More MAIN USAGE fixes

- Fix string literal breakage caused by original fix[^1] to #1346
- Make quote post-processing only apply when we're dealing with
    a string literal, not with variable names (another reason why
    I think we shouldn't do it:
    https://irclog.perlgeek.de/perl6-dev/2017-12-31#i_15636320 )

[1] e543c890ad58d4c707f73403

src/core/Main.pm

@@ -106,11 +106,22 @@ my sub MAIN_HELPER($retval = 0) {
             for $sub.signature.params -> $param {
                 my $argument;
 
+                my int $literals-as-constraint = 0;
+                my int $total-constraints = 0;
                 my $constraints = ~unique $param.constraint_list.map: {
-                    my \g = .gist;
-                    g.contains('#`(Block|')
-                      ?? 'where { ... }'
-                      !! g.substr: 1, *-1 # remove ( ) parens around name
+                    ++$total-constraints;
+                    nqp::if(
+                      nqp::istype($_, Callable),
+                      'where { ... }',
+                      nqp::stmts(
+                        (my \g = .gist),
+                        nqp::if(
+                          nqp::isconcrete($_),
+                          nqp::stmts(
+                            ++$literals-as-constraint,
+                            g), # we constrained by some literal; gist as is
+                          nqp::substr(g, 1, nqp::chars(g)-2))))
+                          # ^ remove ( ) parens around name in the gist
                 }
                 $_ eq 'where { ... }' and $_ = "$param.type.^name() $_"
                     with $constraints;
@@ -145,16 +156,23 @@ my sub MAIN_HELPER($retval = 0) {
                     }
                 }
                 else {
-                    $argument = "<{
-                        $param.name
-                          ?? $param.usage-name
-                          !! $constraints || $param.type.^name
-                    }>";
-
-                    $argument  = "[$argument ...]"          if $param.slurpy;
-                    $argument  = "[$argument]"              if $param.optional;
-                    $argument .= trans(["'"] => [q|'"'"'|]) if $argument.contains("'");
-                    $argument  = "'$argument'"              if $argument.contains(' ' | '"');
+                    $argument = $param.name
+                        ?? "<$param.usage-name()>"
+                        !! $constraints
+                            ?? ($literals-as-constraint == $total-constraints)
+                                ?? $constraints
+                                !! "<{$constraints}>"
+                            !! "<$param.type.^name()>";
+
+                    $argument  = "[$argument ...]" if $param.slurpy;
+                    $argument  = "[$argument]"     if $param.optional;
+                    if $total-constraints
+                    && $literals-as-constraint == $total-constraints {
+                        $argument .= trans(["'"] => [q|'"'"'|])
+                            if $argument.contains("'");
+                        $argument  = "'$argument'"
+                            if $argument.contains(' ' | '"');
+                    }
                     @positional.push($argument);
                 }
                 @arg-help.push($argument => $param.WHY.contents) if $param.WHY and (@arg-help.grep:{ .key eq $argument}) == Empty;  # Use first defined

t/05-messages/02-errors.t

@@ -147,7 +147,9 @@ throws-like { sprintf "%d" }, X::Str::Sprintf::Directives::Count,
 }
 
 # https://github.com/rakudo/rakudo/issues/1346
-subtest 'USAGE with subsets/where' => {
+subtest 'USAGE with subsets/where and variables with quotes' => {
+    plan 3;
+
     sub uhas (\sig, Mu \c, \desc)  {
         is-run ｢sub MAIN (｣ ~ sig ~ ｢) {}｣,
             :err{.contains: c}, :out(*), :exitcode(*), desc
@@ -167,6 +169,9 @@ subtest 'USAGE with subsets/where' => {
         uhas ｢UInt $ where 42｣, '<UInt where { ... }>',
             'subset + where clauses shown sanely';
     }
+
+    uhas ｢$don't｣, ｢<don't>｣,
+        'variable name does not get special quote treatment';
 }
 
 # vim: ft=perl6 expandtab sw=4