[dev.typeparams] cmd/compile: allow types with the same underlying type to have the same shape

First baby step to sharing the underlying implementation among several types.

Change-Id: I6a156176d2b7f0131a87285a03b881ce380c26ed
Reviewed-on: https://go-review.googlesource.com/c/go/+/338610
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, 35 insertions, 0 deletions

diff --git a/test/typeparam/shape1.go b/test/typeparam/shape1.go
new file mode 100644
index 0000000000..3c9e71ea63
--- /dev/null
+++ b/test/typeparam/shape1.go
@@ -0,0 +1,33 @@
+// 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.
+
+package main
+
+type I interface {
+	foo() int
+}
+
+// There should be a single instantiation of f in this program.
+func f[T I](x T) int {
+	return x.foo()
+}
+
+type squarer int
+
+func (x squarer) foo() int {
+	return int(x*x)
+}
+
+type doubler int
+
+func (x doubler) foo() int {
+	return int(2*x)
+}
+
+func main() {
+	println(f(squarer(5)))
+	println(f(doubler(5)))
+}
diff --git a/test/typeparam/shape1.out b/test/typeparam/shape1.out
new file mode 100644
index 0000000000..28391fde66
--- /dev/null
+++ b/test/typeparam/shape1.out
@@ -0,0 +1,2 @@
+25
+10