__traits(getMember, this, ...) in nested function does not capture enclosing this

[niy-ati]

Summary

When a nested function accesses a struct field only through __traits(getMember, this, "field"), the compiler does not register a nested reference to the enclosing member this. As a result, vthis is omitted from the closure frame, and the delegate segfaults at runtime.

A direct field access (this.field) in the same position works correctly because expressionSemantic on a DotVarExp with ThisExp calls checkNestedReference. But getMember with wantsym returns the member VarDeclaration without keeping a ThisExp use, so the nested capture is never recorded.

Reproducer

struct Struct
{
    int field = 42;

    auto nestedThunk()
    {
        int inner()
        {
            return __traits(getMember, this, "field");
        }
        return &inner;
    }
}

void main()
{
    auto s = Struct();
    auto dg = s.nestedThunk();
    assert(dg() == 42);  // segfault: vthis missing from closure frame
}

Replacing __traits(getMember, this, "field") with this.field makes it work.

Expected behaviour

The delegate returned by nestedThunk should capture this and return 42.

Where

compiler/src/dmd/traits.d, inside the getMember branch of semanticTraits: after expressionSemantic on the resolved member, when the lookup object is this, checkNestedReference should be called on te.var (the enclosing member this).

Also affects LDC (ldc-developers/ldc#5203) since it shares the same frontend code.

[kinke]

Works fine with DMD v2.112: https://godbolt.org/z/WTWMb7ExT

So seems invalid.

[FeepingCreature]

I wonder why it works on dmd and fails on ldc if it's a frontend issue.