Fix Issue 19105 - Bogus recursive template expansion via getSymbolsByUDA

[RazvanN7]

Issue 19105 was reported as a compiler bug. The reported code is:

module junk;

import std.traits;

struct A(Args...) {}
struct B {}
alias C = A!( getSymbolsByUDA!(junk, B) );

But reading the docs of getSymbolsByUDAis seems that it was meant to work only with aggregate declarations (not with modules). I added a template constraint that requires that the symbol passed is an aggregated type.

The code fails because while getting the members of junk, the innards of getSymbolsByUDA uses aliases for the list of symbols and at some point alias C ends up aliasing to something that aliases itself.

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	19105	normal	Bogus recursive template expansion via getSymbolsByUDA

Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub fetch digger
dub run digger -- build "master + phobos#7100"

[thewilsonator]

This should have that as a test then.

[RazvanN7]

@thewilsonator ok. done.

[Biotronic]

This is the wrong fix for 19105. 19105 is in fact a DMD issue, as can be showed by this code:

import std.traits;

struct A(Args...) {}

struct S1 {
    // Recursive template expansion
    alias C = A!( getSymbolsByUDA!(S1, A) );
}
struct S2 {
    // precompute the result:
    alias tmp = getSymbolsByUDA!(S2, A);
    // No recursive template expansion
    alias C = A!( getSymbolsByUDA!(S2, A) );
}

The problem seems to be with caching in the compiler, and the only thing this PR has changed is it's made valid use cases fail to compile.

Here's a reduced test case for the behavior, which doesn't use getSymbolsByUDA:

template test(alias T) {
    enum i = __traits(compiles, __traits(getMember, T, "C"));
    alias test = int;
}

struct A(Args...) {}

struct S1 {
    // Recursive template expansion
    alias C = A!(test!S1);
}
struct S2 {
    // No error
    alias a = test!S2;
    alias C = A!(test!S2);
}

[PetarKirov]

CC @RazvanN7

[zorael]

Ping. Still in 2.088 beta