APICompat is showing noisy diffs where it shouldn't (regarding Parameter Modifiers)

[ahsonkhan]

These two netfx dlls are showing up in ApiCompat runs even when they are identical:

• Microsoft.VisualC.STLCLR.dll
• System.Printing.dll

The diffs themselves are all around ParameterModifiersCannotChange and seem non-actionable anyway:

netfx
Compat issues with assembly Microsoft.VisualC.STLCLR:
ParameterModifiersCannotChange : Custom modifiers on parameter '_Right' on method 'Microsoft.VisualC.StlClr.GenericPair<TValue1, TValue2>..ctor(Microsoft.VisualC.StlClr.GenericPair<TValue1, TValue2>)' are 'System.Runtime.CompilerServices.IsImplicitlyDereferenced' in the implementation but 'System.Runtime.CompilerServices.IsImplicitlyDereferenced' in the contract.
ParameterModifiersCannotChange : Custom modifiers on parameter '_Right' on method 'Microsoft.VisualC.StlClr.GenericPair<TValue1, TValue2>.op_Assign(Microsoft.VisualC.StlClr.GenericPair<TValue1, TValue2>)' are 'System.Runtime.CompilerServices.IsImplicitlyDereferenced' in the implementation but 'System.Runtime.CompilerServices.IsImplicitlyDereferenced' in the contract.
ParameterModifiersCannotChange : Custom modifiers on parameter '_Right' on method 'Microsoft.VisualC.StlClr.GenericPair<TValue1, TValue2>.swap(Microsoft.VisualC.StlClr.GenericPair<TValue1, TValue2>)' are 'System.Runtime.CompilerServices.IsImplicitlyDereferenced' in the implementation but 'System.Runtime.CompilerServices.IsImplicitlyDereferenced' in the contract.
ParameterModifiersCannotChange : Custom modifiers on parameter '_Where' on method 'Microsoft.VisualC.StlClr.IDeque<TValue>.erase(Microsoft.VisualC.StlClr.Generic.ContainerRandomAccessIterator<TValue>, 
...
Compat issues with assembly System.Printing:
ParameterModifiersCannotChange : Custom modifiers on parameter 'attribRef' on method 'System.Printing.IndexedProperties.PrintBooleanProperty.op_Implicit(System.Printing.IndexedProperties.PrintBooleanProperty)' are 'System.Runtime.CompilerServices.IsImplicitlyDereferenced' in the implementation but 'System.Runtime.CompilerServices.IsImplicitlyDereferenced' in the contract.
ParameterModifiersCannotChange : Custom modifiers on parameter 'attribRef' on method 'System.Printing.IndexedProperties.PrintByteArrayProperty.op_Implicit(System.Printing.IndexedProperties.PrintByteArrayProperty)' are 'System.Runtime.CompilerServices.IsImplicitlyDereferenced' in the implementation but 'System.Runtime.CompilerServices.IsImplicitlyDereferenced' in the contract.
ParameterModifiersCannotChange : Custom modifiers on parameter 'attribRef' on method 'System.Printing.IndexedProperties.PrintDateTimeProperty.op_Implicit(System.Printing.IndexedProperties.PrintDateTimeProperty)' are 'System.Runtime.CompilerServices.IsImplicitlyDereferenced' in the implementation but 'System.Runtime.CompilerServices.IsImplicitlyDereferenced' in the contract.
ParameterModifiersCannotChange : Custom modifiers on parameter 'attribRef' on method 'System.Printing.IndexedProperties.PrintDriverProperty.op_Implicit(System.Printing.IndexedProperties.PrintDriverProperty)' are 'System.Runtime.CompilerServices.IsImplicitlyDereferenced' in the implementation but 'System.Runtime.CompilerServices.IsImplicitlyDereferenced' in the contract.
...
Total Issues: 151

Comparing all netfx refs corefx\bin\ref\netfx, net45+, ...

See: dotnet/corefx#35557

fc "..\master\bin\ref\netfx\Microsoft.VisualC.STLCLR.dll" "..\branch\bin\ref\netfx\Microsoft.VisualC.STLCLR.dll"
Comparing files ..\MASTER\BIN\REF\NETFX\Microsoft.VisualC.STLCLR.dll and ..\BRANCH\BIN\REF\NETFX\MICROSOFT.VISUALC.STLCLR.DLL
FC: no differences encountered

To reproduce:

• Build corefx (all configuration, or netfx config)
• Run APICompat on the bin\ref\netfx folder against itself.
  dotnet "Microsoft.DotNet.ApiCompat.dll" "bin\ref\netfx" -i "bin\ref\netfx"

cc @ericstj

[ericstj]

Simple repro:

dotnet c:\Users\ericstj\.nuget\packages\microsoft.dotnet.apicompat\1.0.0-beta.19121.4\tools\netcoreapp2.1\Microsoft.DotNet.ApiCompat.dll "C:\Program Files (x86)\Reference Assemblies\Microsoft\Framework\.NETFramework\v4.7.2\System.Printing.dll" -i "C:\Program Files (x86)\Reference Assemblies\Microsoft\Framework\.NETFramework\v4.7.2"

Suspect a bad comparator here:

arcade/src/Microsoft.DotNet.ApiCompat/Rules/Compat/ParameterModifiersCannotChange.cs

Lines 66 to 72 in 767154a

	var union = implParam.CustomModifiers.Union(contractParam.CustomModifiers);
	if (implParam.CustomModifiers.Count() != union.Count())
	{
	differences.AddIncompatibleDifference(this,
	$"Custom modifiers on parameter '{implParam.Name.Value}' on method '{implMethod.FullName()}' are '{PrintCustomModifiers(implParam.CustomModifiers)}' in the {Implementation} but '{PrintCustomModifiers(contractParam.CustomModifiers)}' in the {Contract}.");
	match = false;
	}

Perhaps that's not doing an actual equality check when evaluating the union.

[ericstj]

This appears to have been fixed by @dougbu in https://github.com/dotnet/arcade/pull/4585/files#diff-ab264297f1e0d7296c8d69384a224e589d95ced59d203663d968dcc20f7d1e13R68

Note that the error may still appear if ApiCompat is not able to see that the types on both sides are equivalent, so in my repro above it will still fail unless I pass --contract-depends

dotnet c:\Users\erics\.nuget\packages\microsoft.dotnet.apicompat\6.0.0-beta.21281.1\tools\netcoreapp3.1\Microsoft.DotNet.ApiCompat.dll "C:\Program Files (x86)\Reference Assemblies\Microsoft\Framework\.NETFramework\v4.7.2\System.Printing.dll" --contract-depends "C:\Program Files (x86)\Reference Assemblies\Microsoft\Framework\.NETFramework\v4.7.2" -i "C:\Program Files (x86)\Reference Assemblies\Microsoft\Framework\.NETFramework\v4.7.2"
Total Issues: 0