The go.mod parser treats the characters ( and ) as separate tokens if they appear after whitespace characters or after the end of another token. They are not treated as separate tokens if they appear after other non-whitespace characters though.

require()  // scanned as 1 token:  "require()"
require () // scanned as 3 tokens: "require" "(" ")"

This is making it difficult to add new syntax, such as the version ranges suggested in #24031. It's also a problem that parentheses are the only recognized bracket character.

retract (v1.0.0, v1.1.0) // scanned as 4 tokens: "retract" "(" "v1.0.0," "v1.1.0)"
retract [v1.0.0, v1.1.0] // scanned as 3 tokens: "retract" "[v1.0.0," "v1.10]"

Ideally, the bracket characters (( ) [ ] { }) and the comma (,) would be scanned as separate characters when unquoted, regardless of surrounding whitespace characters.

retract [v1.0.0, v1.1.0] // should be scanned as 6 tokens: "retract" "[" "v1.0.0" "," "v1.1.0" "]"

None of these characters are allowed in module paths or versions, but they are allowed in file paths and may appear on the right side of a replace directive. So for the sake of compatibility, we should make this change in two steps:

• In Go 1.15:
  • The characters [ ] { } , should be scanned as separate tokens when preceded by whitespace. These are not valid at the beginning of any token today (including file paths), so this should be safe to do immediately. The characters ( ) are already scanned this way.
  • For directives added after 1.14 (i.e., retract), the characters ( ) [ ] { } , are scanned as separate tokens, even when preceded by non-whitespace characters in all directives. Old versions of the go command don't need to scan these directives correctly.
  • Tokens that will be scanned differently in the future will be quoted whenever go.mod is formatted. So the path ./foo() will be automatically rewritten as "./foo()".
• In Go 1.17 (when 1.15 is no longer supported):
  • For all directives, the characters ( ) [ ] { } , are scanned as separate tokens, even when preceded by non-whitespace characters in all directives.

Blocks #24031
Related #38144

cc @bcmills @matloob @rsc