llvm/llvm-project

Improve -Wparentheses for arbitrary bool contexts

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#33,528 opened on Aug 14, 2017

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Description
Bugzilla Link 34180
Version unspecified
OS Linux
CC @m4tx,@zygoloid

Extended Description

https://godbolt.org/g/Ca2Xtv

bool f(int x) { return x = 0; }

GCC gives:

:2:16: warning: suggest parentheses around assignment used as truth value [-Wparentheses] return x = 0; ^

This would have caught a bug in r310794 (https://reviews.llvm.org/rL310794)

I don't know for sure, but from the reduced test case, it seems that GCC is just seeing when an assignment is used in a bool-typed context? E.g. just bool(x = 0) seems to trigger it. https://godbolt.org/g/HyBZHM Looking at the two examples, it looks like ImplicitCastExpr to bool is probably the thing to look for. ImplicitCastExpr 0x4fce318 <col:12, col:16> '_Bool' ImplicitCastExpr 0x4fce578 <col:17, col:21> '_Bool'

Contributor guide

Tech stack
cpp
Domain
tooling
Issue type
feature
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
4
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
over 1 week
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
stale
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
Clang ASTC++compiler warning infrastructureimplicit cast expressions
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
20
Research direction
Investigate how Clang currently generates warnings for assignments used as truth values. The issue suggests that checking for ImplicitCastExpr to bool could trigger the Wparentheses warning. Look at the existing implementation in SemaChecking.cpp or similar files in the Clang source. Examine the provided test case and the linked godbolt examples. The goal is to extend the warning to cover cases where an assignment is used in a boolean context, such as a return statement with bool return type.