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>

2 files changed, 83 insertions, 3 deletions

diff --git a/src/cmd/compile/internal/typecheck/subr.go b/src/cmd/compile/internal/typecheck/subr.go
index 8d05356543..73f83f65e4 100644
--- a/src/cmd/compile/internal/typecheck/subr.go
+++ b/src/cmd/compile/internal/typecheck/subr.go
@@ -1163,8 +1163,8 @@ func (ts *Tsubster) typ1(t *types.Type) *types.Type {
 		}
 
 	case types.TINTER:
-		newt = ts.tinter(t)
-		if newt == t && !targsChanged {
+		newt = ts.tinter(t, targsChanged)
+		if newt == t {
 			newt = nil
 		}
 
@@ -1324,11 +1324,20 @@ func (ts *Tsubster) tstruct(t *types.Type, force bool) *types.Type {
 }
 
 // tinter substitutes type params in types of the methods of an interface type.
-func (ts *Tsubster) tinter(t *types.Type) *types.Type {
+func (ts *Tsubster) tinter(t *types.Type, force bool) *types.Type {
 	if t.Methods().Len() == 0 {
+		if t.HasTParam() {
+			// For an empty interface, we need to return a new type,
+			// since it may now be fully instantiated (HasTParam
+			// becomes false).
+			return types.NewInterface(t.Pkg(), nil)
+		}
 		return t
 	}
 	var newfields []*types.Field
+	if force {
+		newfields = make([]*types.Field, t.Methods().Len())
+	}
 	for i, f := range t.Methods().Slice() {
 		t2 := ts.typ1(f.Type)
 		if (t2 != f.Type || f.Nname != nil) && newfields == nil {
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")
+	}
+}