go/types: don't update package-external types when checking validity

The recently added type-validity check uses a new field of Named
types for marking (to detect cycles). That field was modified even
if the type was not part of the current package or belonged to the
Universe scope (error type). This led to race conditions if the
package's type was imported by multiple, concurrently type-checked
packages.

A test would be nice but it's a bit cumbersome to set one up.
Verified manually that package-external types are left alone.

Fixes #35049.

Change-Id: I51686bef47fcca48b99b91ecb1b2e9d58e135ea6
Reviewed-on: https://go-review.googlesource.com/c/go/+/202483Reviewed-by: Bryan C. Mills <bcmills@google.com>

src/go/types/decl.go

@@ -311,19 +311,29 @@ func (check *Checker) validType(typ Type, path []Object) typeInfo {
 		}
 
 	case *Named:
+		// don't touch the type if it is from a different package or the Universe scope
+		// (doing so would lead to a race condition - was issue #35049)
+		if t.obj.pkg != check.pkg {
+			return valid
+		}
+
 		// don't report a 2nd error if we already know the type is invalid
 		// (e.g., if a cycle was detected earlier, via Checker.underlying).
 		if t.underlying == Typ[Invalid] {
 			t.info = invalid
 			return invalid
 		}
+
 		switch t.info {
 		case unknown:
 			t.info = marked
-			t.info = check.validType(t.orig, append(path, t.obj))
+			t.info = check.validType(t.orig, append(path, t.obj)) // only types of current package added to path
 		case marked:
 			// cycle detected
 			for i, tn := range path {
+				if t.obj.pkg != check.pkg {
+					panic("internal error: type cycle via package-external type")
+				}
 				if tn == t.obj {
 					check.cycleError(path[i:])
 					t.info = invalid