feat: (experiment) have synth order defer "mixin" instances #7313
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This PR modifies the instance synth order checker to detect projections-like instances, rather having special support only for constructor field projections. An instance is projection-like if the first instance implicit argument has a type that's a constant applied to all the preceding arguments in order — these arguments are assumed to be the parameters of the type. In the future the `class` command will make all its generated parent projections use implicits for all parameters, making the generated projections all consistent.
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This PR is a draft that's trying a new approach to what I was doing in #7310
The idea is that we can detect "mixin" arguments, assume that they're more likely than not to be inferable from the instance argument they're mixed into, and defer their synthesis.
In general, mixin arguments get promoted to be instance implicit, even if they were implicit.