go/types, types2: replace setDefType with pending type check

Given a type definition of the form:

  type T RHS

The setDefType function would set T.fromRHS as soon as we knew its
top-level type. For instance, in:

  type S struct { ... }

S.fromRHS is set to a struct type before type-checking anything inside
the struct.

This permit access to the (incomplete) RHS type in a cyclic type
declaration. Accessing this information is fraught (as it's incomplete),
but was used for reporting certain types of cycles.

This CL replaces setDefType with a check that ensures no value of type
T is used before its RHS is set up.

This CL is strictly more complete than what setDefType achieved. For
instance, it enables correct reporting for the below cycles:

  type A [unsafe.Sizeof(A{})]int

  var v any = 42
  type B [v.(B)]int

  func f() C {
    return C{}
  }
  type C [unsafe.Sizeof(f())]int

Fixes #76383
Fixes #76384

Change-Id: I9dfab5b708013b418fa66e43362bb4d8483fedec
Reviewed-on: https://go-review.googlesource.com/c/go/+/724140
Auto-Submit: Mark Freeman <markfreeman@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

1 files changed, 2 insertions, 5 deletions

diff --git a/test/fixedbugs/issue18392.go b/test/fixedbugs/issue18392.go
index 32c39c3a7f..9e4d48d6ea 100644
--- a/test/fixedbugs/issue18392.go
+++ b/test/fixedbugs/issue18392.go
@@ -6,9 +6,6 @@
 
 package p
 
-type A interface {
-	// TODO(mdempsky): This should be an error, but this error is
-	// nonsense. The error should actually mention that there's a
-	// type loop.
-	Fn(A.Fn) // ERROR "type A has no method Fn|A.Fn undefined|A.Fn is not a type"
+type A interface { // ERROR "invalid recursive type"
+	Fn(A.Fn)
 }