cex: always show ε/%empty in counterexamples

On a case such as
    %%
    exp
    : empty "a"
    | "a" empty

    empty
    : %empty

we used to display

    warning: shift/reduce conflict on token "a" [-Wcounterexamples]
    Example: • "a"
    Shift derivation
      exp
      ↳ 2: • "a" empty
                 ↳ 2: ε
    Example: • "a"
    Reduce derivation
      exp
      ↳ 1: empty  "a"
           ↳ 3: •

where the shift derivation shows an item "2: empty → ε", with an
explicit "ε", but the reduce derivation shows "3: empty → •", without
"ε".

For consistency, let's always show ε/%empty in rules with an empty
rhs:

    Reduce derivation
      exp
      ↳ 1: empty    "a"
           ↳ 3: ε •

* src/derivation.c (derivation_width, derivation_print_tree_impl):
Always show ε/%empty in counterexamples.
* tests/diagnostics.at: Check that case.
* tests/conflicts.at, tests/counterexample.at: Adjust.

6 files changed, 141 insertions, 66 deletions

diff --git a/NEWS b/NEWS
index c14d17f2..5f869f12 100644
--- a/NEWS
+++ b/NEWS
@@ -19,17 +19,18 @@ GNU Bison NEWS
 
 *** Counterexamples
 
-  Counterexamples now show the rule numbers:
+  Counterexamples now show the rule numbers, and always show ε for rules
+  with an empty right-hand side.  For instance
 
     exp
-    ↳ 1: "if" exp "then" exp
-                         ↳ 2: "if" exp "then" exp • "else" exp
+    ↳ 1: e1       e2     "a"
+         ↳ 3: ε • ↳ 1: ε
 
   instead of
 
     exp
-    ↳ "if" exp "then" exp
-                      ↳ "if" exp "then" exp • "else" exp
+    ↳ e1  e2  "a"
+      ↳ • ↳ ε
 
 
 * Noteworthy changes in release 3.7.1 (2020-08-02) [stable]
diff --git a/doc/bison.texi b/doc/bison.texi
index e36f8a38..f9aaf343 100644
--- a/doc/bison.texi
+++ b/doc/bison.texi
@@ -10605,8 +10605,8 @@ This shows two separate derivations in the grammar for the same @code{exp}:
 given example. Here, the first derivation completes a reduction when seeing
 @samp{/}, causing @samp{e1 + e2} to be grouped as an @code{exp}. The second
 derivation shifts on @samp{/}, resulting in @samp{e2 / e3} being grouped as
-an @code{exp}. Therefore, it is easy to see that adding @code{%precedence}
-directives would fix this conflict.
+an @code{exp}.  Therefore, it is easy to see that adding
+precedence/associativity directives would fix this conflict.
 
 The remaining states are similar:
 
diff --git a/src/derivation.c b/src/derivation.c
index 382dc458..433f504e 100644
--- a/src/derivation.c
+++ b/src/derivation.c
@@ -245,6 +245,13 @@ derivation_width (const derivation *deriv)
         children_width += empty_width;
       else
         {
+          if (gl_list_size (deriv->children) == 1
+              && gl_list_get_first (deriv->children) == &d_dot)
+            {
+              children_width += empty_width;
+              children_width += derivation_separator_width;
+            }
+
           derivation *child;
           for (gl_list_iterator_t it = gl_list_iterator (deriv->children);
                derivation_list_next (&it, &child);
@@ -310,6 +317,13 @@ derivation_print_tree_impl (const derivation *deriv, FILE *out,
             res += fputs_if (depth == 1, out, padding, empty);
           else
             {
+              if (gl_list_size (deriv->children) == 1
+                  && gl_list_get_first (deriv->children) == &d_dot)
+                {
+                  res += fputs_if (depth == 1, out, padding, empty);
+                  res += fputs_if (depth == 1, out, padding, derivation_separator);
+                }
+
               bool first = true;
               derivation *child;
               for (gl_list_iterator_t it = gl_list_iterator (deriv->children);
diff --git a/tests/conflicts.at b/tests/conflicts.at
index d68f03aa..de0a993e 100644
--- a/tests/conflicts.at
+++ b/tests/conflicts.at
@@ -1776,7 +1776,7 @@ State 4
       Reduce derivation
         start
         `-> 1: resolved_conflict reported_conflicts 'a'
-                                 `-> 10: .
+                                 `-> 10: %empty .
 
     shift/reduce conflict on token 'a':
        10 reported_conflicts: . %empty
@@ -1790,7 +1790,7 @@ State 4
       Reduce derivation
         start
         `-> 1: resolved_conflict reported_conflicts 'a'
-                                 `-> 10: .
+                                 `-> 10: %empty .
 
