Fix GIMPLE verification failure in LTO mode on Ada code

The issue is that tree_is_indexable doesn't return the same result for
a FIELD_DECL with QUAL_UNION_TYPE and the QUAL_UNION_TYPE, resulting
in two instances of the QUAL_UNION_TYPE in the bytecode.  The result
for the type is the correct one (false, since it is variably modified)
while the result for the field is falsely true because:

  else if (TREE_CODE (t) == FIELD_DECL
           && lto_variably_modified_type_p (DECL_CONTEXT (t)))
    return false;

is not satisfied.  The reason for this is that the DECL_QUALIFIER of
fields of a QUAL_UNION_TYPE depends on a discriminant in Ada, which
means that the size of the type does too (CONTAINS_PLACEHOLDER_P),
which in turn means that it is reset to a mere PLACEHOLDER_EXPR by
free_lang_data, which finally means that the size of DECL_CONTEXT is
too, so RETURN_TRUE_IF_VAR is false.

In other words, the CONTAINS_PLACEHOLDER_P property of the DECL_QUALIFIER
of fields of a QUAL_UNION_TYPE hides the variably_modified_type_p property
of  these fields, if you look from the outside.

	PR middle-end/93961
	* tree.c (variably_modified_type_p) <RECORD_TYPE>: Recurse into
	fields whose type is a qualified union.

1 files changed, 10 insertions, 0 deletions

diff --git a/gcc/tree.c b/gcc/tree.c
index 66d52c7..905563f 100644
--- a/gcc/tree.c
+++ b/gcc/tree.c
@@ -9206,8 +9206,18 @@ variably_modified_type_p (tree type, tree fn)
 	    RETURN_TRUE_IF_VAR (DECL_SIZE (t));
 	    RETURN_TRUE_IF_VAR (DECL_SIZE_UNIT (t));
 
+	    /* If the type is a qualified union, then the DECL_QUALIFIER
+	       of fields can also be an expression containing a variable.  */
 	    if (TREE_CODE (type) == QUAL_UNION_TYPE)
 	      RETURN_TRUE_IF_VAR (DECL_QUALIFIER (t));
+
+	    /* If the field is a qualified union, then it's only a container
+	       for what's inside so we look into it.  That's necessary in LTO
+	       mode because the sizes of the field tested above have been set
+	       to PLACEHOLDER_EXPRs by free_lang_data.  */
+	    if (TREE_CODE (TREE_TYPE (t)) == QUAL_UNION_TYPE
+		&& variably_modified_type_p (TREE_TYPE (t), fn))
+	      return true;
 	  }
       break;