Show the actual type of RHS on type error when LHS is a pattern with a type annotation

[travv0]

Description

Previously, when a pattern on the LHS had a type annotation that didn't match the binding's type, a type error would report the annotation type as the type of the RHS. This PR separates it into two steps, so the binding type is checked against the annotation type, and then the RHS's type is checked against the binding's type. (Hopefully I worded this okay, I'm new to typechecking stuff.)

Fixes #18849

Checklist

• Test cases added

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[github-actions]

❗ Release notes required

@travv0,

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Change path	Release notes path	Description
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[auduchinok]

Consider

let a: T = b

If I understand it correctly, during the analysis the compiler rewrites it to

let a = (b: T)

T may be a partially annotated type, e.g. _ list in the source, and then it can be inferred to something like int list after the type checking of the b expression. Can we propagate this inferred type here instead of type checking T from scratch? Would there be downsides?

[travv0]

Sorry if I'm misunderstanding what you're saying here, but the problem this addresses is that when it's rewritten to let a = (b: T), if a is a pattern and b doesn't typecheck, it reports the annotated type of a as the type of b instead of the actual type of b (in the specific scenario where the type annotation doesn't fit the pattern).