Add a new diagnostic type: NoMethodByNil

Add a new diagnostic type; NoMethodByNil, that is emitted to no method
calls for the objects having optional types.  It's separated from
NoMethod to control the visiblity of these diagnostics individually.

It's useful to find the methods not having appropriate types on checking
the Ruby application by filtering NoMethod diagnostics.

lib/steep/diagnostic/ruby.rb

@@ -303,6 +303,30 @@ def header_line
         end
       end
 
+      class NoMethodByNil < Base
+        attr_reader :type
+        attr_reader :method
+
+        def initialize(node:, type:, method:)
+          loc = case node.type
+                when :send
+                  loc = _ = nil
+                  loc ||= node.loc.operator if node.loc.respond_to?(:operator)
+                  loc ||= node.loc.selector if node.loc.respond_to?(:selector)
+                  loc
+                when :block
+                  node.children[0].loc.selector
+                end
+          super(node: node, location: loc || node.loc.expression)
+          @type = type
+          @method = method
+        end
+
+        def header_line
+          "Type `#{type}` does not have method `#{method}`"
+        end
+      end
+
       class ReturnTypeMismatch < Base
         attr_reader :expected
         attr_reader :actual
@@ -1035,6 +1059,7 @@ def self.default
             MethodReturnTypeAnnotationMismatch => :hint,
             MultipleAssignmentConversionError => :hint,
             NoMethod => :error,
+            NoMethodByNil => :error,
             ProcHintIgnored => :hint,
             ProcTypeExpected => :hint,
             RBSError => :information,
@@ -1093,6 +1118,7 @@ def self.strict
             MethodReturnTypeAnnotationMismatch => :error,
             MultipleAssignmentConversionError => :error,
             NoMethod => :error,
+            NoMethodByNil => :error,
             ProcHintIgnored => :information,
             ProcTypeExpected => :error,
             RBSError => :error,
@@ -1151,6 +1177,7 @@ def self.lenient
             MethodReturnTypeAnnotationMismatch => nil,
             MultipleAssignmentConversionError => nil,
             NoMethod => :information,
+            NoMethodByNil => :information,
             ProcHintIgnored => nil,
             ProcTypeExpected => nil,
             RBSError => :information,

lib/steep/type_construction.rb

@@ -3387,13 +3387,20 @@ def type_send_interface(node, interface:, receiver:, receiver_type:, method_name
           end
         end
 
+        interface_type = interface&.type || receiver_type
+        if interface_type.is_a?(AST::Types::Union) && interface_type.types.any? {|type| type.is_a?(AST::Types::Nil) }
+          error = Diagnostic::Ruby::NoMethodByNil.new(node: node, method: method_name, type: interface_type)
+        else
+          error = Diagnostic::Ruby::NoMethod.new(node: node, method: method_name, type: interface_type)
+        end
+
         constr.add_call(
           TypeInference::MethodCall::NoMethodError.new(
             node: node,
             context: context.call_context,
             method_name: method_name,
             receiver_type: receiver_type,
-            error: Diagnostic::Ruby::NoMethod.new(node: node, method: method_name, type: interface&.type || receiver_type)
+            error: error
           )
         )
       end

sig/steep/diagnostic/ruby.rbs

@@ -139,6 +139,9 @@ module Steep
         def header_line: () -> ::String
       end
 
+      class NoMethodByNil < NoMethod
+      end
+
       class ReturnTypeMismatch < Base
         attr_reader expected: untyped