Current behavior:
---
struct S0 { @disable this(); }
struct S  { int i; S0 s0; }

static assert(!__traits(compiles, { S0 s; }));
static assert(!__traits(compiles, { S0 s = S0(); }));
static assert( __traits(compiles, { S0 s = S0.init; }));
static assert(!__traits(compiles, {
    struct S1 { S0 s0; this(int) { } }
}));

static assert(!__traits(compiles, { S s; }));
static assert( __traits(compiles, { S s = S(5, S0.init); }));
static assert(!__traits(compiles, { S s = S(); }));     // assert failure
static assert(!__traits(compiles, { S s = S(5); }));    // assert failure
static assert(!__traits(compiles, { S x = { }; }));     // assert failure
static assert(!__traits(compiles, { S x = { i:1 }; })); // assert failure
---


It looks inconsistent because this is disabled:
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struct S1 {
    int i;
    S0 s0;

    // Error: constructor main.S1.this field s0 must be initialized in constructor
    this(int i) { this.i = i; }
}
---