Accept a beginless range of values for a bare Array type

CHANGELOG.md

@@ -56,6 +56,7 @@
 * [#2989](https://github.com/ruby-grape/grape/pull/2989): Leave the API format untouched when `format` rejects one with no content type - [@ericproulx](https://github.com/ericproulx).
 * [#2990](https://github.com/ruby-grape/grape/pull/2990): Name a nested array element by its own indices when a Hash scope sits between two Array scopes - [@ericproulx](https://github.com/ericproulx).
 * [#2991](https://github.com/ruby-grape/grape/pull/2991): Require exactly one name for a `requires` or `optional` block - [@ericproulx](https://github.com/ericproulx).
+* [#2992](https://github.com/ruby-grape/grape/pull/2992): Accept a beginless range of `values` for a bare `Array` type - [@ericproulx](https://github.com/ericproulx).
 * Your contribution here.
 
 ### 4.0.1 (2026-09-15)

lib/grape/validations/validations_spec.rb

@@ -154,7 +154,9 @@ def guess_coerce_type(coerce_type, *values_list)
 
         values_list.each do |values|
           next if !values || values.is_a?(Proc)
-          return values.first.class if values.is_a?(Range) || !values.empty?
+          # A beginless Range has no first element to ask.
+          return (values.begin || values.end).class if values.is_a?(Range)
+          return values.first.class unless values.empty?
         end
         coerce_type
       end

spec/grape/validations/params_scope_spec.rb

@@ -183,6 +183,21 @@ def initialize(value)
       end.not_to raise_error
     end
 
+    it 'accepts beginless and endless range values' do
+      expect do
+        subject.params do
+          requires :low, type: Array, values: ..5
+          requires :high, type: Array, values: 1..
+        end
+      end.not_to raise_error
+    end
+
+    it 'still raises for a beginless range whose end is not of the member type' do
+      expect do
+        subject.params { requires :numbers, type: Array, values: ..5, except_values: ['a'] }
+      end.to raise_error Grape::Exceptions::IncompatibleOptionValues
+    end
+
     it 'accepts an array containing only allowed values, given as a literal array' do
       subject.params do
         optional :periods, type: Array, values: %w[day month]