bug#47302: 27.1; calc math-format-number formatting for floats without decimals is unusual

[Jelle Licht]

Hey folks,

I run into the following behaviour when using calc in Emacs 27.1 (using
`emacs -Q'):

--8<---------------cut here---------------start------------->8---
(calc-eval "123.0") ; => "123."
(calc-eval "0.0") ; => "0."
--8<---------------cut here---------------end--------------->8---

I would expect this output to be either "123.0" or "123" and "0.0" or
"0" respectively.

I have attached a super simple patch that fixes this issue for me, but I
am not actually certain of the following:

- Was this formatting quirk perhaps there with a purpose? 
- Does my 'fix' have unintended consequences?

Thanks,
 Jelle

>From 88e52b073510c86b582487a65fa8ddf52a48e7dd Mon Sep 17 00:00:00 2001
From: Jelle Licht <jlicht@fsfe.org>
Date: Sun, 21 Mar 2021 22:39:41 +0100
Subject: [PATCH] (calc): Fix formatting of floats without decimals

* lisp/calc/calc.el (math-format-number): Fix formatting for floats
  without decimals.
---
 lisp/calc/calc.el | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/lisp/calc/calc.el b/lisp/calc/calc.el
index ec09abb34c..fac9f70915 100644
--- a/lisp/calc/calc.el
+++ b/lisp/calc/calc.el
@@ -3193,8 +3193,9 @@ math-format-number
                   ((= dpos 0)
                    (setq str (concat "0" point str)))
                   ((and (<= exp 0) (> dpos 0))
-                   (setq str (concat (substring str 0 dpos) point
-                                     (substring str dpos))))
+                    (when (> len dpos)
+                     (setq str (concat (substring str 0 dpos) point
+                                       (substring str dpos)))))
                   ((> exp 0)
                    (setq str (concat str (make-string exp ?0) point)))
                   (t   ; (< dpos 0)
-- 
2.31.0