c++: fix constexpr cast from void* diag issue [PR116741]

The result of build_fold_indirect_ref can be a COMPONENT_REF in
which case using DECL_SOURCE_LOCATION will crash.  Look at its op1
instead.

	PR c++/116741

gcc/cp/ChangeLog:

	* constexpr.cc (cxx_eval_constant_expression) <case CONVERT_EXPR>: If
	the result of build_fold_indirect_ref is a COMPONENT_REF, use its op1.
	Check DECL_P before calling inform.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp26/constexpr-voidptr4.C: New test.

Reviewed-by: Jason Merrill <jason@redhat.com>

diff --git a/gcc/cp/constexpr.cc b/gcc/cp/constexpr.cc
index c3668b0d7d3f3a3f3cb2bff328c17676966ebdb2..f6fd059be4665ca1bbceba43fcbb71a33fac8f3d 100644 (file)
--- a/gcc/cp/constexpr.cc
+++ b/gcc/cp/constexpr.cc
@@ -8201,8 +8201,11 @@ cxx_eval_constant_expression (const constexpr_ctx *ctx, tree t,
                              TREE_TYPE (op), TREE_TYPE (TREE_TYPE (sop)),
                              TREE_TYPE (type));
                    tree obj = build_fold_indirect_ref (sop);
-                   inform (DECL_SOURCE_LOCATION (obj),
-                           "pointed-to object declared here");
+                   if (TREE_CODE (obj) == COMPONENT_REF)
+                     obj = TREE_OPERAND (obj, 1);
+                   if (DECL_P (obj))
+                     inform (DECL_SOURCE_LOCATION (obj),
+                             "pointed-to object declared here");
                  }
                *non_constant_p = true;
                return t;

diff --git a/gcc/testsuite/g++.dg/cpp26/constexpr-voidptr4.C b/gcc/testsuite/g++.dg/cpp26/constexpr-voidptr4.C
new file mode 100644 (file)
index 0000000..53563c9
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp26/constexpr-voidptr4.C
@@ -0,0 +1,29 @@
+// PR c++/116741
+// { dg-do compile { target c++26 } }
+
+struct S {
+  int foo;    // { dg-message "pointed-to object" }
+};
+
+struct S2 {
+  int foo;    // { dg-message "pointed-to object" }
+};
+
+struct X {
+  S2 s;
+};
+
+constexpr float f1() {
+  S s;
+  void* p = &s.foo;
+  return *static_cast<float*>(p); // { dg-error "not allowed in a constant expression" }
+}
+
+constexpr float f2() {
+  X x;
+  void* p = &x.s.foo;
+  return *static_cast<float*>(p); // { dg-error "not allowed in a constant expression" }
+}
+
+constexpr auto x1 = f1();
+constexpr auto x2 = f2();