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misc-redundant-expression: expression with calls to a non-pure function is considered as always-true #35853

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pavelkryukov opened this issue Feb 24, 2018 · 8 comments
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misc-redundant-expression: expression with calls to a non-pure function is considered as always-true #35853

pavelkryukov opened this issue Feb 24, 2018 · 8 comments
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bugzilla Issues migrated from bugzilla clang-tidy

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@pavelkryukov
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pavelkryukov commented Feb 24, 2018 •
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Bugzilla Link 36505
Version unspecified
OS Windows NT
Attachments Example of always-true false positive with calls to a non-pure function
CC @EugeneZelenko,@Xazax-hun

Extended Description

Clang-Tidy produces a false positive warning about always-true expression (misc-redundant-expression check) if expression contains consequent calls to a non-pure function which have different return values:

$> clang-tidy -checks='*' ldfile.c -- -Wall
15 warnings generated.
./ldfile.c:11:32: warning: logical expression is always true [misc-redundant-expression]
    if ((token = yylex()) != 1 || (token = yylex()) != 2) {
                               ^
Suppressed 14 warnings (14 in non-user code).

$> clang ldfile.c -Wall -Wextra && ./a.out
false

The code is taken from GNU Binutils (/ld/ldfile.c) source file.
@pavelkryukov
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Issue is reproducable in Clang-Tidy 6.0.0

@pavelkryukov
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Still there in Clang-Tidy 7.0.0

@pavelkryukov
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Reproduced with Clang-Tidy 9.0.1

@pavelkryukov
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Same thing for Clang-Tidy 10.
Godbolt example: https://godbolt.org/z/w7PggB

@pavelkryukov
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Reproduced with Clang-Tidy 11.0.0: https://godbolt.org/z/6nWMMa

@llvmbot llvmbot transferred this issue from llvm/llvm-bugzilla-archive Dec 10, 2021
@pavelkryukov
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@EugeneZelenko @Xazax-hun I’ll try to get more attention, as wrong fix-it can be generated because of this.

@pavelkryukov pavelkryukov mentioned this issue Mar 26, 2022
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@FabianWolff
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This is unrelated to #54011 and even intended to some extent, I believe: Almost every function call might have side effects (and misc-redundant-expression does not look into the called function), but taking this into account would prevent many legitimate warnings (arising from typos etc.) from firing, so the false positives here are tolerated.

Instead, to fix your example, I'd focus on the assignments. I've opened D122535 to this end.

@pavelkryukov
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Thank you!

mem-frob pushed a commit to draperlaboratory/hope-llvm-project that referenced this issue Oct 7, 2022
@FabianWolff
[clang-tidy] Never consider assignments as equivalent in `misc-redund…
98ed063 
…ant-expression` check

Fixes llvm/llvm-project#35853.

Reviewed By: aaron.ballman

Differential Revision: https://reviews.llvm.org/D122535
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