Showing wrong compiler error in this case.

Code

// If we remove this enum, the compiler error seems reasonable.
enum Terminal<E> {
    Error(E),
}

struct From<I>(I);

fn repeat<T>(value: T, n: usize) -> From<_>
where
    T: Clone,
{
    From(std::iter::repeat_n(value, n))
}

Current output

[{
	"resource": "/Users/yawang/Documents/rust-test/src/lib.rs",
	"owner": "rustc",
	"code": {
		"value": "Click for full compiler diagnostic",
		"target": {
			"$mid": 1,
			"path": "/diagnostic message [0]",
			"scheme": "rust-analyzer-diagnostics-view",
			"query": "0",
			"fragment": "file:///Users/yawang/Documents/rust-test/src/lib.rs"
		}
	},
	"severity": 8,
	"message": "cycle detected when computing the variances for items in this crate\n...which again requires computing the variances for items in this crate, completing the cycle\nsee https://rustc-dev-guide.rust-lang.org/overview.html#queries and https://rustc-dev-guide.rust-lang.org/query.html for more information",
	"source": "rustc",
	"startLineNumber": 1,
	"startColumn": 1,
	"endLineNumber": 1,
	"endColumn": 1,
	"relatedInformation": [
		{
			"startLineNumber": 7,
			"startColumn": 1,
			"endLineNumber": 9,
			"endColumn": 14,
			"message": "...which requires computing function signature of `repeat`...",
			"resource": "/Users/yawang/Documents/rust-test/src/lib.rs"
		},
		{
			"startLineNumber": 7,
			"startColumn": 1,
			"endLineNumber": 9,
			"endColumn": 14,
			"message": "...which requires type-checking `repeat`...",
			"resource": "/Users/yawang/Documents/rust-test/src/lib.rs"
		},
		{
			"startLineNumber": 5,
			"startColumn": 1,
			"endLineNumber": 5,
			"endColumn": 15,
			"message": "...which requires computing the variances of `From`...",
			"resource": "/Users/yawang/Documents/rust-test/src/lib.rs"
		},
		{
			"startLineNumber": 1,
			"startColumn": 1,
			"endLineNumber": 1,
			"endColumn": 17,
			"message": "cycle used when computing the variances of `Terminal`",
			"resource": "/Users/yawang/Documents/rust-test/src/lib.rs"
		}
	]
}]

Desired output

[{
	"resource": "/Users/yawang/Documents/rust-test/src/lib.rs",
	"owner": "rustc",
	"code": {
		"value": "Click for full compiler diagnostic",
		"target": {
			"$mid": 1,
			"path": "/diagnostic message [1]",
			"scheme": "rust-analyzer-diagnostics-view",
			"query": "1",
			"fragment": "file:///Users/yawang/Documents/rust-test/src/lib.rs"
		}
	},
	"severity": 8,
	"message": "the placeholder `_` is not allowed within types on item signatures for return types\nnot allowed in type signatures",
	"source": "rustc",
	"startLineNumber": 7,
	"startColumn": 42,
	"endLineNumber": 7,
	"endColumn": 43
},{
	"resource": "/Users/yawang/Documents/rust-test/src/lib.rs",
	"owner": "rustc",
	"code": {
		"value": "Click for full compiler diagnostic",
		"target": {
			"$mid": 1,
			"path": "/diagnostic message [0]",
			"scheme": "rust-analyzer-diagnostics-view",
			"query": "0",
			"fragment": "file:///Users/yawang/Documents/rust-test/src/lib.rs"
		}
	},
	"severity": 8,
	"message": "type annotations needed\ncannot infer type for struct `From<_>`",
	"source": "rustc",
	"startLineNumber": 10,
	"startColumn": 1,
	"endLineNumber": 12,
	"endColumn": 2
}]

Rationale and extra context

No response

Other cases

Rust Version

rustc 1.86.0 (05f9846f8 2025-03-31)
binary: rustc
commit-hash: 05f9846f893b09a1be1fc8560e33fc3c815cfecb
commit-date: 2025-03-31
host: aarch64-apple-darwin
release: 1.86.0
LLVM version: 19.1.7

Anything else?

No response

Reproduce the case in src/lib.rs using rustc 1.86.0 and compare the current diagnostic with the desired output. Start by investigating the compiler query cycle reported in the diagnostic and the handling of the _ return type in repeat. Done means the invalid placeholder diagnostic appears at the return type and the separate inference error is reported for From<_>.