Fix a bug where "ambiguous first argument" diagnostic was not emitted

lib/parser/lexer.rl

@@ -1357,21 +1357,21 @@ class Parser::Lexer
       w_space* '?'
       => { fhold; fgoto expr_beg; };
 
-               # a %{1}, a %[1] (but not "a %=1=" or "a % foo")
-               # a /foo/ (but not "a / foo" or "a /=foo")
-      w_space+ ( [%/] ( c_any - c_space_nl - '=' ) # /
-               # a <<HEREDOC
-               | '<<'
-               )
-      => { fhold; fhold; fgoto expr_beg; };
-
-      # x /1
-      # Ambiguous regexp literal.
-      w_space+ '/'
+      # a %{1}, a %[1] (but not "a %=1=" or "a % foo")
+      # a /foo/ (but not "a / foo" or "a /=foo")
+      # a <<HEREDOC
+      w_space+ %{ tm = p }
+      ( [%/] ( c_any - c_space_nl - '=' ) # /
+      | '<<'
+      )
       => {
-        diagnostic :warning, :ambiguous_literal, nil, range(@te - 1, @te)
+        if tok(tm, tm + 1) == '/'
+          # Ambiguous regexp literal.
+          diagnostic :warning, :ambiguous_literal, nil, range(tm, tm + 1)
+        end
 
-        fhold; fgoto expr_beg;
+        p = tm - 1
+        fgoto expr_beg;
       };
 
       # x *1

test/parse_helper.rb

@@ -161,6 +161,23 @@ def assert_diagnoses(diagnostic, code, source_maps='', versions=ALL_VERSIONS)
     end
   end
 
+  def refute_diagnoses(code, versions=ALL_VERSIONS)
+    with_versions(versions) do |version, parser|
+      source_file = Parser::Source::Buffer.new('(refute_diagnoses)')
+      source_file.source = code
+
+      begin
+        parser = parser.parse(source_file)
+      rescue Parser::SyntaxError
+        # do nothing; the diagnostic was reported
+      end
+
+      assert_empty @diagnostics,
+                   "(#{version}) emits no diagnostics, not\n" \
+                   "#{@diagnostics.map(&:render).join("\n")}"
+    end
+  end
+
   SOURCE_MAP_DESCRIPTION_RE =
       /(?x)
        ^(?# $1 skip)            ^(\s*)

test/test_parser.rb

@@ -2578,6 +2578,16 @@ def test_send_plain_cmd
         |~~~~~~~~~~~~ expression})
   end
 
+  def test_send_plain_cmd_ambiguous_literal
+    assert_diagnoses(
+      [:warning, :ambiguous_literal],
+      %q{m /foo/},
+      %q{  ^ location})
+
+    refute_diagnoses(
+      %q{m %[1]})
+  end
+
   def test_send_plain_cmd_ambiguous_prefix
     assert_diagnoses(
       [:warning, :ambiguous_prefix, { :prefix => '+' }],