[gcc r16-7182] fold-const: Fix implicit cast to nullptr_t typedef [PR123790]

[Jakub Jelinek]

https://gcc.gnu.org/g:cffe5fd7c98babd85acf86f3e7f8d02b04416ec2

commit r16-7182-gcffe5fd7c98babd85acf86f3e7f8d02b04416ec2
Author: Jakub Jelinek <jakub@redhat.com>
Date:   Fri Jan 30 11:40:17 2026 +0100

    fold-const: Fix implicit cast to nullptr_t typedef [PR123790]
    
    The following testcase is incorrectly rejected, because since r15-6744
    it adds build_nop to preserve the exact typedef type and
    cxx_eval_constant_expression can't fold NOP_EXPR from integer_zerop of one
    NULLPTR_TYPE to another NULLPTR_TYPE, while cxx_eval_constant_expression
    relies on fold to do such folding.
    
    I see 3 options to fix that, one is deal with this in the r15-6744 spot
    and special case NULLPTR_TYPE there and build_zero_cst instead of build_nop
    (with similar condition like in the patch below), another possibility is
    special case this in cxx_eval_constant_expression, and
    another one (done in this patch) is to handle this in fold-const.cc -
    fold_convert_loc and also in fold_convert_const.
    
    2026-01-30  Jakub Jelinek  <jakub@redhat.com>
    
            PR c++/123790
            * fold-const.cc (fold_convert_const): Handle conversion of
            integer_zerop to NULLPTR_TYPE.
            (fold_convert_loc): Likewise.
    
            * g++.dg/cpp0x/nullptr47.C: New test.

Diff:
---
 gcc/fold-const.cc                      |  6 ++++++
 gcc/testsuite/g++.dg/cpp0x/nullptr47.C | 10 ++++++++++
 2 files changed, 16 insertions(+)

diff --git a/gcc/fold-const.cc b/gcc/fold-const.cc
index 47cd0053d31a..82d8ef5092e8 100644
--- a/gcc/fold-const.cc
+++ b/gcc/fold-const.cc
@@ -2566,6 +2566,8 @@ fold_convert_const (enum tree_code code, tree type, tree arg1)
 	  return v.build ();
 	}
     }
+  else if (TREE_CODE (type) == NULLPTR_TYPE && integer_zerop (arg1))
+    return build_zero_cst (type);
   return NULL_TREE;
 }
 
@@ -2788,6 +2790,10 @@ fold_convert_loc (location_t loc, tree type, tree arg)
       tem = fold_ignored_result (arg);
       return fold_build1_loc (loc, NOP_EXPR, type, tem);
 
+    case NULLPTR_TYPE:
+      if (integer_zerop (arg))
+	return build_zero_cst (type);
+      /* FALLTHRU */
     default:
       if (TYPE_MAIN_VARIANT (type) == TYPE_MAIN_VARIANT (orig))
 	return fold_build1_loc (loc, NOP_EXPR, type, arg);
diff --git a/gcc/testsuite/g++.dg/cpp0x/nullptr47.C b/gcc/testsuite/g++.dg/cpp0x/nullptr47.C
new file mode 100644
index 000000000000..4c4071b65248
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/nullptr47.C
@@ -0,0 +1,10 @@
+// PR c++/123790
+// { dg-do compile { target c++11 } }
+
+int
+main ()
+{
+  using nullptr_t = decltype (nullptr);
+  constexpr nullptr_t zero = nullptr;
+  constexpr nullptr_t other_zero = zero;
+}