@jaredpar Roslyn update introduced a build break in a very old dynamic test:

src/libraries/System.Runtime/tests/System.Dynamic.Runtime.Tests/Dynamic.Context/Conformance.dynamic.context.operator.regclass.cs(4383,29): error CS0039: (NETCORE_ENGINEERING_TELEMETRY=Build) Cannot convert type 'void' to 'object' via a reference conversion, boxing conversion, unboxing conversion, wrapping conversion, or null type conversion

I think @chsienki was looking into it in #98559

Roslyn update introduced a build break in a very old dynamic test

class C
{
    static object M1()
    {
        var a = "";
        dynamic da = a;
        return M2(da) as object;
    }

    static void M2(string s) { }
}

This scenario behaves differently after Params Collections - Adjust binding in presence of dynamic arguments (71421). There's a new error:

// (7,16): error CS0039: Cannot convert type 'void' to 'object' via a reference conversion, boxing conversion, unboxing conversion, wrapping conversion, or null type conversion
//         return M2(da) as object;
Diagnostic(ErrorCode.ERR_NoExplicitBuiltinConv, "M2(da) as object").WithArguments("void", "object").WithLocation(7, 16)

@AlekseyTs is that expected?

is that expected?

I would say yes. There is only one applicable candidate. Therefore, compile time resolution is performed. Therefore, result type of the invocation is not dynamic.

@AlekseyTs can the old behavior be achieved by adding a second overload here to force the more dynamic interpretation? Example:

static void M2(int i) { }

can the old behavior be achieved by adding a second overload here to force the more dynamic interpretation?

Yes, https://sharplab.io/#v2:EYLgZgpghgLgrgJwgZwLQAUoKgW2QYQHsAbYiAYxgEtCA7ZAGhhCuIB8ABAJgAJ8BYAFABvIT3E8OARgBsPQsABWFGDwCyUgBQBKMRNGCJRngDcsPKDwC8PAES2A3HuM8AJgE9auKuTeWbUE6GLpIA7Opcmq5Q2hbI8koqQUYAvkLOkrKSACwRmtIADDzIscI8acHi0nIcuWqRVLSqJTxlFSlAA=

I assume this no longer compiling is also considered a bug fix?
https://sharplab.io/#v2:EYLgtghglgdgPgAQMwAIECYUGEUG8CwAUCiWqggCwoCyAFNQJ5YA2EAzmygMYCUeRpQSgAmDGBDBQuIlAF4UFANwChJLgG0kAGhEBdOQvTLipAL5FzhIsjSZGLdmyIETJG6PGTpAFwAWUNnVYbxQADx0PCSk9OQA+EWNTIA=

public class C {
    public void M(MyClass c) {
        dynamic d = 4;
        c[3, d] = 42;
    }
}

public class MyClass
{
    public dynamic this[int x, dynamic d] => d;
}

It previously compiled, but now it fails to compile with:

Property or indexer 'MyClass.this[int, dynamic]' cannot be assigned to -- it is read only

(I pushed a commit to workaround the dynamic changes to the other PR.)

I assume this no longer compiling is also considered a bug fix?

A language design change.

Closing in favor of #98559