[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>

src/cmd/compile/internal/reflectdata/reflect.go

@@ -1920,7 +1920,7 @@ func methodWrapper(rcvr *types.Type, method *types.Field, forItab bool) *obj.LSy
 			// Target method uses shaped names.
 			targs2 := make([]*types.Type, len(targs))
 			for i, t := range targs {
-				targs2[i] = typecheck.Shaped[t]
+				targs2[i] = typecheck.Shapify(t)
 			}
 			targs = targs2
 

src/cmd/compile/internal/typecheck/subr.go

@@ -1348,32 +1348,26 @@ func Shapify(t *types.Type) *types.Type {
 	if t.IsShape() {
 		return t // TODO: is this right?
 	}
-	if s := Shaped[t]; s != nil {
+	// Map all types with the same underlying type to the same shape.
+	u := t.Underlying()
+
+	if s := shaped[u]; s != nil {
 		return s //TODO: keep?
 	}
 
-	// For now, there is a 1-1 mapping between regular types and shape types.
 	sym := Lookup(fmt.Sprintf(".shape%d", snum))
 	snum++
-	name := ir.NewDeclNameAt(t.Pos(), ir.OTYPE, sym)
+	name := ir.NewDeclNameAt(u.Pos(), ir.OTYPE, sym)
 	s := types.NewNamed(name)
-	s.SetUnderlying(t.Underlying())
+	s.SetUnderlying(u)
 	s.SetIsShape(true)
 	name.SetType(s)
 	name.SetTypecheck(1)
 	// TODO: add methods to s that the bound has?
-	Shaped[t] = s
+	shaped[u] = s
 	return s
 }
 
 var snum int
 
-var Shaped = map[*types.Type]*types.Type{}
-
-func ShapifyList(targs []*types.Type) []*types.Type {
-	r := make([]*types.Type, len(targs))
-	for i, t := range targs {
-		r[i] = Shapify(t)
-	}
-	return r
-}
+var shaped = map[*types.Type]*types.Type{}

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)))
+}

test/typeparam/shape1.out

@@ -0,0 +1,2 @@
+25
+10