Improve the error message for an inappropriate column definition list.

The existing message about "a column definition list is only allowed for
functions returning "record"" could be given in some cases where it was
fairly confusing; in particular, a function with multiple OUT parameters
*does* return record according to pg_proc.  Break it down into a couple
more cases to deliver a more on-point complaint.  Per complaint from
Bruce Momjian.

Discussion: https://postgr.es/m/798909.1600562993@sss.pgh.pa.us

src/backend/parser/parse_relation.c

@@ -1737,16 +1737,46 @@ addRangeTableEntryForFunction(ParseState *pstate,
 
 		/*
 		 * A coldeflist is required if the function returns RECORD and hasn't
-		 * got a predetermined record type, and is prohibited otherwise.
+		 * got a predetermined record type, and is prohibited otherwise.  This
+		 * can be a bit confusing, so we expend some effort on delivering a
+		 * relevant error message.
 		 */
 		if (coldeflist != NIL)
 		{
-			if (functypclass != TYPEFUNC_RECORD)
-				ereport(ERROR,
-						(errcode(ERRCODE_SYNTAX_ERROR),
-						 errmsg("a column definition list is only allowed for functions returning \"record\""),
-						 parser_errposition(pstate,
-											exprLocation((Node *) coldeflist))));
+			switch (functypclass)
+			{
+				case TYPEFUNC_RECORD:
+					/* ok */
+					break;
+				case TYPEFUNC_COMPOSITE:
+				case TYPEFUNC_COMPOSITE_DOMAIN:
+
+					/*
+					 * If the function's raw result type is RECORD, we must
+					 * have resolved it using its OUT parameters.  Otherwise,
+					 * it must have a named composite type.
+					 */
+					if (exprType(funcexpr) == RECORDOID)
+						ereport(ERROR,
+								(errcode(ERRCODE_SYNTAX_ERROR),
+								 errmsg("a column definition list is redundant for a function with OUT parameters"),
+								 parser_errposition(pstate,
+													exprLocation((Node *) coldeflist))));
+					else
+						ereport(ERROR,
+								(errcode(ERRCODE_SYNTAX_ERROR),
+								 errmsg("a column definition list is redundant for a function returning a named composite type"),
+								 parser_errposition(pstate,
+													exprLocation((Node *) coldeflist))));
+					break;
+				default:
+					ereport(ERROR,
+							(errcode(ERRCODE_SYNTAX_ERROR),
+							 errmsg("a column definition list is only allowed for functions returning \"record\""),
+							 parser_errposition(pstate,
+												exprLocation((Node *) coldeflist))));
+					break;
+			}
 		}
 		else
 		{

src/test/regress/expected/rangefuncs.out

@@ -2109,6 +2109,19 @@ select * from testrngfunc();
  7.136178 | 7.14
 (1 row)
 
+-- Check a couple of error cases while we're here
+select * from testrngfunc() as t(f1 int8,f2 int8);  -- fail, composite result
+ERROR:  a column definition list is redundant for a function returning a named composite type
+LINE 1: select * from testrngfunc() as t(f1 int8,f2 int8);
+                                         ^
+select * from pg_get_keywords() as t(f1 int8,f2 int8);  -- fail, OUT params
+ERROR:  a column definition list is redundant for a function with OUT parameters
+LINE 1: select * from pg_get_keywords() as t(f1 int8,f2 int8);
+                                             ^
+select * from sin(3) as t(f1 int8,f2 int8);  -- fail, scalar result type
+ERROR:  a column definition list is only allowed for functions returning "record"
+LINE 1: select * from sin(3) as t(f1 int8,f2 int8);
+                                  ^
 drop type rngfunc_type cascade;
 NOTICE:  drop cascades to function testrngfunc()
 --

src/test/regress/sql/rangefuncs.sql

@@ -629,6 +629,11 @@ explain (verbose, costs off)
 select * from testrngfunc();
 select * from testrngfunc();
 
+-- Check a couple of error cases while we're here
+select * from testrngfunc() as t(f1 int8,f2 int8);  -- fail, composite result
+select * from pg_get_keywords() as t(f1 int8,f2 int8);  -- fail, OUT params
+select * from sin(3) as t(f1 int8,f2 int8);  -- fail, scalar result type
+
 drop type rngfunc_type cascade;
 
 --