llvm/llvm-project

Decomposition Declaration diagnostic for missing initializer is pretty unclear

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#90,107 opened on Apr 25, 2024

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clang:diagnosticsgood first issuequality-of-implementation

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Description

Given the following code:

#include <utility>

std::pair<int, double> MyFunc();

void foo() {
    auto [A, B] C = MyFunc();
}

Our diagnostic is just:

<source>:9:10: error: decomposition declaration '[A, B]' requires an initializer
    9 |     auto [A, B] C = MyFunc();
      |          ^
<source>:9:16: error: expected ';' at end of declaration
    9 |     auto [A, B] C = MyFunc();
      |                ^
      |                ;
2 errors generated.
Compiler returned: 1

https://godbolt.org/z/h9zKT5zGe

No compilers are particularly good at this diagnostic, but all the others at least highlight the C as the mistake, whereas Clang's error is pretty opaque/points at the opening bracket.

I'd prefer we move the 'arrow' to the position where the initializer is expected. Additionally, some improved diagnostic wording that highlights what went wrong better would be appreciated.

Contributor guide

Research direction
Inspect the diagnostic emission for decomposition declarations in Clang's parser (lib/Parse/ParseDecl.cpp). Identify where the caret location is set and improve the diagnostic to point at the unexpected identifier after the structured binding declaration. Also enhance the wording to clarify that an initializer is expected after the '[...]'.
Tech stack
cpp
Domain
tooling
Issue type
Bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
2
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1-2 days
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Active
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Clear
Prerequisites
C++Clang developmentGit
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
70