fix #21478: Ref parameter overload not called when struct implements copy constructor

[ibuclaw]

If a struct implemented a copy constructor, then an rvalue opAssign would be given a better match score than the lvalue opAssign regardless of the ref-ness of the argument.

struct S
{
    this(ref return scope S);
    ref S opAssign(const ref S);
    ref S opAssign(const S);
}

s1 = s2;  // lowered as:  S.opAssign(&s1, *S.this(s2));

This change optimizes the assignment as:

s1 = s2;  // lowered as: S.opAssign(&s1, &s2);

Essentially, this is same as C++ semantics on object that implements rule of five.

[dlang-bot]

Thanks for your pull request, @ibuclaw!

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Testing this PR locally

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

dub run digger -- build "stable + dmd#21493"

[ibuclaw]

Looks like this has found a wrong @nogc diagnostic coming from a speculative context.

source/mir/algebraic.d(2659,19): Error: `@nogc` function `mir.algebraic.__unittest_L2622_C24` cannot call non-@nogc constructor `mir.algebraic.Algebraic!(C4, S4).Algebraic.__ctor!(immutable(Algebraic!(C4, S4)), S4).this`
    immutable b = immutable V(s);
                  ^
source/mir/algebraic.d(2046,50):        and allocating with `new` makes it fail to infer `@nogc`
            enum CanCompile = __traits(compiles, new Q(forward!args));

Changing new to scope new suppresses this, but that shouldn't be necessary.

--- a/source/mir/algebraic.d
+++ b/source/mir/algebraic.d
@@ -2043,7 +2043,7 @@ struct Algebraic(T__...)
         template CanCompile(T)
         {
             alias Q = CopyTypeQualifiers!(This, T);
-            enum CanCompile = __traits(compiles, new Q(forward!args));
+            enum CanCompile = __traits(compiles, { scope q = new Q(forward!args); });
         }
 
         alias TargetType = Filter!(CanCompile, _ReflectionTypes);

[ibuclaw]

Blocked by #21504

[thewilsonator]

dependant PR merged. Please rebase to restart the CI.