Bug 16567 - MCS non-uniqueness condition for type inference is too aggressively causing inference failure
Summary: MCS non-uniqueness condition for type inference is too aggressively causing i...
Status: RESOLVED FIXED
Alias: None
Product: Compilers
Classification: Mono
Component: C# ()
Version: unspecified
Hardware: PC Windows
: --- normal
Target Milestone: ---
Assignee: Marek Safar
URL:
Depends on:
Blocks:
 
Reported: 2013-12-03 11:54 UTC by Brett van Swelm
Modified: 2013-12-06 10:40 UTC (History)
1 user (show)

Tags:
Is this bug a regression?: ---
Last known good build:

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Status:
RESOLVED FIXED

Description Brett van Swelm 2013-12-03 11:54:41 UTC
With "D:\Applications\Mono-3.0.9\bin" and
"C:\Windows\Microsoft.NET\Framework\v4.0.30319" in the path on my Windows 7
machine:

===== Test.cs =====
class Alpha { }
class Bravo { }
interface ICharlie<in T> { }
class Delta :
    ICharlie<Alpha>,
    ICharlie<Bravo>
{
    static void Echo<U>(ICharlie<U> icu, U u)
    { }

    static void Main()
    {
        Echo(new Delta(), new Alpha());
    }
}
===== end =====

$ csc Test.cs
Microsoft (R) Visual C# Compiler version 4.0.30319.18408
for Microsoft (R) .NET Framework 4.5
Copyright (C) Microsoft Corporation. All rights reserved.

$ mcs Test.cs
Test.cs(13,9): error CS0411: The type arguments for method `Delta.Echo<U>(ICharlie<U>, U)' cannot be inferred from the usage. Try specifying the type arguments explicitly
Compilation failed: 1 error(s), 0 warnings

We suspect the "non-uniqueness causing failure" logic commented at https://github.com/mono/mono/blob/master/mcs/mcs/generic.cs#L3454 is the cause. The non-uniqueness should result in no inference being made, not immediate inference failure.

Thanks,

Brett van Swelm | Senior Engineer
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Comment 1 Marek Safar 2013-12-06 10:40:00 UTC
Fixed in master