c++: generic lambda and -fsanitize=vla-bound [PR93822]

Within the generic lambda the VLA capture proxy VAR_DECL has DECL_VALUE_EXPR
which is a NOP_EXPR to the VLA type of the proxy.  The problem here was that
when instantiating we were tsubsting that type twice, once for the type of
the DECL and once for the type of the NOP_EXPR, and getting two
different (though equivalent) types.  Then gimplify_type_sizes fixed up the
type of the DECL, but that didn't affect the type of the NOP_EXPR, leading
to sadness.

Fixed by directly reusing the type from the DECL.

gcc/cp/ChangeLog
2020-05-01  Jason Merrill  <jason@redhat.com>

	PR c++/93822
	* pt.c (tsubst_decl): Make sure DECL_VALUE_EXPR continues to have
	the same type as the variable.

diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
index 4673c27468e6321a4eb854ed3e71c26a2da2330d..2d38310f1fb5825089c325359f6dee3581389fa4 100644 (file)
--- a/gcc/cp/pt.c
+++ b/gcc/cp/pt.c
@@ -13817,6 +13817,11 @@ tsubst_decl (tree t, tree args, tsubst_flags_t complain)
            if (DECL_HAS_VALUE_EXPR_P (t))
              {
                tree ve = DECL_VALUE_EXPR (t);
+               /* If the DECL_VALUE_EXPR is converted to the declared type,
+                  preserve the identity so that gimplify_type_sizes works.  */
+               bool nop = (TREE_CODE (ve) == NOP_EXPR);
+               if (nop)
+                 ve = TREE_OPERAND (ve, 0);
                ve = tsubst_expr (ve, args, complain, in_decl,
                                  /*constant_expression_p=*/false);
                if (REFERENCE_REF_P (ve))
@@ -13824,6 +13829,10 @@ tsubst_decl (tree t, tree args, tsubst_flags_t complain)
                    gcc_assert (TREE_CODE (type) == REFERENCE_TYPE);
                    ve = TREE_OPERAND (ve, 0);
                  }
+               if (nop)
+                 ve = build_nop (type, ve);
+               else
+                 gcc_checking_assert (TREE_TYPE (ve) == type);
                SET_DECL_VALUE_EXPR (r, ve);
              }
            if (CP_DECL_THREAD_LOCAL_P (r)