dotnet/roslyn

CS0311 (cannot be used as type parameter) at wrong location

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#11,607 opened on May 27, 2016

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 (2 comments) (0 reactions) (0 assignees)C# (20,414 stars) (4,257 forks)batch import
Area-CompilersBugConcept-Diagnostic Clarityhelp wanted

Description

    class B<T1, T2>
        where T1 : B<T1, T2>.B1
        where T2 : B<T1, T2>.B2
    {
        public class B1 { }
        public class B2 { }
    }

    class D : B<D.D1, D.D2>
    {
        public class D1 { } // error expected here
        public class D2 : B2 { } // error shown here
    }

Actual CS0311 The type 'D.D1' cannot be used as type parameter 'T1' in the generic type or method 'B<T1, T2>'. There is no implicit reference conversion from 'D.D1' to 'B<D.D1, D.D2>.B1'. underlined D2

Expected: The same error but at D1 line.

Version: 2.0.0.60428

Contributor guide

Tech stack
csharp
Domain
tooling
Issue type
bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
4
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
half day
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
stale
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
C# generics and constraint checkingRoslyn compiler basics
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
30
Research direction
The bug is in error location for CS0311 in the Roslyn compiler. Investigate the constraint checking pass in src/Compilers/CSharp/Portable/Binder/ or related files. Search for 'CS0311' in the source to find where the error is emitted and modify the location to point to the correct type parameter.