llvm/llvm-project

Clang elides the wrong part of the template type in error message.

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#180046 opened on Feb 5, 2026

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clang:frontendgood first issue

Description

Repro: https://godbolt.org/z/9bYzd3WPG

Error message:error: no viable conversion from returned value of type 'S3<S2<...>>' to function return type 'S3<S2<...>>'

Expected: error: no viable conversion from returned value of type 'S3<const S2<int>>' to function return type 'S3<S2<int>>'

Contributor guide

Tech stack
cpp
Domain
toolingdeveloper experience
Issue type
bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
3
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
1-2 days
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
active
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
C++ templatesClang compiler internals
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
45
Research direction
Investigate the code in Clang's Sema that handles template type printing during error message generation. Look at the function that prints types with template arguments, likely in SemaOverload.cpp or AST/TypePrinter. The issue shows that const qualifiers are being elided in some parts of the type but not others. Check how the type comparison and printing logic decides where to insert '...' and ensure const is included in the printed type.