Incorrect Name Qualification In AST / Diagnostics For Alias CTAD

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Introduction

In the realm of compiler development, the Abstract Syntax Tree (AST) plays a crucial role in representing the source code in a structured and easily analyzable format. However, when it comes to diagnostics, the AST can sometimes lead to incorrect name qualification, resulting in misleading error messages. This article delves into the issue of incorrect name qualification in AST/diagnostics for alias CTAD (Class Template Argument Deduction) and explores potential solutions.

The Problem

Consider the following example:

template<typename U> struct A {
  U t;
};

template<typename V> A(V) -> A<V>;

namespace foo {
  template<class Y> using Alias = A<Y>;
}

foo::Alias t = 0;

When compiled, this code produces the following error message:

error: no viable conversion from 'int' to 'foo::A<int>' (aka 'A<int>')

However, the issue lies in the fact that the foo:: qualification is applied to A in the AST, where it cannot be found. This is because the foo:: qualification is applied to Alias in the source code, which is in the foo namespace.

The Issues

There are several issues at play here:

  1. Elaborated Type: The elaborated type foo::Alias is not correctly represented in the AST. Instead, it is represented as A<int>, which is the deduced template specialization (DTST) of A.
  2. Printing DTST: When printing a DTST that has already been deduced, we choose to print it as the deduced type instead of the original template specialization. This is done for diagnostic reasons, but it is incorrect for -ast-print and cannot be controlled with a policy.
  3. Alias CTAD: For alias CTAD, we do not build a guide that contains the alias template specialization Alias<int>. Instead, the guide contains A<int>.

Potential Solutions

There are two potential solutions to this issue:

  • Fix 3: If we fix the issue with alias CTAD, the guide would contain Alias<int>, and the error message would be printed as intended: 'foo::Alias<int>' (aka 'A<int>').
  • Stop doing 2: If we stop printing DTST as the deduced type, the error message would be printed as 'foo::Alias' (aka 'A<int>'). This solution does not include much information, but it is correctly qualified, and the 'aka' gives the missing information.

Conclusion

In conclusion, the issue of incorrect name qualification in AST/diagnostics for alias CTAD is a complex problem that requires careful consideration of the representation of elaborated types, printing of DTST, and alias CTAD. By understanding the root causes of the issue and exploring potential solutions, we can improve the accuracy and clarity of error messages in compilers.

Recommendations

Based on the analysis, we recommend the following:

  • Fix 3: Implement a fix for alias CTAD to include the alias template specialization in the guide.
  • Stop doing 2: Consider stopping the practice of printing DTST as the deduced, especially for -ast-print.
  • Improve diagnostics: Improve the diagnostics to provide more accurate and informative error messages.

Q: What is the issue with incorrect name qualification in AST/diagnostics for alias CTAD?

A: The issue lies in the fact that the foo:: qualification is applied to A in the AST, where it cannot be found. This is because the foo:: qualification is applied to Alias in the source code, which is in the foo namespace.

Q: What are the root causes of the issue?

A: There are several root causes of the issue:

  1. Elaborated Type: The elaborated type foo::Alias is not correctly represented in the AST. Instead, it is represented as A<int>, which is the deduced template specialization (DTST) of A.
  2. Printing DTST: When printing a DTST that has already been deduced, we choose to print it as the deduced type instead of the original template specialization. This is done for diagnostic reasons, but it is incorrect for -ast-print and cannot be controlled with a policy.
  3. Alias CTAD: For alias CTAD, we do not build a guide that contains the alias template specialization Alias<int>. Instead, the guide contains A<int>.

Q: What are the potential solutions to the issue?

A: There are two potential solutions to the issue:

  • Fix 3: If we fix the issue with alias CTAD, the guide would contain Alias<int>, and the error message would be printed as intended: 'foo::Alias<int>' (aka 'A<int>').
  • Stop doing 2: If we stop printing DTST as the deduced type, the error message would be printed as 'foo::Alias' (aka 'A<int>'). This solution does not include much information, but it is correctly qualified, and the 'aka' gives the missing information.

Q: What are the recommendations for improving diagnostics?

A: Based on the analysis, we recommend the following:

  • Fix 3: Implement a fix for alias CTAD to include the alias template specialization in the guide.
  • Stop doing 2: Consider stopping the practice of printing DTST as the deduced, especially for -ast-print.
  • Improve diagnostics: Improve the diagnostics to provide more accurate and informative error messages.

Q: How can I reproduce the issue?

A: You can reproduce the issue by using the following code:

template<typename U> struct A {
  U t;
};

template<typename V> A(V) -> A<V>;

namespace foo {
  template<class Y> using Alias = A<Y>;
}

foo::Alias t = 0;

Q: What are the implications of the issue?

A: The issue can lead to misleading error messages, making it difficult for developers to diagnose and fix issues in their code. By improving diagnostics, we can make it easier for developers to write correct code and reduce the time spent on debugging.

Q: How can I contribute to improving diagnostics?

A: You can contribute to improving diagnostics by:

  • Reporting issues and providing feedback on the diagnostics. *ating in discussions and providing suggestions for improving diagnostics.
  • Contributing code to improve the diagnostics.

By working together, we can improve the accuracy and clarity of error messages in compilers, making it easier for developers to write correct code and reduce the time spent on debugging.