From 8097a1b472b7ae39f4c0011bad6952b68cd76362 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Juli=C3=A1n=20Pinz=C3=B3n=20Eslava?= <julian@buildkite.com>
Date: Thu, 19 Feb 2026 12:42:18 +1100
Subject: [PATCH] Fix ERB expression compilation when code contains a heredoc

When an ERB output tag contains a heredoc (e.g. `<%= method_call <<~GRAPHQL, variables %>`),
the closing parenthesis in the compiled Ruby must appear on its own line after the heredoc
terminator. Without this, the compiled Ruby is syntactically invalid.

Amp-Thread-ID: https://ampcode.com/threads/T-019c738c-52b4-772e-ab48-9bfb6b31e8f0
Co-authored-by: Amp <amp@ampcode.com>
---
 lib/herb/engine.rb                                  | 12 ++++++++++--
 sig/herb/engine.rbs                                 |  2 ++
 test/engine/engine_test.rb                          | 13 +++++++++++++
 ..._valid_Ruby_46fb4e6ec5bd55cc1041f3f88ad9508c.txt | 12 ++++++++++++
 4 files changed, 37 insertions(+), 2 deletions(-)
 create mode 100644 test/snapshots/engine/engine_test/test_0018_heredoc_with_trailing_arguments_compiles_to_valid_Ruby_46fb4e6ec5bd55cc1041f3f88ad9508c.txt

diff --git a/lib/herb/engine.rb b/lib/herb/engine.rb
index 1c75fdad6..cbc2b5c55 100644
--- a/lib/herb/engine.rb
+++ b/lib/herb/engine.rb
@@ -213,13 +213,17 @@ def add_expression(indicator, code)
 
     def add_expression_result(code)
       with_buffer {
-        @src << " << (" << code << comment_aware_newline(code) << ").to_s"
+        @src << " << (" << code
+        @src << "\n" if heredoc?(code)
+        @src << comment_aware_newline(code) << ").to_s"
       }
     end
 
     def add_expression_result_escaped(code)
       with_buffer {
-        @src << " << " << @escapefunc << "((" << code << comment_aware_newline(code) << "))"
+        @src << " << " << @escapefunc << "((" << code
+        @src << "\n" if heredoc?(code)
+        @src << comment_aware_newline(code) << "))"
       }
     end
 
@@ -247,6 +251,10 @@ def comment_aware_newline(code)
       code.include?("#") ? "\n" : ""
     end
 
+    def heredoc?(code)
+      code.match?(/<<[~-]?\s*['"`]?\w/)
+    end
+
     def add_postamble(postamble)
       terminate_expression
       @src << postamble
diff --git a/sig/herb/engine.rbs b/sig/herb/engine.rbs
index a904f6ed7..c7eff1298 100644
--- a/sig/herb/engine.rbs
+++ b/sig/herb/engine.rbs
@@ -53,6 +53,8 @@ module Herb
 
     def comment_aware_newline: (untyped code) -> untyped
 
+    def heredoc?: (untyped code) -> untyped
+
     def add_postamble: (untyped postamble) -> untyped
 
     def with_buffer: () ?{ (?) -> untyped } -> untyped
diff --git a/test/engine/engine_test.rb b/test/engine/engine_test.rb
index 97d0b7376..48af2239d 100644
--- a/test/engine/engine_test.rb
+++ b/test/engine/engine_test.rb
@@ -164,5 +164,18 @@ class EngineTest < Minitest::Spec
 
       assert_compiled_snapshot(template, escape: false)
     end
+
+    test "heredoc with trailing arguments compiles to valid Ruby" do
+      template = <<~ERB
+        <%= method_call <<~GRAPHQL, variables
+          query {
+            field
+          }
+        GRAPHQL
+        %>
+      ERB
+
+      assert_compiled_snapshot(template)
+    end
   end
 end
diff --git a/test/snapshots/engine/engine_test/test_0018_heredoc_with_trailing_arguments_compiles_to_valid_Ruby_46fb4e6ec5bd55cc1041f3f88ad9508c.txt b/test/snapshots/engine/engine_test/test_0018_heredoc_with_trailing_arguments_compiles_to_valid_Ruby_46fb4e6ec5bd55cc1041f3f88ad9508c.txt
new file mode 100644
index 000000000..cc0970090
--- /dev/null
+++ b/test/snapshots/engine/engine_test/test_0018_heredoc_with_trailing_arguments_compiles_to_valid_Ruby_46fb4e6ec5bd55cc1041f3f88ad9508c.txt
@@ -0,0 +1,12 @@
+---
+source: "Engine::EngineTest#test_0018_heredoc with trailing arguments compiles to valid Ruby"
+input: "{source: \"<%= method_call <<~GRAPHQL, variables\\n  query {\\n    field\\n  }\\nGRAPHQL\\n%>\\n\", options: {}}"
+---
+_buf = ::String.new; _buf << (method_call <<~GRAPHQL, variables
+  query {
+    field
+  }
+GRAPHQL
+).to_s; _buf << '
+'.freeze;
+_buf.to_s