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Assertion abort on `{{a[()}}` (unchecked expected after a failed subscript)

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Assessment

Difficulty
2/5
Estimated time
1-3 hours
Newbie friendliness
88/100
Issue type
Bug
Clarity
Clearly specified
Activity status
Active
Tech stack
cpp
Domain
compilers

Research direction

Start in src/expression_parser.cpp:333-336 and reproduce the abort with {{a[()]}}, also checking the other examples in the issue. Trace the ParseSubscript result before the ParseCall path; done means Template::Load returns a parse error for the unterminated subscript instead of aborting or reaching undefined behaviour.

Written by the indexing model from the issue text.

Description

The parser dereferences the result of ParseSubscript without checking whether it holds an error. When the subscript fails and the next token is (, the error value reaches ParseCall, and expected::operator* asserts.

Affected: master 79af49f. The same code is present in releases 1.1.0, 1.2.1 and 1.3.2.

Reproduction

Template:

{{a[()}}

Loading it aborts:

Assertion `has_value()' failed.
#0 ... nonstd::expected_lite::expected<...>::operator*() at third_party/nonstd/include/nonstd/expected.hpp:2254
#1 jinja2::ExpressionParser::ParseValueExpression(jinja2::LexScanner&) at src/expression_parser.cpp:336

{{endmacro[()}} and {{a[(]()}} abort the same way.

Expected: Template::Load returns a parse error for the unterminated subscript, as Jinja2 does.

Actual: the process aborts before a parse error is returned. With -DNDEBUG the assert compiles out and the same line reads the error storage, which is undefined behaviour.

Issue #252 reports the same class of unchecked expected access from a different parse path.

Root cause

src/expression_parser.cpp:333-336:

if (tok == '[' || tok == '.')
    valueRef = ParseSubscript(lexer, *valueRef);
if (lexer.EatIfEqual('('))
    valueRef = ParseCall(lexer, *valueRef);

ParseSubscript returns an expected that can hold a ParseError. The ParseCall line dereferences that result without checking it first.

Dominant language
C++
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