[gcc r15-3674] c++: fix constexpr cast from void* diag issue [PR116741]

[Marek Polacek via Gcc-cvs]

https://gcc.gnu.org/g:d6d8445c85509b66a59aa6247ad7b2cfeab17725

commit r15-3674-gd6d8445c85509b66a59aa6247ad7b2cfeab17725
Author: Marek Polacek <pola...@redhat.com>
Date:   Tue Sep 17 14:34:30 2024 -0400

    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 <ja...@redhat.com>

Diff:
---
 gcc/cp/constexpr.cc                             |  7 ++++--
 gcc/testsuite/g++.dg/cpp26/constexpr-voidptr4.C | 29 +++++++++++++++++++++++++
 2 files changed, 34 insertions(+), 2 deletions(-)

diff --git a/gcc/cp/constexpr.cc b/gcc/cp/constexpr.cc
index c3668b0d7d3f..f6fd059be466 100644
--- 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
index 000000000000..53563c928f27
--- /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();