diff --git a/spec/compiler/parser/parser_spec.cr b/spec/compiler/parser/parser_spec.cr
index 8cef168f4453..529c5a3effe1 100644
--- a/spec/compiler/parser/parser_spec.cr
+++ b/spec/compiler/parser/parser_spec.cr
@@ -257,19 +257,19 @@ module Crystal
     %w(
       bar? bar!
     ).each do |name|
-      assert_syntax_error "def foo(#{name}); end", "invalid param name", 1, 14
-      assert_syntax_error "def foo(foo #{name}); end", "invalid param name", 1, 17
+      assert_syntax_warning "def foo(#{name}); end", "invalid parameter name: #{name}", 1, 14
+      assert_syntax_warning "def foo(foo #{name}); end", "invalid parameter name: #{name}", 1, 17
       it_parses "def foo(#{name} foo); end", Def.new("foo", [Arg.new("foo", external_name: name.to_s)])
 
-      assert_syntax_error "macro foo(#{name}); end", "invalid param name", 1, 16
-      assert_syntax_error "macro foo(foo #{name}); end", "invalid param name", 1, 19
+      assert_syntax_warning "macro foo(#{name}); end", "invalid parameter name: #{name}", 1, 16
+      assert_syntax_warning "macro foo(foo #{name}); end", "invalid parameter name: #{name}", 1, 19
       it_parses "macro foo(#{name} foo); end", Macro.new("foo", [Arg.new("foo", external_name: name.to_s)], body: MacroLiteral.new(" "))
 
       it_parses "foo(#{name})", Call.new(nil, "foo", [name.call] of ASTNode)
       it_parses "foo #{name}", Call.new(nil, "foo", [name.call] of ASTNode)
 
-      assert_syntax_error "foo { |#{name})| }", "invalid param name", 1, 12
-      assert_syntax_error "foo { |(#{name}))| }", "invalid param name", 1, 13
+      assert_syntax_warning "foo { |#{name}| }", "invalid parameter name: #{name}", 1, 12
+      assert_syntax_warning "foo { |(#{name})| }", "invalid parameter name: #{name}", 1, 13
     end
 
     it_parses "def self.foo\n1\nend", Def.new("foo", body: 1.int32, receiver: "self".var)
diff --git a/spec/support/syntax.cr b/spec/support/syntax.cr
index e1fd8f43d951..a1360b61e4e3 100644
--- a/spec/support/syntax.cr
+++ b/spec/support/syntax.cr
@@ -158,6 +158,16 @@ def assert_syntax_error(str, message = nil, line = nil, column = nil, metafile =
   end
 end
 
+def assert_syntax_warning(str, message = nil, line = nil, column = nil, metafile = __FILE__, metaline = __LINE__, metaendline = __END_LINE__)
+  it "says syntax warning on #{str.inspect}", metafile, metaline, metaendline do
+    warnings = WarningCollection.new
+    parse str, false, nil, warnings
+    if warnings.infos.find { |info| info.ends_with?(message) }.nil?
+      fail "Expected warnings to include '#{message}'"
+    end
+  end
+end
+
 def parse(string, wants_doc = false, filename = nil, warnings = nil)
   parser = Parser.new(string, warnings: warnings)
   parser.warnings = warnings if warnings
diff --git a/src/compiler/crystal/syntax/parser.cr b/src/compiler/crystal/syntax/parser.cr
index 1ebfa28fe09d..2c1476755875 100644
--- a/src/compiler/crystal/syntax/parser.cr
+++ b/src/compiler/crystal/syntax/parser.cr
@@ -3887,9 +3887,11 @@ module Crystal
       do_next_token = true
       found_string_literal = false
       invalid_internal_name = nil
+      external_name_token = nil
 
       if allow_external_name && (@token.type.ident? || string_literal_start?)
         name_location = @token.location
+        external_name_token = @token.dup
         if @token.type.ident?
           if @token.keyword? && invalid_internal_name?(@token.value)
             invalid_internal_name = @token.dup
@@ -3921,7 +3923,7 @@ module Crystal
           raise "when specified, external name must be different than internal name", @token
         end
 
-        check_valid_param_name(arg_name)
+        check_valid_param_name
 
         uses_arg = false
         do_next_token = true
@@ -3986,7 +3988,10 @@ module Crystal
             raise "cannot use '#{invalid_internal_name}' as a parameter name", invalid_internal_name
           end
           arg_name = external_name
-          check_valid_param_name(arg_name)
+          if external_name_token.nil?
+            raise "missing external name token"
+          end
+          check_valid_param_name(external_name_token)
         else
           unexpected_token
         end
@@ -4036,9 +4041,10 @@ module Crystal
       end
     end
 
-    def check_valid_param_name(param_name)
-      if param_name.ends_with?(/\?|!/)
-        raise "invalid param name"
+    def check_valid_param_name(token : Token = @token)
+      param_name = token.value.to_s
+      if param_name[-1]?.in?('?', '!')
+        warnings.add_warning_at(token.location, "invalid parameter name: #{param_name}")
       end
     end
 
@@ -4360,7 +4366,7 @@ module Crystal
             end
 
             arg_name = @token.value.to_s
-            check_valid_param_name(arg_name)
+            check_valid_param_name
 
             if all_names.includes?(arg_name)
               raise "duplicated block parameter name: #{arg_name}", @token
@@ -4382,7 +4388,7 @@ module Crystal
                 end
 
                 sub_arg_name = @token.value.to_s
-                check_valid_param_name(sub_arg_name)
+                check_valid_param_name
 
                 if all_names.includes?(sub_arg_name)
                   raise "duplicated block parameter name: #{sub_arg_name}", @token