llvm/llvm-project

[clang] Poor diagnostic with wrong template argument type

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#121,235 opened on Dec 27, 2024

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clang:diagnosticsconfirmedenhancementgood first issue

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Description

Clang diagnostic for invalid bool-to-enum template parameter conversion lacks failure detail when compared to GCC, as the following C++ snippet:

enum endianness { little = 0, big };

template <endianness e>
inline void foo() {}

void bar() {
    foo<false>();
}

Fails as follows on latest clang:

<source>:7:5: error: no matching function for call to 'foo'
    7 |     foo<false>();
      |     ^~~~~~~~~~
<source>:4:13: note: candidate template ignored: invalid explicitly-specified argument for template parameter 'big_endian'
    4 | inline void foo() {}

GCC explicitly indicates the conversion failure instead:

<source>:4:13: note: template argument deduction/substitution failed:
<source>:7:15: error: could not convert 'false' from 'bool' to 'endianness'
    7 |     foo<false>();
      |     ~~~~~~~~~~^~
      |               |
      |               bool

Godbolt: https://godbolt.org/z/hnazTn3M7 (cc/ @shafik).

Contributor guide

Research direction
Locate the diagnostic emission in Clang's Sema for template argument deduction failure. Modify it to include the specific conversion failure, similar to GCC's error. Focus on the code path when an implicitly convertible template argument fails due to type mismatch.
Tech stack
cpp
Domain
backend
Issue type
Bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
3
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Half day
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Active
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Clear
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C++Git
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70