[EvgenyGuguev]

What & why

Fixes #27700.

In compile-time templates ($tmpl / $veb.html()), the documented @@ escape
for a literal @ rendered incorrectly whenever it preceded a complex
at-expression (ident(...), ident[...], ident.field):

• @@get('/x') rendered as @{get('/x')} instead of the literal @get('/x').

This is a real-world pain point for Datastar hypermedia apps written with
veb, where HTML attributes carry action verbs
(data-on:click="@get('/increment')") that must be escaped as @@get(...)` in
a V template.

Root cause

In rewrite_complex_template_at_expressions (vlib/v/parser/tmpl.v), the
@@ branch wrote a single @ and advanced the index by only 1, leaving the
second @ to be re-parsed as the start of a complex at-expression (wrapped
into @{...}). The bare-ident case (@@world) already worked because
find_tmpl_complex_at_expr_end returns -1 for a bare identifier, so the
existing escape test never exercised the complex-suffix case.

The fix

Write both @ characters and advance the index by 2, deferring the
@@ → @ collapse to insert_template_code (which already handles it).

Tests

• Added a focused regression test:
  test_escape_double_at_before_complex_at_expression covering
  @@get('/x') and @@post('/save').
• The existing @@world / @{expr} / @url behaviors are unchanged.
• v test vlib/v/tests/ → 2175 passed, 6 skipped, 0 failures (full template
  suite green, including the new regression). GitHub CI runs the complete
  suite on this PR.

[medvednikov]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Can't wait for the next one!

Reviewed commit: cbb060ff35

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[medvednikov]

Thanks!