diff --git a/gcc/tree.h b/gcc/tree.h
index 8bd0580397d5..5df8b84687f1 100644
--- a/gcc/tree.h
+++ b/gcc/tree.h
@@ -1679,6 +1679,8 @@ struct GTY(()) tree_constructor {
   (CAN_HAVE_LOCATION_P ((NODE)) ? (NODE)->exp.locus : UNKNOWN_LOCATION)
 #define SET_EXPR_LOCATION(NODE, LOCUS) EXPR_CHECK ((NODE))->exp.locus = (LOCUS)
 #define EXPR_HAS_LOCATION(NODE) (EXPR_LOCATION (NODE) != UNKNOWN_LOCATION)
+/* The location to be used in a diagnostic about this expression.  Do not
+   use this macro if the location will be assigned to other expressions.  */
 #define EXPR_LOC_OR_HERE(NODE) (EXPR_HAS_LOCATION (NODE) ? (NODE)->exp.locus : input_location)
 #define EXPR_FILENAME(NODE) LOCATION_FILE (EXPR_CHECK ((NODE))->exp.locus)
 #define EXPR_LINENO(NODE) LOCATION_LINE (EXPR_CHECK (NODE)->exp.locus)