Confirmed.

<<< Unknown tree: c_maybe_const_expr
  
  SAVE_EXPR <*s++ > >>>

Any complex type will cause this ICE.

The problem is that c_fully_fold_internal doesn't recurse on the RHS of a MODIFY_EXPR:
      /* The RHS of a MODIFY_EXPR was fully folded when building that
         expression for the sake of conversion warnings.  */
While this is true, here RHS was INDIRECT_REF of a POSTINCREMENT_EXPR of PARM_DECL, all with COMPLEX_TYPE or pointers to that and it was fully folded.
But afterwards when this RHS is convert()ed to the LHS type, c_common_truthvalue_conversion is called and introduces new C_MAYBE_CONST_EXPRs
into it.
I wonder if we couldn't swap the order of c_fully_fold and convert_for_assignment
in build_modify_expr:
      newrhs = c_fully_fold (newrhs, false, NULL);
      if (rhs_semantic_type)
        newrhs = build1 (EXCESS_PRECISION_EXPR, rhs_semantic_type, newrhs);
      newrhs = convert_for_assignment (location, lhstype, newrhs, rhs_origtype,
                                       ic_assign, npc, NULL_TREE, NULL_TREE, 0);
so that it would be fully folding also whatever was added by convert_for_assignment.  Joseph?

Actually, reading that comment again it, the conversion warnings are probably emitted during convert_for_assignment and thus the earlier c_fully_fold is needed.  Perhaps we need another c_fully_fold if convert_for_assignment returned something different from the passed argument, or remove the if (code != MODIFY_EXPR) guard for c_fully_fold_internal on the RHS of a MODIFY_EXPR.

My inclination is that the problem is a missing check of 
in_late_binary_op.  Specifically, that c_common_truthvalue_conversion 
should call save_expr instead of c_save_expr if in_late_binary_op (which 
in turn requires that in_late_binary_op move from c-tree.h/c-typeck.c to 
being defined/declared in c-family code).

It is caused by revision 46547:

http://gcc.gnu.org/ml/gcc-cvs/2009-03/msg00761.html

It is caused by revision 145254:

http://gcc.gnu.org/ml/gcc-cvs/2009-03/msg00761.html

Testing a patch.

Author: jsm28
Date: Fri Nov 19 18:32:57 2010
New Revision: 166951

URL: http://gcc.gnu.org/viewcvs?root=gcc&view=rev&rev=166951
Log:
	PR c/46547
	* c-tree.h (in_late_binary_op): Move to c-family/c-common.h.
	* c-typeck.c (in_late_binary_op): Move to c-family/c-common.c.

c-family:
	* c-common.c (in_late_binary_op): Define.
	(c_common_truthvalue_conversion): Check in_late_binary_op before
	calling c_save_expr.
	* c-common.h (in_late_binary_op): Declare.

testsuite:
	* gcc.c-torture/compile/pr46547-1.c: New test.

Added:
    trunk/gcc/testsuite/gcc.c-torture/compile/pr46547-1.c
Modified:
    trunk/gcc/ChangeLog
    trunk/gcc/c-family/ChangeLog
    trunk/gcc/c-family/c-common.c
    trunk/gcc/c-family/c-common.h
    trunk/gcc/c-tree.h
    trunk/gcc/c-typeck.c
    trunk/gcc/testsuite/ChangeLog

Author: jsm28
Date: Fri Nov 19 20:45:00 2010
New Revision: 166957

URL: http://gcc.gnu.org/viewcvs?root=gcc&view=rev&rev=166957
Log:
	PR c/46547
	* c-common.c (in_late_binary_op): Define.
	(c_common_truthvalue_conversion): Check in_late_binary_op before
	calling c_save_expr.
	* c-common.h (in_late_binary_op): Declare.
	* c-tree.h (in_late_binary_op): Move to c-common.h.
	* c-typeck.c (in_late_binary_op): Move to c-common.c.

testsuite:
	* gcc.c-torture/compile/pr46547-1.c: New test.

Added:
    branches/gcc-4_5-branch/gcc/testsuite/gcc.c-torture/compile/pr46547-1.c
Modified:
    branches/gcc-4_5-branch/gcc/ChangeLog
    branches/gcc-4_5-branch/gcc/c-common.c
    branches/gcc-4_5-branch/gcc/c-common.h
    branches/gcc-4_5-branch/gcc/c-tree.h
    branches/gcc-4_5-branch/gcc/c-typeck.c
    branches/gcc-4_5-branch/gcc/testsuite/ChangeLog

Fixed for 4.5.2 and 4.6.

Revision 166951 breaks bootstrap with obj-c++ enabled:

ld: duplicate symbol _in_late_binary_op in c-family/c-common.o and objcp/objcp-act.o

Author: jakub
Date: Fri Nov 19 23:44:47 2010
New Revision: 166964

URL: http://gcc.gnu.org/viewcvs?root=gcc&view=rev&rev=166964
Log:
	PR c/46547
	* objc-act.c (in_late_binary_op): Remove.

Modified:
    trunk/gcc/objc/ChangeLog
    trunk/gcc/objc/objc-act.c

Author: jakub
Date: Fri Nov 19 23:46:57 2010
New Revision: 166965

URL: http://gcc.gnu.org/viewcvs?root=gcc&view=rev&rev=166965
Log:
	PR c/46547
	* objc-act.c (in_late_binary_op): Remove.

Modified:
    branches/gcc-4_5-branch/gcc/objc/ChangeLog
    branches/gcc-4_5-branch/gcc/objc/objc-act.c