Improve USAGE message for subset/where params

Fixes #1346

- Display `where` constraints as `where { ... }` instead of
    `-> ;; $_? is raw { #`(Block|83868104) ... }`. Looks like we
    did this awhile back too, but the detection of custom wheres got
    broken when .gist of blocks got changed
- Display subset names instead of their nominal types in named params

src/core/Main.pm

@@ -105,6 +105,16 @@ my sub MAIN_HELPER($retval = 0) {
 
             for $sub.signature.params -> $param {
                 my $argument;
+
+                my $constraints = ~unique $param.constraint_list.map: {
+                    my \g = .gist;
+                    g.contains('#`(Block|')
+                      ?? 'where { ... }'
+                      !! g.substr: 1, *-1 # remove ( ) parens around name
+                }
+                $_ eq 'where { ... }' and $_ = "$param.type.^name() $_"
+                    with $constraints;
+
                 if $param.named {
                     if $param.slurpy {
                         if $param.name { # ignore anon *%
@@ -116,7 +126,9 @@ my sub MAIN_HELPER($retval = 0) {
                         my @names  = $param.named_names.reverse;
                         $argument  = @names.map({($^n.chars == 1 ?? '-' !! '--') ~ $^n}).join('|');
                         if $param.type !=== Bool {
-                            $argument ~= "=<{$param.type.^name}>";
+                            $argument ~= "=<{
+                                $constraints || $param.type.^name
+                            }>";
                             if Metamodel::EnumHOW.ACCEPTS($param.type.HOW) {
                                 my $options = $param.type.^enum_values.keys.sort.Str;
                                 $argument ~= $options.chars > 50
@@ -133,11 +145,11 @@ my sub MAIN_HELPER($retval = 0) {
                     }
                 }
                 else {
-                    my $constraints  = $param.constraint_list.map(*.gist).join(' ');
-                    my $simple-const = $constraints && $constraints !~~ /^_block/;
-                    $argument = $param.name   ?? "<$param.usage-name()>" !!
-                                $simple-const ??       $constraints                !!
-                                                 '<' ~ $param.type.^name     ~ '>' ;
+                    $argument = "<{
+                        $param.name
+                          ?? $param.usage-name
+                          !! $constraints || $param.type.^name
+                    }>";
 
                     $argument  = "[$argument ...]"          if $param.slurpy;
                     $argument  = "[$argument]"              if $param.optional;

t/05-messages/02-errors.t

@@ -2,7 +2,7 @@ use lib <t/packages/>;
 use Test;
 use Test::Helpers;
 
-plan 21;
+plan 22;
 
 # RT #132295
 
@@ -146,4 +146,27 @@ throws-like { sprintf "%d" }, X::Str::Sprintf::Directives::Count,
         'error message when :deleting from natively typed array';
 }
 
+# https://github.com/rakudo/rakudo/issues/1346
+subtest 'USAGE with subsets/where' => {
+    sub uhas (\sig, Mu \c, \desc)  {
+        is-run ｢sub MAIN (｣ ~ sig ~ ｢) {}｣,
+            :err{.contains: c}, :out(*), :exitcode(*), desc
+    }
+
+    subtest 'named params' => {
+        uhas ｢UInt :$x!｣,          '<UInt>', 'mentions subset name';
+        uhas ｢Int :$x! where 42｣,  '<Int where { ... }>',
+            'Type + where clauses shown sanely';
+        uhas ｢UInt :$x! where 42｣, '<UInt where { ... }>',
+            'subset + where clauses shown sanely';
+    }
+    subtest 'anon positional params' => {
+        uhas ｢UInt $｣,          '<UInt>', 'mentions subset name';
+        uhas ｢Int $ where 42｣,  '<Int where { ... }>',
+            'where clauses shown sanely';
+        uhas ｢UInt $ where 42｣, '<UInt where { ... }>',
+            'subset + where clauses shown sanely';
+    }
+}
+
 # vim: ft=perl6 expandtab sw=4