[gcc r11-8765] stor-layout: Avoid DECL_BIT_FIELD_REPRESENTATIVE with NULL TREE_TYPE [PR101172]

[Jakub Jelinek <jakub@gcc.gnu.org>]

https://gcc.gnu.org/g:2b99794806d83e47249fd7c05bf521caee1fdcd4

commit r11-8765-g2b99794806d83e47249fd7c05bf521caee1fdcd4
Author: Jakub Jelinek <jakub@redhat.com>
Date:   Thu Jun 24 12:22:14 2021 +0200

    stor-layout: Avoid DECL_BIT_FIELD_REPRESENTATIVE with NULL TREE_TYPE [PR101172]
    
    finish_bitfield_representative has an early out if the field after a
    bitfield has error_mark_node type, but that early out leads to TREE_TYPE
    of the DECL_BIT_FIELD_REPRESENTATIVE being NULL, which breaks assumptions
    on code that uses the DECL_BIT_FIELD_REPRESENTATIVE during error-recovery.
    
    The following patch instead sets TREE_TYPE of the representative to
    error_mark_node, something the users can deal with better.  At this point
    the representative can be set as DECL_BIT_FIELD_REPRESENTATIVE for multiple
    bitfields, so making sure that we clear the DECL_BIT_FIELD_REPRESENTATIVE
    instead would be harder (but doable, e.g. with the error_mark_node TREE_TYPE
    set by this patch set some flag in the caller and if the flag is there, walk
    all the fields once again and clear all DECL_BIT_FIELD_REPRESENTATIVE that
    have error_mark_node TREE_TYPE).
    
    2021-06-24  Jakub Jelinek  <jakub@redhat.com>
    
            PR middle-end/101172
            * stor-layout.c (finish_bitfield_representative): If nextf has
            error_mark_node type, set repr type to error_mark_node too.
    
            * gcc.dg/pr101172.c: New test.
    
    (cherry picked from commit 65371066d8967560e3508af4a804e0ddb90acee7)

Diff:
---
 gcc/stor-layout.c               |  5 ++++-
 gcc/testsuite/gcc.dg/pr101172.c | 20 ++++++++++++++++++++
 2 files changed, 24 insertions(+), 1 deletion(-)

diff --git a/gcc/stor-layout.c b/gcc/stor-layout.c
index 9f850c3951e..242452f2acf 100644
--- a/gcc/stor-layout.c
+++ b/gcc/stor-layout.c
@@ -2086,7 +2086,10 @@ finish_bitfield_representative (tree repr, tree field)
       /* If there was an error, the field may be not laid out
          correctly.  Don't bother to do anything.  */
       if (TREE_TYPE (nextf) == error_mark_node)
-	return;
+	{
+	  TREE_TYPE (repr) = error_mark_node;
+	  return;
+	}
       maxsize = size_diffop (DECL_FIELD_OFFSET (nextf),
 			     DECL_FIELD_OFFSET (repr));
       if (tree_fits_uhwi_p (maxsize))
diff --git a/gcc/testsuite/gcc.dg/pr101172.c b/gcc/testsuite/gcc.dg/pr101172.c
new file mode 100644
index 00000000000..b9d098bb644
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/pr101172.c
@@ -0,0 +1,20 @@
+/* PR middle-end/101172 */
+/* { dg-do compile } */
+/* { dg-options "-O2" } */
+
+union U
+{
+  int a[3];
+  struct
+  {
+    int a : 3;
+    struct this_struct var;	/* { dg-error "field 'var' has incomplete type" } */
+  } b;
+};
+
+const union U hello = {.a = {1, 2, 3}};
+
+void foo()
+{
+  int x = hello.b.a;
+}