IntelliSense loses the type of Action<T> parameter only while typing

[datvm]

Version Used:

Steps to Reproduce:

1. Create a new C# Console App (top-level statement) and put in this code:

AddConfig(new()
{
    A = a =>
    {

    },
});

static void AddConfig(Config config) { }

class Config
{
    public Action<A>? A { get; set; }
}

class A
{
    public string Foo1 { get; set; } = "";
}

2. Try to use a, like typing a.Foo1 = "something". IntelliSense cannot shows any member of a and the type of a is ?

3. Without accessing to a, it correctly detects a as A. The moment a is access/typed however, it becomes ? again.

4. Manually typing a complete statement would result in correct type assertment again, however, pressing Enter and type another statement with a and the problem happens again.

The behavior is similar to #68027 so I wonder if it's the same issue. This only affects IntelliSense, the code can be compiled successfully if it's typed manully.

I have also just realized it only happen to the automatic new() statement. Explicitly type new Config() on the first line would fix this issue.

Diagnostic Id: None

Expected Behavior:

a's type should be kept consistent while typing.

Actual Behavior:

a loses its type while typing.

[sharwell]

/cc @genlu @jcouv

[jcouv]

The root cause is likely a compiler issue (ability to recover in the presence of an error in a lambda). I'm not sure whether that root cause can be addressed or whether it will need to be worked around from the IDE side.

    [Fact]
    public void TODO2()
    {
        var src = """
#nullable enable
using System;

AddConfig(new()
    {
        A = a =>
        {
            a
        },
    });

static void AddConfig(Config config) { }

class Config
{
    public Action<A>? A { get; set; }
}

class A
{
    public string Goo1 { get; set; } = "";
}
""";

        var comp = CreateCompilation(src, targetFramework: TargetFramework.Net70);
        comp.VerifyDiagnostics(
            // (8,14): error CS1002: ; expected
            //             a
            Diagnostic(ErrorCode.ERR_SemicolonExpected, "").WithLocation(8, 14));

        var tree = comp.SyntaxTrees.Single();
        var model = comp.GetSemanticModel(tree);
        var a = GetSyntax<IdentifierNameSyntax>(tree, "a");
        Assert.Equal("? a", model.GetSymbolInfo(a).Symbol.ToTestDisplayString()); // Should be "A a" parameter
    }

[CyrusNajmabadi]

This def feels like an odd case for the compiler to not be able to get good semantics with (esp. as there are no overloads goign on).

Given:

AddConfig(new()
    {
        A = a =>
        {
            a
        },
    });

I would expect new() ... to be target typed with Config. Which means 'A' will be Action<A>. So a => will be target typed with Action<A>, So a will be a parameter with type A. So a. should properly give Goo. :)

Given the simplicity of the scenario, i would really want compiler to give reasonable results here :)

Thanks!