On second thought, explain why date_trunc("week") on interval values is

not supported in the error message, rather than the docs.

doc/src/sgml/func.sgml

@@ -7050,11 +7050,6 @@ date_trunc('<replaceable>field</replaceable>', <replaceable>source</replaceable>
     </simplelist>
    </para>
 
-   <para>
-    <literal>week</literal> is not supported for <type>interval</>
-    values because months usually have fractional weeks.
-   </para>
-
    <para>
     Examples:
 <screen>

src/backend/utils/adt/timestamp.c

@@ -3710,10 +3710,17 @@ interval_trunc(PG_FUNCTION_ARGS)
 					break;
 
 				default:
-					ereport(ERROR,
-							(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
-							 errmsg("interval units \"%s\" not supported",
-									lowunits)));
+					if (val == DTK_WEEK)
+						ereport(ERROR,
+								(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+								 errmsg("interval units \"%s\" not supported "
+									"because months usually have fractional weeks",
+										lowunits)));
+					else
+						ereport(ERROR,
+								(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+								 errmsg("interval units \"%s\" not supported",
+										lowunits)));
 			}
 
 			if (tm2interval(tm, fsec, result) != 0)