[dev.typeparams] cmd/compile/internal/types2: use correct type parameter list in missingMethod

For #46275

Change-Id: Iaed9d8ba034ad793e5c57f2be174f01a535fee95
Reviewed-on: https://go-review.googlesource.com/c/go/+/321232
Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>

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

@@ -333,6 +333,9 @@ func (check *Checker) missingMethod(V Type, T *Interface, static bool) (method,
 			if len(ftyp.tparams) != len(mtyp.tparams) {
 				return m, f
 			}
+			if !acceptMethodTypeParams && len(ftyp.tparams) > 0 {
+				panic("internal error: method with type parameters")
+			}
 
 			// If the methods have type parameters we don't care whether they
 			// are the same or not, as long as they match up. Use unification
@@ -386,6 +389,9 @@ func (check *Checker) missingMethod(V Type, T *Interface, static bool) (method,
 		if len(ftyp.tparams) != len(mtyp.tparams) {
 			return m, f
 		}
+		if !acceptMethodTypeParams && len(ftyp.tparams) > 0 {
+			panic("internal error: method with type parameters")
+		}
 
 		// If V is a (instantiated) generic type, its methods are still
 		// parameterized using the original (declaration) receiver type
@@ -413,7 +419,20 @@ func (check *Checker) missingMethod(V Type, T *Interface, static bool) (method,
 		// TODO(gri) is this always correct? what about type bounds?
 		// (Alternative is to rename/subst type parameters and compare.)
 		u := newUnifier(check, true)
-		u.x.init(ftyp.tparams)
+		if len(ftyp.tparams) > 0 {
+			// We reach here only if we accept method type parameters.
+			// In this case, unification must consider any receiver
+			// and method type parameters as "free" type parameters.
+			assert(acceptMethodTypeParams)
+			// We don't have a test case for this at the moment since
+			// we can't parse method type parameters. Keeping the
+			// unimplemented call so that we test this code if we
+			// enable method type parameters.
+			unimplemented()
+			u.x.init(append(ftyp.rparams, ftyp.tparams...))
+		} else {
+			u.x.init(ftyp.rparams)
+		}
 		if !u.unify(ftyp, mtyp) {
 			return m, f
 		}

src/cmd/compile/internal/types2/testdata/fixedbugs/issue46275.go2

@@ -0,0 +1,26 @@
+// Copyright 2021 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 issue46275
+
+type N[T any] struct {
+        *N[T]
+        t T
+}
+
+func (n *N[T]) Elem() T {
+        return n.t
+}
+
+type I interface {
+        Elem() string
+}
+
+func _() {
+        var n1 *N[string]
+        var _ I = n1
+        type NS N[string]
+        var n2 *NS
+        var _ I = n2
+}