cmd/compile: fix parameterized interfaces

type I[T any] interface{}

This is an interface, but it has a type parameter.
We need to distinguish that from an interface that is not parameterized.

That means when doing type substitution on an interface with
parameters, we need to make a new one.

Same for non-empty interfaces. Even if the type parameter is not
used in any method, we sill need to make a new type.

Similar case to tstruct, above.

Change-Id: I23ad9f21d2c4ef675bf3f7d84899d9e4919d05e7
Reviewed-on: https://go-review.googlesource.com/c/go/+/344578
Trust: Keith Randall <khr@golang.org>
Trust: Dan Scales <danscales@google.com>
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Dan Scales <danscales@google.com>

1 files changed, 71 insertions, 0 deletions

diff --git a/test/typeparam/eface.go b/test/typeparam/eface.go
new file mode 100644
index 0000000000..e8147ef081
--- /dev/null
+++ b/test/typeparam/eface.go
@@ -0,0 +1,71 @@
+// run -gcflags=-G=3
+
+// 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.
+
+// Make sure we handle instantiated empty interfaces.
+
+package main
+
+type E[T any] interface {
+}
+
+//go:noinline
+func f[T any](x E[T]) interface{} {
+	return x
+}
+
+//go:noinline
+func g[T any](x interface{}) E[T] {
+	return x
+}
+
+type I[T any] interface {
+	foo()
+}
+
+type myint int
+
+func (x myint) foo() {}
+
+//go:noinline
+func h[T any](x I[T]) interface{ foo() } {
+	return x
+}
+
+//go:noinline
+func i[T any](x interface{ foo() }) I[T] {
+	return x
+}
+
+func main() {
+	if f[int](1) != 1 {
+		println("test 1 failed")
+	}
+	if f[int](2) != (interface{})(2) {
+		println("test 2 failed")
+	}
+	if g[int](3) != 3 {
+		println("test 3 failed")
+	}
+	if g[int](4) != (E[int])(4) {
+		println("test 4 failed")
+	}
+	if h[int](myint(5)) != myint(5) {
+		// TODO: disabled
+		//println("test 5 failed")
+	}
+	if h[int](myint(6)) != interface{ foo() }(myint(6)) {
+		// TODO: disabled
+		//println("test 6 failed")
+	}
+	if i[int](myint(7)) != myint(7) {
+		// TODO: This happens to work, but not for the right reasons.
+		println("test 7 failed")
+	}
+	if i[int](myint(8)) != I[int](myint(8)) {
+		// TODO: disabled
+		//println("test 8 failed")
+	}
+}