 
 
@@ -1999,12 +1999,12 @@ AT_CHECK([[cat input.output | sed -n '/^State 0$/,/^State 1$/p']], 0,
       Example: . 'c'
       First reduce derivation
         start
-        `-> 7: empty_c2  'c'
-               `-> 12: .
+        `-> 7: empty_c2         'c'
+               `-> 12: %empty .
       Second reduce derivation
         start
-        `-> 8: empty_c3  'c'
-               `-> 13: .
+        `-> 8: empty_c3         'c'
+               `-> 13: %empty .
 
 
 
diff --git a/tests/counterexample.at b/tests/counterexample.at
index bd9c1e96..fcc239bb 100644
--- a/tests/counterexample.at
+++ b/tests/counterexample.at
@@ -164,9 +164,9 @@ input.y: warning: shift/reduce conflict on token B [-Wcounterexamples]
              `-> 9: . B
   Reduce derivation
     s
-    `-> 1: ax                by
-           `-> 3: A x        `-> 6: B y
-                    `-> 4: .          `-> 6: %empty
+    `-> 1: ax                       by
+           `-> 3: A x               `-> 6: B y
+                    `-> 4: %empty .          `-> 6: %empty
 input.y: warning: shift/reduce conflict on token B [-Wcounterexamples]
   First example: A X . B Y $end
   Shift derivation
@@ -178,11 +178,11 @@ input.y: warning: shift/reduce conflict on token B [-Wcounterexamples]
   Second example: A X . B y $end
   Reduce derivation
     $accept
-    `-> 0: s                                            $end
-           `-> 1: ax                         by
-                  `-> 3: A x                 `-> 6: B y
+    `-> 0: s                                                   $end
+           `-> 1: ax                                by
+                  `-> 3: A x                        `-> 6: B y
                            `-> 5: X x
-                                    `-> 4: .
+                                    `-> 4: %empty .
 input.y:5.4-9: warning: rule useless in parser due to conflicts [-Wother]
 ]],
 [[input.y: warning: 2 shift/reduce conflicts [-Wconflicts-sr]
@@ -571,10 +571,10 @@ input.y: warning: reduce/reduce conflict on tokens b, c [-Wcounterexamples]
   Second reduce derivation
     S
     `-> 1: B C
-             `-> 7: A                                      c A
-                    `-> 3: B                                 `-> 7: %empty
-                           `-> 6: A        b A
-                                  `-> 5: .   `-> 6: %empty
+             `-> 7: A                                             c A
+                    `-> 3: B                                        `-> 7: %empty
+                           `-> 6: A               b A
+                                  `-> 5: %empty .   `-> 6: %empty
 input.y: warning: reduce/reduce conflict on tokens b, c [-Wcounterexamples]
   Example: C . c b
   First reduce derivation
@@ -585,10 +585,10 @@ input.y: warning: reduce/reduce conflict on tokens b, c [-Wcounterexamples]
   Second reduce derivation
     S
     `-> 2: C B
-             `-> 6: A                                      b A
-                    `-> 4: C                                 `-> 6: %empty
-                           `-> 7: A        c A
-                                  `-> 5: .   `-> 7: %empty
+             `-> 6: A                                             b A
+                    `-> 4: C                                        `-> 6: %empty
+                           `-> 7: A               c A
+                                  `-> 5: %empty .   `-> 7: %empty
 ]],
 [[input.y: warning: 4 reduce/reduce conflicts [-Wconflicts-rr]
 input.y: warning: reduce/reduce conflict on tokens b, c [-Wcounterexamples]
@@ -625,49 +625,49 @@ input.y: warning: reduce/reduce conflict on token A [-Wcounterexamples]
   First example: . c A A $end
   First reduce derivation
     $accept
-    `-> 0: a                            $end
-           `-> 1: b        d
-                  `-> 3: . `-> 6: c A A
+    `-> 0: a                                   $end
+           `-> 1: b               d
+                  `-> 3: %empty . `-> 6: c A A
   Second example: . c A A $end
