Catch syntax error in generated column definition

The syntax

    GENERATED BY DEFAULT AS (expr)

is not allowed but we have to accept it in the grammar to avoid
shift/reduce conflicts because of the similar syntax for identity
columns.  The existing code just ignored this, incorrectly.  Add an
explicit error check and a bespoke error message.
Reported-by: Justin Pryzby <pryzby@telsasoft.com>

src/backend/parser/gram.y

@@ -3505,6 +3505,19 @@ ColConstraintElem:
 					n->raw_expr = $5;
 					n->cooked_expr = NULL;
 					n->location = @1;
+
+					/*
+					 * Can't do this in the grammar because of shift/reduce
+					 * conflicts.  (IDENTITY allows both ALWAYS and BY
+					 * DEFAULT, but generated columns only allow ALWAYS.)  We
+					 * can also give a more useful error message and location.
+					 */
+					if ($2 != ATTRIBUTE_IDENTITY_ALWAYS)
+						ereport(ERROR,
+								(errcode(ERRCODE_SYNTAX_ERROR),
+								 errmsg("for a generated column, GENERATED ALWAYS must be specified"),
+								 parser_errposition(@2)));
+
 					$$ = (Node *)n;
 				}
 			| REFERENCES qualified_name opt_column_list key_match key_actions

src/test/regress/expected/generated.out

@@ -85,6 +85,11 @@ CREATE TABLE gtest_err_7d (a int PRIMARY KEY, b int GENERATED ALWAYS AS (generat
 ERROR:  set-returning functions are not allowed in column generation expressions
 LINE 1: ...7d (a int PRIMARY KEY, b int GENERATED ALWAYS AS (generate_s...
                                                              ^
+-- GENERATED BY DEFAULT not allowed
+CREATE TABLE gtest_err_8 (a int PRIMARY KEY, b int GENERATED BY DEFAULT AS (a * 2) STORED);
+ERROR:  for a generated column, GENERATED ALWAYS must be specified
+LINE 1: ...E gtest_err_8 (a int PRIMARY KEY, b int GENERATED BY DEFAULT...
+                                                             ^
 INSERT INTO gtest1 VALUES (1);
 INSERT INTO gtest1 VALUES (2, DEFAULT);
 INSERT INTO gtest1 VALUES (3, 33);  -- error

src/test/regress/sql/generated.sql

@@ -37,6 +37,9 @@ CREATE TABLE gtest_err_7b (a int PRIMARY KEY, b int GENERATED ALWAYS AS (row_num
 CREATE TABLE gtest_err_7c (a int PRIMARY KEY, b int GENERATED ALWAYS AS ((SELECT a)) STORED);
 CREATE TABLE gtest_err_7d (a int PRIMARY KEY, b int GENERATED ALWAYS AS (generate_series(1, a)) STORED);
 
+-- GENERATED BY DEFAULT not allowed
+CREATE TABLE gtest_err_8 (a int PRIMARY KEY, b int GENERATED BY DEFAULT AS (a * 2) STORED);
+
 INSERT INTO gtest1 VALUES (1);
 INSERT INTO gtest1 VALUES (2, DEFAULT);
 INSERT INTO gtest1 VALUES (3, 33);  -- error