Auto merge of #61825 - Centril:tauv-infer-fix, r=petrochenkov

type_alias_enum_variants: fix #61801; allow a path pattern to infer

Fix #61801.

Given a type-relative path pattern referring to an enum variant through a type alias, allow inferring the generic argument applied in the expectation set by the scrutinee of a `match` expression.

Similar issues may exist for `let` statements but I don't know how to test for that since `PhantomData<T>` is necessary...)

The gist of the problem here was that `resolve_ty_and_res_ufcs` was called twice which is apparently no good... It is possible that this PR is papering over some deeper problem, but that is beyond my knowledge of the compiler.

r? @petrochenkov
cc @eddyb @alexreg
cc https://github.com/rust-lang/rust/pull/61682
cc https://github.com/rust-lang/rust/issues/49683

1 files changed, 30 insertions, 0 deletions

diff --git a/src/test/ui/type-alias-enum-variants/issue-61801-path-pattern-can-infer.rs b/src/test/ui/type-alias-enum-variants/issue-61801-path-pattern-can-infer.rs
new file mode 100644
index 00000000000..21be61acb0c
--- /dev/null
+++ b/src/test/ui/type-alias-enum-variants/issue-61801-path-pattern-can-infer.rs
@@ -0,0 +1,30 @@
+// In this regression test we check that a path pattern referring to a unit variant
+// through a type alias is successful in inferring the generic argument.
+
+// compile-pass
+
+#![feature(type_alias_enum_variants)]
+
+enum Opt<T> {
+    N,
+    S(T),
+}
+
+type OptAlias<T> = Opt<T>;
+
+fn f1(x: OptAlias<u8>) {
+    match x {
+        OptAlias::N // We previously failed to infer `T` to `u8`.
+            => (),
+        _ => (),
+    }
+
+    match x {
+        <
+            OptAlias<_> // And we failed to infer this type also.
+        >::N => (),
+        _ => (),
+    }
+}
+
+fn main() {}