c++: improve constexpr type mismatch diagnostic

This diagnostic failed to specify the actual type of the object being
accessed through a glvalue of an incompatible type, and could also use to
indicate where that object comes from.

gcc/cp/ChangeLog:

	* constexpr.cc (cxx_eval_indirect_ref): Improve diagnostic.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp26/constexpr-new3.C: Tweak diagnostic.

2 files changed, 18 insertions, 5 deletions

diff --git a/gcc/cp/constexpr.cc b/gcc/cp/constexpr.cc
index 65b91ec..b8ac454 100644
--- a/gcc/cp/constexpr.cc
+++ b/gcc/cp/constexpr.cc
@@ -7179,10 +7179,23 @@ cxx_eval_indirect_ref (const constexpr_ctx *ctx, tree t,
 			  (TREE_TYPE (TREE_TYPE (sub)), TREE_TYPE (t)));
 	      /* DR 1188 says we don't have to deal with this.  */
 	      if (!ctx->quiet)
-		error_at (cp_expr_loc_or_input_loc (t),
-			  "accessing value of %qE through a %qT glvalue in a "
-			  "constant expression", build_fold_indirect_ref (sub),
-			  TREE_TYPE (t));
+		{
+		  auto_diagnostic_group d;
+		  error_at (cp_expr_loc_or_input_loc (t),
+			    "accessing value of %qT object through a %qT "
+			    "glvalue in a constant expression",
+			    TREE_TYPE (TREE_TYPE (sub)), TREE_TYPE (t));
+		  tree ob = build_fold_indirect_ref (sub);
+		  if (DECL_P (ob))
+		    {
+		      if (DECL_ARTIFICIAL (ob))
+			inform (DECL_SOURCE_LOCATION (ob),
+				"%qT object created here", TREE_TYPE (ob));
+		      else
+			inform (DECL_SOURCE_LOCATION (ob),
+				"%q#D declared here", ob);
+		    }
+		}
 	      *non_constant_p = true;
 	      return t;
 	    }
diff --git a/gcc/testsuite/g++.dg/cpp26/constexpr-new3.C b/gcc/testsuite/g++.dg/cpp26/constexpr-new3.C
index 6a06a6e..7466199 100644
--- a/gcc/testsuite/g++.dg/cpp26/constexpr-new3.C
+++ b/gcc/testsuite/g++.dg/cpp26/constexpr-new3.C
@@ -37,7 +37,7 @@ baz ()
 {
   std::allocator<int> a;
   auto b = a.allocate (2);
-  new (b) long (42);		// { dg-error "accessing value of 'heap ' through a 'long int' glvalue in a constant expression" }
+  new (b) long (42);		// { dg-error "accessing value of 'int [2]' object through a 'long int' glvalue in a constant expression" }
   a.deallocate (b, 2);
   return true;
 }