internal/types: add test for cycles in value context

Exposition is also added to outline a difference between syntax which
can / cannot produce values of incomplete types.

For us to enforce non-nilness of type RHS and remove the pending type
mechanism, I suspect we would need to add completeness guards to
the syntax which *can*.

Enforcing non-nilness of type RHS currently breaks the below test
cases, but I suspect that is simply an implementation artifact.
In other words, they just call Underlying at a bad time.

  - T0
  - T3
  - T6 / T7
  - T10
  - T12

If we also remove pendingType, all of these test cases break; again,
we would need guards in the appropriate syntax logic.

Change-Id: Ibe22042232e542de1d38b923dd1d5cc50dce08cb
Reviewed-on: https://go-review.googlesource.com/c/go/+/734600
TryBot-Bypass: Mark Freeman <markfreeman@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>
Auto-Submit: Mark Freeman <markfreeman@google.com>

src/internal/types/testdata/check/cycles6.go

@@ -0,0 +1,71 @@
+// Copyright 2026 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+import "unsafe"
+
+// Below are the pieces of syntax corresponding to functions which can produce a
+// type T without first having a value of type T. Notice that each causes a
+// value of type T to be passed to unsafe.Sizeof while T is incomplete.
+
+// literal on type
+type T0 /* ERROR "invalid recursive type" */ [unsafe.Sizeof(T0{})]int
+// literal on value                                                             (not applicable)
+// literal on pointer                                                           (not applicable)
+
+// call on type
+type T1 /* ERROR "invalid recursive type" */ [unsafe.Sizeof(T1(42))]int
+// call on value
+func f2() T2
+type T2 /* ERROR "invalid recursive type" */ [unsafe.Sizeof(f2())]int
+// call on pointer                                                              (not applicable)
+
+// assert on type
+var i3 interface{}
+type T3 /* ERROR "invalid recursive type" */ [unsafe.Sizeof(i3.(T3))]int
+// assert on value                                                              (not applicable)
+// assert on pointer                                                            (not applicable)
+
+// receive on type                                                              (not applicable)
+// receive on value
+func f4() <-chan T4
+type T4 /* ERROR "invalid recursive type" */ [unsafe.Sizeof(<-f4())]int
+// receive on pointer                                                           (not applicable)
+
+// star on type                                                                 (not applicable)
+// star on value                                                                (not applicable)
+// star on pointer
+func f5() *T5
+type T5 /* ERROR "invalid recursive type" */ [unsafe.Sizeof(*f5())]int
+
+// Below is additional syntax which interacts with incomplete types. Notice that
+// each of the below falls into 1 of 3 cases:
+//   1. It cannot produce a value of (incomplete) type T.
+//   2. It can, but only because it already has a value of type T.
+//   3. It can, but only because it performs an implicit dereference.
+
+// select on type                                                               (case 1)
+// select on value                                                              (case 2)
+type T6 /* ERROR "invalid recursive type" */ struct {
+	f T7
+}
+type T7 [unsafe.Sizeof(T6{}.f)]int
+// select on pointer                                                            (case 3)
+type T8 /* ERROR "invalid recursive type" */ struct {
+	f T9
+}
+type T9 [unsafe.Sizeof(new(T8).f)]int
+
+// slice on type                                                                (not applicable)
+// slice on value                                                               (case 2)
+type T10 /* ERROR "invalid recursive type" */ [unsafe.Sizeof(T10{}[:])]int
+// slice on pointer                                                             (case 3)
+type T11 /* ERROR "invalid recursive type" */ [unsafe.Sizeof(new(T11)[:])]int
+
+// index on type                                                                (case 1)
+// index on value                                                               (case 2)
+type T12 /* ERROR "invalid recursive type" */ [unsafe.Sizeof(T12{}[42])]int
+// index on pointer                                                             (case 3)
+type T13 /* ERROR "invalid recursive type" */ [unsafe.Sizeof(new(T13)[42])]int