Skip coherence checks for error types

[sanxiyn]

Fix#30589.

[rust-highfive]

r? @alexcrichton

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[brson]

r? @nikomatsakis

[arielb1]

I suspect #29954 treats the issue closer to the root.

[sanxiyn]

This is a fallout of #30320. I don't see how #29954 helps. Both ICE says ty_is_local invoked on unexpected type: [type error], but the source of error types is different. There is no unification going on here and the type is an error type from the start because it was undeclared.

[arielb1]

@sanxiyn

Didn't notice that you are only working at the outer level. In that case, you probably want to check for trait_ref.references_error() (or self_ty.references_error() for inherent impls) to avoid problems with error types in inner places. r+ modulo that + test that ICEs your current example.

[nikomatsakis]

see also #30676

[bors]

☔ The latest upstream changes (presumably #30676) made this pull request unmergeable. Please resolve the merge conflicts.

[nikomatsakis]

Closing since this was subsumed into #30676.