[florianvoss-commit]

Summary

Add support for implicit concatenation of adjacent string literals in Minja expressions.

This allows valid Jinja expressions such as:

{{ raise_exception(
    "first part "
    "second part "
    "third part"
) }}

The implementation extends Minja's existing constant parser to consume adjacent quoted literals separated by whitespace and combine them into a single string value.

Fixes #27

Motivation

Google's canonical Gemma 4 chat template uses adjacent string literals inside a raise_exception() call:

https://huggingface.co/google/gemma-4-E2B-it/blob/main/chat_template.jinja#L257-L260

The template was published by the Google Gemma Engineering Team and is used by the public google/gemma-4-E2B-it model.

Minja currently parses the first literal and then expects a comma or closing parenthesis. It therefore fails on the second literal with:

Expected closing parenthesis in call args

This prevents runtimes using Minja from loading the canonical Gemma 4 template.

Implementation

• Continue consuming string literals after parsing the first literal when they are separated only by whitespace.
• Combine their contents into the existing Minja Value.
• Keep the behavior within the native Minja parser; no template preprocessing or model-specific workaround is introduced.
• Support mixed single and double quotes as well as multiline adjacency.

Tests

Added native C++ syntax coverage for:

• General adjacent literal concatenation with mixed quote styles.
• The exact multiline raise_exception() structure used by Gemma 4.
• Verification that the exception receives the fully concatenated message, proving parsing completed successfully.

Validation performed:

• Native test-syntax suite passes.
• Sources compile cleanly with -Wall -Wextra -pedantic -Werror.
• Sources also compile successfully with a AArch64 cross-compiler.

Scope

This adds syntax that is actively used by a publicly available LLM chat
template, consistent with Minja's restricted contribution scope.