druntime 2.087 regression? (was: Old codebase stops compiling at 2.087)

[James Blachly]

TL;DR druntime regression? 
https://gist.github.com/jblachly/78c5762bbfea65b09e7a1417ad763019

---

Our team has an older codebase that compiled fine up until frontend 
version 2.087 (builds with 2.086.1).  Now, compiler (DMD2 and LDC2) 
complains about implicit conversion of shared(AA) to const(shared(AA)).

The codebase is private (could possibly make it public soon) but the 
error involves an AA inside a shared struct.

```
/home/james/dmd2.087/dmd2/linux/bin64/../../src/druntime/import/object.d(3453,36): 
Error: cannot implicitly convert expression aa of type 
shared(GSeqAllele[string]) to const(shared(GSeqAllele)[string])

source/polytyper/gseq.d(231,33): Error: template instance 
`object.values!(shared(GSeqAllele[string]), shared(GSeqAllele), string)` 
error instantiating

/home/james/dmd2.087/dmd2/linux/bin64/../../src/druntime/import/object.d(3402,36): 
Error: cannot implicitly convert expression aa of type 
shared(GSeqAllele[string]) to const(shared(GSeqAllele)[string])

source/polytyper/gseq.d(280,35): Error: template instance 
`object.keys!(shared(GSeqAllele[string]), shared(GSeqAllele), string)` 
error instantiating
```
(line breaks added for clarity)

The two triggering lines both iterate over a built-in AA

gseq.d:231:        foreach (a; this.alleles.values)
gseq.d:208:        foreach (key; this.alleles.keys)

Looking at the changelog for 2.087 ( 
https://dlang.org/changelog/2.087.0.html ) I do not see any obvious 
culprits. git blame suggests this is the responsible commit:

https://github.com/dlang/druntime/commit/af86e7db58f4b792e45bdc3ee43c17aadc9d54ab#diff-a68e58fcf0de5aa198fcaceafe4e8cf9L3430-R3454

 From this change, it looks like the runtime now cannot handle a shared 
AA -- is this right? A minimal reproducible example is here: 
https://gist.github.com/jblachly/78c5762bbfea65b09e7a1417ad763019

Thanks in advance for help
James

PS: Why is this commit ("object: Declare private struct AA and fix aaA 
function signatures ") not mentioned in the changelog?