retain the non-matching value in a PatternMismatch error

kernel/errors.ay

@@ -1,6 +1,7 @@
 MethodFail message :=
-  ("method '" + @method-name to-s + "' did not understand " +
-  "its arguments (non-exhaustive patterns)")
+  "method '" + @method-name to-s + "' did not understand " +
+    "its arguments (non-exhaustive patterns)"
 
 PatternMismatch message :=
-  "irrefutable pattern failed for " + @pattern to-s
+  "pattern of type `" + @pattern class name split("::") last +
+    "' did not match value `" + @value inspect + "'"

lib/patterns.rb

@@ -1,6 +1,7 @@
 class PatternMismatch < RuntimeError
-  def initialize(p)
+  def initialize(p, v)
     @pattern = p
+    @value = v
   end
 end
 
@@ -52,7 +53,7 @@ def assign(g, expr, set = false)
     # try pattern-matching, erroring on failure
     # effect on the stack: top value removed
     def match(g, set = false, locals = {})
-      error = g.new_label
+      mismatch = g.new_label
       done = g.new_label
 
       local_names.each do |n|
@@ -61,16 +62,20 @@ def match(g, set = false, locals = {})
 
       g.dup
       matches?(g)
-      g.gif error
+      g.gif mismatch
+
       deconstruct(g, locals)
       g.goto done
 
-      error.set!
-      g.pop
+      mismatch.set!
       g.push_self
-      g.push_const :PatternMismatch
+      g.swap
+      g.push_cpath_top
+      g.find_const :PatternMismatch
+      g.swap
       construct(g)
-      g.send :new, 1
+      g.swap
+      g.send :new, 2
       g.allow_private
       g.send :raise, 1
       g.pop