go/types, types2: remove incorrect assertion (don't crash)

The removed assertion was never incorrect, as signatures may
be from methods in interfaces, and (some) interfaces set the
receivers of their methods (so we have a position for error
reporting).

This CL changes the issue below from a release blocker to an
issue for Go 1.19.

For #51593.

Change-Id: I0c5f2913b397b9ab557ed74a80cc7a715e840412
Reviewed-on: https://go-review.googlesource.com/c/go/+/391615
Trust: Robert Griesemer <[email protected]>
Run-TryBot: Robert Griesemer <[email protected]>
Reviewed-by: Robert Findley <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>

src/cmd/compile/internal/types2/infer.go

@@ -739,8 +739,6 @@ func (w *cycleFinder) typ(typ Type) {
 	//      in signatures where they are handled explicitly.
 
 	case *Signature:
-		// There are no "method types" so we should never see a recv.
-		assert(t.recv == nil)
 		if t.params != nil {
 			w.varList(t.params.vars)
 		}

src/cmd/compile/internal/types2/testdata/fixedbugs/issue51593.go2

@@ -0,0 +1,13 @@
+// Copyright 2022 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
+
+func f[P interface{ m(R) }, R any]() {}
+
+type T = interface { m(int) }
+
+func _() {
+	_ = f[ /* ERROR cannot infer R */ T] // don't crash in type inference
+}

src/go/types/infer.go

@@ -738,8 +738,6 @@ func (w *cycleFinder) typ(typ Type) {
 	//      in signatures where they are handled explicitly.
 
 	case *Signature:
-		// There are no "method types" so we should never see a recv.
-		assert(t.recv == nil)
 		if t.params != nil {
 			w.varList(t.params.vars)
 		}

src/go/types/testdata/fixedbugs/issue51593.go2

@@ -0,0 +1,13 @@
+// Copyright 2022 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
+
+func f[P interface{ m(R) }, R any]() {}
+
+type T = interface { m(int) }
+
+func _() {
+	_ = f /* ERROR cannot infer R */ [T] // don't crash in type inference
+}