De-obsolete if-let' and when-let'

For the following release it is planned to make `if-let*' and
`when-let*' aliases for `if-let' and `when-let'.  For now we revert
declaring `if-let' and `when-let' obsolete and tweak the docstrings.

* lisp/emacs-lisp/subr-x.el (if-let*, when-let*): Make docstrings
refer to those of `if-let' and `when-let'.
(if-let, when-let): De-obsolete.  Rewrite documentation.

etc/NEWS

@@ -1305,12 +1305,8 @@ current buffer or the self-insertion takes place within a comment.
 ** The alist 'ucs-names' is now a hash table.
 
 ---
-** 'if-let' and 'when-let' are subsumed by 'if-let*' and 'when-let*'.
+** 'if-let' and 'when-let' now support binding lists as defined by the
-The incumbent 'if-let' and 'when-let' are now marked obsolete.
+SRFI-2 (Scheme Request for Implementation 2).
-'if-let*' and 'when-let*' do not accept the single tuple special case.
-New macro 'and-let*' is an implementation of the Scheme SRFI-2 syntax
-of the same name.  'if-let*' and 'when-let*' now accept the same
-binding syntax as 'and-let*'.
 
 ---
 ** 'C-up', 'C-down', 'C-left' and 'C-right' are now defined in term

lisp/emacs-lisp/subr-x.el

@@ -123,15 +123,8 @@ If ELT is of the form ((EXPR)), listify (EXPR) with a dummy symbol."
 
 (defmacro if-let* (varlist then &rest else)
   "Bind variables according to VARLIST and eval THEN or ELSE.
-Each binding is evaluated in turn, and evaluation stops if a
+This is like `if-let' but doesn't handle a VARLIST of the form
-binding value is nil.  If all are non-nil, the value of THEN is
+\(SYMBOL SOMETHING) specially."
-returned, or the last form in ELSE is returned.
-
-Each element of VARLIST is a list (SYMBOL VALUEFORM) which binds
-SYMBOL to the value of VALUEFORM.  An element can additionally
-be of the form (VALUEFORM), which is evaluated and checked for
-nil; i.e. SYMBOL can be omitted if only the test result is of
-interest."
   (declare (indent 2)
            (debug ((&rest [&or symbolp (symbolp form) (form)])
                    form body)))
@@ -144,11 +137,8 @@ interest."
 
 (defmacro when-let* (varlist &rest body)
   "Bind variables according to VARLIST and conditionally eval BODY.
-Each binding is evaluated in turn, and evaluation stops if a
+This is like `when-let' but doesn't handle a VARLIST of the form
-binding value is nil.  If all are non-nil, the value of the last
+\(SYMBOL SOMETHING) specially."
-form in BODY is returned.
-
-VARLIST is the same as in `if-let*'."
   (declare (indent 1) (debug if-let*))
   (list 'if-let* varlist (macroexp-progn body)))
 
@@ -168,12 +158,25 @@ are non-nil, then the result is non-nil."
 
 (defmacro if-let (spec then &rest else)
   "Bind variables according to SPEC and eval THEN or ELSE.
-Like `if-let*' except SPEC can have the form (SYMBOL VALUEFORM)."
+Each binding is evaluated in turn, and evaluation stops if a
+binding value is nil.  If all are non-nil, the value of THEN is
+returned, or the last form in ELSE is returned.
+
+Each element of SPEC is a list (SYMBOL VALUEFORM) which binds
+SYMBOL to the value of VALUEFORM.  An element can additionally be
+of the form (VALUEFORM), which is evaluated and checked for nil;
+i.e. SYMBOL can be omitted if only the test result is of
+interest.  It can also be of the form SYMBOL, then the binding of
+SYMBOL is checked for nil.
+
+As a special case, a SPEC of the form \(SYMBOL SOMETHING) is
+interpreted like \((SYMBOL SOMETHING)). This exists for backward
+compatibility with the old syntax that accepted only one
+binding."
   (declare (indent 2)
            (debug ([&or (&rest [&or symbolp (symbolp form) (form)])
                         (symbolp form)]
-                   form body))
+                   form body)))
-           (obsolete "use `if-let*' instead." "26.1"))
   (when (and (<= (length spec) 2)
              (not (listp (car spec))))
     ;; Adjust the single binding case
@@ -182,9 +185,12 @@ Like `if-let*' except SPEC can have the form (SYMBOL VALUEFORM)."
 
 (defmacro when-let (spec &rest body)
   "Bind variables according to SPEC and conditionally eval BODY.
-Like `when-let*' except SPEC can have the form (SYMBOL VALUEFORM)."
+Each binding is evaluated in turn, and evaluation stops if a
-  (declare (indent 1) (debug if-let)
+binding value is nil.  If all are non-nil, the value of the last
-           (obsolete "use `when-let*' instead." "26.1"))
+form in BODY is returned.
+
+The variable list SPEC is the same as in `if-let'."
+  (declare (indent 1) (debug if-let))
   (list 'if-let spec (macroexp-progn body)))
 
 (defsubst hash-table-empty-p (hash-table)