[PATCH] c++: Fix ICE during explanation of failed __has_unique_object_representation [PR126867]

Hi!

The following testcase ICEs since r16-5520 added explain
arguments to type_has_unique_obj_representations and
record_has_unique_obj_representations.  Most of the location_t
arguments in that change look correct, either DECL_SOURCE_LOCATION
of some FIELD_DECL or in type_has_unique_obj_representations use
loc, which is initialized to
  location_t loc = input_location;
  if (tree m = TYPE_MAIN_DECL (t))
    loc = DECL_SOURCE_LOCATION (m);
But in record_has_unique_obj_representations we don't have
a variable like that and t at that point is the RECORD_TYPE
or UNION_TYPE, so using DECL_SOURCE_LOCATION on it will surely ICE
in a checking compiler.
location_of call does the above dances, so it seems easiest to
call it rather than duplicate it by hand.

Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk/16.3?

2026-08-17  Jakub Jelinek  <[email protected]>

	PR c++/126867
	* tree.cc (record_has_unique_obj_representations): Use
	location_of (const_cast <tree> (t)) rather than
	DECL_SOURCE_LOCATION (t).

	* g++.dg/cpp1z/has-unique-obj-representations6.C: New test.

--- gcc/cp/tree.cc.jj	2026-07-24 18:38:54.337025261 +0200
+++ gcc/cp/tree.cc	2026-08-17 12:32:40.551645418 +0200
@@ -5274,7 +5274,7 @@ record_has_unique_obj_representations (c
 	    inform (DECL_SOURCE_LOCATION (last_named_field),
 		    "padding occurs after %qD", last_named_field);
 	  else
-	    inform (DECL_SOURCE_LOCATION (t),
+	    inform (location_of (const_cast <tree> (t)),
 		    "%qT has padding and no data fields", t);
 	}
       return false;
--- gcc/testsuite/g++.dg/cpp1z/has-unique-obj-representations6.C.jj	2026-08-17 12:37:22.573052447 +0200
+++ gcc/testsuite/g++.dg/cpp1z/has-unique-obj-representations6.C	2026-08-17 12:38:16.061371108 +0200
@@ -0,0 +1,10 @@
+// PR c++/126867
+// { dg-do compile { target c++17 } }
+
+template <class T>
+constexpr bool U = __has_unique_object_representations (T);
+
+struct S {};		// { dg-message "'S' does not have unique object representations, because" }
+			// { dg-message "'S' has padding and no data fields" "" { target *-*-* } .-1 }
+
+static_assert (U <S>);	// { dg-error "static assertion failed" }

	Jakub