inference is not generalizing in a complete way

[nikomatsakis]

Reading the code with @lcnr and @compiler-errors we found that...

• the "occurs check" in the inference code is overly simplistic when it comes to aliases
• see also generalization is incomplete for aliases trait-system-refactor-initiative#8

General sketch of the algorithm we want

• walk down the non-variable type T and look for aliases
• if the alias references a variable bound in T, then return "ambiguous, can't make progress"
• otherwise, replace it with an inference variable (and a new resulting goal)

[nikomatsakis]

@rustbot assign @lcnr

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[compiler-errors]

@rustbot assign @lcnr

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[compiler-errors]

@rustbot assign @lcnr

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[nikomatsakis]

🎉 😢