Clarify the behavior of `#__PURE__` for tagged templates

[sapphi-red]

The spec does not clarify whether #__PURE__ applies to tagged templates.

/* #__PURE__ */ foo``

Tool	result	playground
terser	❌
Rollup	❌	link
esbuild	❌	link
SWC	✅	link
Oxc	❌	link

✅: removes the expression
❌: does not remove the expression

Currently, the behaviors are aligned expect for SWC.

I think we should clarify that #__PURE__ does not apply for tagged templates.

[antfu]

I personally consider tagged templates a syntax sugar to function calls, which in that case, #__PURE__ should be able to mark it as pure and remove it. I would consider that the other bundlers are missing the handling for this case, and we should clear that and ask the bundlers to support that. WDYT? Maybe we should cc the bundler maintainers to put their opinions on this?

[sapphi-red]

Found some related issues:

• Support __PURE__ annotations on tagged template literals terser/terser#975
• Support for __PURE__ annotations on tagged template literals rollup/rollup#4035
• Tree shake template literals evanw/esbuild#3472
• implement annotations mishoo/UglifyJS#4763 (comment)

[sapphi-red]

It seems rollup/rollup#4035 (comment) has some discussion about the problem with

/*#__PURE__*/ makeMacro`some code`("arg for some code")
/*#__PURE__*/ makeMacro("some thing")`Hello`

/*@__PURE__*/foo.bar(effect1())`baz ${effect2()}`, moo();