Forward reference error in recursive template

[dlangBugzillaToGithub]

Jack Applegame (@japplegame) reported this on 2017-03-19T09:56:25Z

Transferred from https://issues.dlang.org/show_bug.cgi?id=17267

CC List

• Radu Racariu (@rracariu)
• Simen Kjaeraas

Description

This code compiles:
struct A(T) {
    T* ptr;
    void m() {
        pragma(msg, T.sizeof);
    }
}

struct B {
    int i;
    A!B a;
}

Following should compile, but doesn't (struct foo.B no size because of forward reference):

// example 1 (struct definition inside function)
struct A(T) {
    T* ptr;
    void m() {
        pragma(msg, T.sizeof);
    }
}

void foo() {
    static struct B {
        int i;
        A!B a;
    }
}

// example 2 (sizeof outside member function)
struct A(T) {
    T* ptr;
    pragma(msg, T.sizeof);
}

struct B {
    int i;
    A!B a;
}

[dlangBugzillaToGithub]

slavo5150 commented on 2017-11-25T06:50:30Z

*** Issue 17230 has been marked as a duplicate of this issue. ***

[dlangBugzillaToGithub]

simen.kjaras commented on 2017-12-27T10:00:52Z

Another, possibly simplified example:

// Mixed templates
struct S1 {
    S2!()* arr;
}
struct S2()
{
    // struct S1 no size because of forward reference
    private byte[S1.sizeof] payload;
}

It seems to be caused by some ordering issues in DMD, where it tries to calculate the size of S1, notices that it depends on a templated type, and just up and dies.

Note also that these variants compile:

// No templates
struct S3 {
    S4* arr;
}
struct S4 {
    private byte[S3.sizeof] payload;
}

// All templates
struct S5() {
    S6!()* arr;
}
struct S6() {
    private byte[S5!().sizeof] payload;
}
S6!() dummy; // Force instantiation

// Mixed templates, revisited
struct S7() {
    S8* arr;
}
struct S8
{
    private byte[S7!().sizeof] payload;
}