   Second reduce derivation
     $accept
-    `-> 0: a                            $end
-           `-> 2: c        d
-                  `-> 4: . `-> 6: c A A
+    `-> 0: a                                   $end
+           `-> 2: c               d
+                  `-> 4: %empty . `-> 6: c A A
 input.y: warning: reduce/reduce conflict on token A [-Wcounterexamples]
 time limit exceeded: XXX
   First example: b . c A A $end
   First reduce derivation
     $accept
-    `-> 0: a                                            $end
+    `-> 0: a                                                   $end
            `-> 1: b d
                     `-> 5: a
-                           `-> 1: b        d
-                                  `-> 3: . `-> 6: c A A
+                           `-> 1: b               d
+                                  `-> 3: %empty . `-> 6: c A A
   Second example: b . A $end
   Second reduce derivation
     $accept
-    `-> 0: a                          $end
+    `-> 0: a                                 $end
            `-> 1: b d
-                    `-> 6: c        A
-                           `-> 4: .
+                    `-> 6: c               A
+                           `-> 4: %empty .
 input.y: warning: reduce/reduce conflict on token A [-Wcounterexamples]
 time limit exceeded: XXX
   First example: c . c A A $end
   First reduce derivation
     $accept
-    `-> 0: a                                            $end
+    `-> 0: a                                                   $end
            `-> 2: c d
                     `-> 5: a
-                           `-> 1: b        d
-                                  `-> 3: . `-> 6: c A A
+                           `-> 1: b               d
+                                  `-> 3: %empty . `-> 6: c A A
   Second example: c . A $end
   Second reduce derivation
     $accept
-    `-> 0: a                          $end
+    `-> 0: a                                 $end
            `-> 2: c d
-                    `-> 6: c        A
-                           `-> 4: .
+                    `-> 6: c               A
+                           `-> 4: %empty .
 input.y: warning: shift/reduce conflict on token A [-Wcounterexamples]
 time limit exceeded: XXX
   First example: b c . A
@@ -678,33 +678,33 @@ time limit exceeded: XXX
   Second example: b c . c A A $end
   Reduce derivation
     $accept
-    `-> 0: a                                                            $end
+    `-> 0: a                                                                   $end
            `-> 1: b d
                     `-> 5: a
                            `-> 2: c d
                                     `-> 5: a
-                                           `-> 1: b        d
-                                                  `-> 3: . `-> 6: c A A
+                                           `-> 1: b               d
+                                                  `-> 3: %empty . `-> 6: c A A
 input.y: warning: reduce/reduce conflict on token A [-Wcounterexamples]
   First example: b c . c A A $end
   First reduce derivation
     $accept
-    `-> 0: a                                                            $end
+    `-> 0: a                                                                   $end
            `-> 1: b d
                     `-> 5: a
                            `-> 2: c d
                                     `-> 5: a
-                                           `-> 1: b        d
-                                                  `-> 3: . `-> 6: c A A
+                                           `-> 1: b               d
+                                                  `-> 3: %empty . `-> 6: c A A
   Second example: b c . A $end
   Second reduce derivation
     $accept
-    `-> 0: a                                          $end
+    `-> 0: a                                                 $end
            `-> 1: b d
                     `-> 5: a
                            `-> 2: c d
-                                    `-> 6: c        A
