toke.c: explicitly diagnose 'catch (my $e)'

Previously, this would produce

    Can't redeclare "my" in "our" at -e line 1, near "(my"

Fixes #23222.

Author: mauke

pod/perldiag.pod

@@ -1485,10 +1485,23 @@ missing.  You need to figure out where your CRTL misplaced its environ
 or define F<PERL_ENV_TABLES> (see L<perlvms>) so that environ is not
 searched.
 
+=item Can't redeclare catch variable as "%s"
+
+(F) A C<my>, C<our> or C<state> keyword was used with the exception variable in
+a C<catch> block:
+
+    try { ... }
+    catch (my $e) { ... }
+    # or catch (our $e) { ... }
+    # or catch (state $e) { ... }
+
+This is not valid syntax. C<catch> takes a bare variable name, which is
+automatically lexically declared.
+
 =item Can't redeclare "%s" in "%s"
 
-(F) A "my", "our" or "state" declaration was found within another declaration,
-such as C<my ($x, my($y), $z)> or C<our (my $x)>.
+(F) A C<my>, C<our> or C<state> declaration was found within another
+declaration, such as C<my ($x, my($y), $z)> or C<our (my $x)>.
 
 =item Can't "redo" outside a loop block
 

toke.c

@@ -7186,17 +7186,37 @@ yyl_do(pTHX_ char *s, I32 orig_keyword)
     OPERATOR(KW_DO);
 }
 
+static const char *
+declarator_name(I32 k) {
+    switch (k) {
+        case KEY_my:    return "my";
+        case KEY_state: return "state";
+        case KEY_our:   return "our";
+        case KEY_field: return "field";
+        case KEY_catch: return "catch";
+        default: return "???";
+    }
+}
+
 static int
 yyl_my(pTHX_ char *s, I32 my)
 {
+    assert(my == KEY_my || my == KEY_state || my == KEY_our);
     if (PL_in_my) {
         PL_bufptr = s;
-        yyerror(form(
-                          "Can't redeclare \"%s\" in \"%s\"",
-                           my       == KEY_my    ? "my" :
-                           my       == KEY_state ? "state" : "our",
-                           PL_in_my == KEY_my    ? "my" :
-                           PL_in_my == KEY_state ? "state" : "our"));
+        if (PL_in_my == KEY_catch) {
+            yyerror(form(
+                "Can't redeclare catch variable as \"%s\"",
+                    declarator_name(my)
+            ));
+        } else {
+            assert(PL_in_my == KEY_my || PL_in_my == KEY_state || PL_in_my == KEY_our);
+            yyerror(form(
+                "Can't redeclare \"%s\" in \"%s\"",
+                    declarator_name(my),
+                    declarator_name(PL_in_my)
+            ));
+        }
     }
     PL_in_my = (U16)my;
     s = skipspace(s);