gcc/testsuite/g++.dg/gomp/pr107558.C

@@ -0,0 +1,14 @@
+// PR c++/107558
+// { dg-do compile { target c++11 } }
+// { dg-additional-options "-fmerge-all-constants" }
+// { dg-additional-options "-flto" { target lto } }
+
+int a = 15;
+
+void
+foo ()
+{
+  auto &&l = [&]() { return a; };
+#pragma omp target parallel
+  l ();
+}