[gcc(refs/vendors/redhat/heads/gcc-8-branch)] c++: generic lambda and -fsanitize=vla-bound [PR93822]

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

https://gcc.gnu.org/g:2a4634e6d87fc7094104084a86b038c5ab7d0fe2

commit 2a4634e6d87fc7094104084a86b038c5ab7d0fe2
Author: Jason Merrill <jason@redhat.com>
Date:   Mon May 25 18:38:09 2020 -0400

    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:
---
 gcc/cp/pt.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
index 4673c27468e..2d38310f1fb 100644
--- 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)