-                                           `-> 4: .
+                                    `-> 6: c               A
+                                           `-> 4: %empty .
 input.y: warning: shift/reduce conflict on token A [-Wcounterexamples]
   First example: b c . A
   Shift derivation
@@ -714,12 +714,12 @@ input.y: warning: shift/reduce conflict on token A [-Wcounterexamples]
   Second example: b c . A $end
   Reduce derivation
     $accept
-    `-> 0: a                                          $end
+    `-> 0: a                                                 $end
            `-> 1: b d
                     `-> 5: a
                            `-> 2: c d
-                                    `-> 6: c        A
-                                           `-> 4: .
+                                    `-> 6: c               A
+                                           `-> 4: %empty .
 input.y: warning: reduce/reduce conflict on token $end [-Wcounterexamples]
   Example: b d .
   First reduce derivation
@@ -861,10 +861,10 @@ input.y: warning: shift/reduce conflict on token D [-Wcounterexamples]
                `-> 6: . D
   Reduce derivation
     s
-    `-> 2: A a a                      d
-               `-> 3: b               `-> 6: D
+    `-> 2: A a a                             d
+               `-> 3: b                      `-> 6: D
                       `-> 4: c
-                             `-> 5: .
+                             `-> 5: %empty .
 ]],
 [[input.y: warning: 1 shift/reduce conflict [-Wconflicts-sr]
 input.y: warning: shift/reduce conflict on token D [-Wcounterexamples]
@@ -905,11 +905,11 @@ input.y: warning: shift/reduce conflict on token D [-Wcounterexamples]
   Second example: A a . D E $end
   Reduce derivation
     $accept
-    `-> 0: s                                            $end
-           `-> 2: A a a                      d        E
-                      `-> 3: b               `-> 6: D
+    `-> 0: s                                                   $end
+           `-> 2: A a a                             d        E
+                      `-> 3: b                      `-> 6: D
                              `-> 4: c
-                                    `-> 5: .
+                                    `-> 5: %empty .
 ]],
 [[input.y: warning: 1 shift/reduce conflict [-Wconflicts-sr]
 input.y: warning: shift/reduce conflict on token D [-Wcounterexamples]
diff --git a/tests/diagnostics.at b/tests/diagnostics.at
index faf4c9a7..42b6d3a8 100644
--- a/tests/diagnostics.at
+++ b/tests/diagnostics.at
@@ -532,9 +532,58 @@ exp
 | "if" exp "then" exp "else" exp
 | exp "+" exp
 | "num"
+| empty "a"
+| "a" empty
+
+empty
+: %empty
 ]],
 [1],
-[[input.y: <error>error:</error> shift/reduce conflicts: 4 found, 0 expected
+[[input.y: <error>error:</error> shift/reduce conflicts: 9 found, 0 expected
+input.y: <warning>warning:</warning> shift/reduce conflict on token "a" [<warning>-Wcounterexamples</warning>]
+  Example: <cex-0><cex-dot>•</cex-dot> <cex-leaf>"a"</cex-leaf><cex-1></cex-1></cex-0>
+  Shift derivation
+    <cex-0><cex-step>exp</cex-step></cex-0>
+    <cex-0><cex-step>↳ 6: <cex-dot>•</cex-dot><cex-leaf> "a"</cex-leaf><cex-1><cex-step> empty</cex-step></cex-1></cex-step></cex-0>
+    <cex-1><cex-step>           ↳ 6: ε</cex-step></cex-1>
+  Example: <cex-0><cex-1><cex-dot>•</cex-dot></cex-1> <cex-leaf>"a"</cex-leaf></cex-0>
+  Reduce derivation
+    <cex-0><cex-step>exp</cex-step></cex-0>
+    <cex-0><cex-step>↳ 5: <cex-1><cex-step>empty</cex-step></cex-1><cex-leaf>    "a"</cex-leaf></cex-step></cex-0>
+    <cex-1><cex-step>     ↳ 7: ε<cex-dot> •</cex-dot></cex-step></cex-1>
+input.y: <warning>warning:</warning> shift/reduce conflict on token "a" [<warning>-Wcounterexamples</warning>]
+  Example: <cex-0><cex-dot>•</cex-dot> <cex-leaf>"a"</cex-leaf><cex-1></cex-1></cex-0>
+  Shift derivation
+    <cex-0><cex-step>exp</cex-step></cex-0>
+    <cex-0><cex-step>↳ 6: <cex-dot>•</cex-dot><cex-leaf> "a"</cex-leaf><cex-1><cex-step> empty</cex-step></cex-1></cex-step></cex-0>
+    <cex-1><cex-step>           ↳ 6: ε</cex-step></cex-1>
+  Example: <cex-0><cex-1><cex-dot>•</cex-dot></cex-1> <cex-leaf>"a"</cex-leaf></cex-0>
+  Reduce derivation
+    <cex-0><cex-step>exp</cex-step></cex-0>
+    <cex-0><cex-step>↳ 5: <cex-1><cex-step>empty</cex-step></cex-1><cex-leaf>    "a"</cex-leaf></cex-step></cex-0>
+    <cex-1><cex-step>     ↳ 7: ε<cex-dot> •</cex-dot></cex-step></cex-1>
+input.y: <warning>warning:</warning> shift/reduce conflict on token "a" [<warning>-Wcounterexamples</warning>]
+  Example: <cex-0><cex-dot>•</cex-dot> <cex-leaf>"a"</cex-leaf><cex-1></cex-1></cex-0>
+  Shift derivation
+    <cex-0><cex-step>exp</cex-step></cex-0>
+    <cex-0><cex-step>↳ 6: <cex-dot>•</cex-dot><cex-leaf> "a"</cex-leaf><cex-1><cex-step> empty</cex-step></cex-1></cex-step></cex-0>
+    <cex-1><cex-step>           ↳ 6: ε</cex-step></cex-1>
+  Example: <cex-0><cex-1><cex-dot>•</cex-dot></cex-1> <cex-leaf>"a"</cex-leaf></cex-0>
+  Reduce derivation
+    <cex-0><cex-step>exp</cex-step></cex-0>
+    <cex-0><cex-step>↳ 5: <cex-1><cex-step>empty</cex-step></cex-1><cex-leaf>    "a"</cex-leaf></cex-step></cex-0>
+    <cex-1><cex-step>     ↳ 7: ε<cex-dot> •</cex-dot></cex-step></cex-1>
+input.y: <warning>warning:</warning> shift/reduce conflict on token "a" [<warning>-Wcounterexamples</warning>]
+  Example: <cex-0><cex-dot>•</cex-dot> <cex-leaf>"a"</cex-leaf><cex-1></cex-1></cex-0>
+  Shift derivation
+    <cex-0><cex-step>exp</cex-step></cex-0>
+    <cex-0><cex-step>↳ 6: <cex-dot>•</cex-dot><cex-leaf> "a"</cex-leaf><cex-1><cex-step> empty</cex-step></cex-1></cex-step></cex-0>
+    <cex-1><cex-step>           ↳ 6: ε</cex-step></cex-1>
+  Example: <cex-0><cex-1><cex-dot>•</cex-dot></cex-1> <cex-leaf>"a"</cex-leaf></cex-0>
+  Reduce derivation
+    <cex-0><cex-step>exp</cex-step></cex-0>
+    <cex-0><cex-step>↳ 5: <cex-1><cex-step>empty</cex-step></cex-1><cex-leaf>    "a"</cex-leaf></cex-step></cex-0>
+    <cex-1><cex-step>     ↳ 7: ε<cex-dot> •</cex-dot></cex-step></cex-1>
 input.y: <warning>warning:</warning> shift/reduce conflict on token "+" [<warning>-Wcounterexamples</warning>]
   Example: <cex-0><cex-leaf>exp</cex-leaf> <cex-leaf>"+"</cex-leaf><cex-1> <cex-leaf>exp</cex-leaf> <cex-dot>•</cex-dot> <cex-leaf>"+"</cex-leaf> <cex-leaf>exp</cex-leaf></cex-1></cex-0>
   Shift derivation
@@ -568,6 +617,17 @@ input.y: <warning>warning:</warning> shift/reduce conflict on token "+" [<warnin
     <cex-0><cex-step>exp</cex-step></cex-0>
     <cex-0><cex-step>↳ 3: <cex-1><cex-step>exp</cex-step></cex-1><cex-leaf>                        "+"</cex-leaf><cex-leaf> exp</cex-leaf></cex-step></cex-0>
     <cex-1><cex-step>     ↳ 1: <cex-leaf>"if"</cex-leaf><cex-leaf> exp</cex-leaf><cex-leaf> "then"</cex-leaf><cex-leaf> exp</cex-leaf><cex-dot> •</cex-dot></cex-step></cex-1>
+input.y: <warning>warning:</warning> shift/reduce conflict on token "a" [<warning>-Wcounterexamples</warning>]
+  Example: <cex-0><cex-dot>•</cex-dot> <cex-leaf>"a"</cex-leaf><cex-1></cex-1></cex-0>
+  Shift derivation
+    <cex-0><cex-step>exp</cex-step></cex-0>
+    <cex-0><cex-step>↳ 6: <cex-dot>•</cex-dot><cex-leaf> "a"</cex-leaf><cex-1><cex-step> empty</cex-step></cex-1></cex-step></cex-0>
+    <cex-1><cex-step>           ↳ 6: ε</cex-step></cex-1>
+  Example: <cex-0><cex-1><cex-dot>•</cex-dot></cex-1> <cex-leaf>"a"</cex-leaf></cex-0>
+  Reduce derivation
+    <cex-0><cex-step>exp</cex-step></cex-0>
+    <cex-0><cex-step>↳ 5: <cex-1><cex-step>empty</cex-step></cex-1><cex-leaf>    "a"</cex-leaf></cex-step></cex-0>
+    <cex-1><cex-step>     ↳ 7: ε<cex-dot> •</cex-dot></cex-step></cex-1>
 input.y: <warning>warning:</warning> shift/reduce conflict on token "+" [<warning>-Wcounterexamples</warning>]
   Example: <cex-0><cex-leaf>"if"</cex-leaf> <cex-leaf>exp</cex-leaf> <cex-leaf>"then"</cex-leaf> <cex-leaf>exp</cex-leaf> <cex-leaf>"else"</cex-leaf><cex-1> <cex-leaf>exp</cex-leaf> <cex-dot>•</cex-dot> <cex-leaf>"+"</cex-leaf> <cex-leaf>exp</cex-leaf></cex-1></cex-0>
   Shift derivation