cmd/compile: delay transformAssign if lhs/rhs have typeparam

This also requires that we sometimes delay transformSelect(), if the
assignments in the Comm part of the select have not been transformed.

Fixes #48137

Change-Id: I163aa1f999d1e63616280dca807561b12b2aa779
Reviewed-on: https://go-review.googlesource.com/c/go/+/347915
Trust: Dan Scales <danscales@google.com>
Run-TryBot: Dan Scales <danscales@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

src/cmd/compile/internal/noder/stencil.go

@@ -995,6 +995,9 @@ func (subst *subster) node(n ir.Node) ir.Node {
 			case ir.OSEND:
 				transformSend(m.(*ir.SendStmt))
 
+			case ir.OSELECT:
+				transformSelect(m.(*ir.SelectStmt))
+
 			}
 		}
 

src/cmd/compile/internal/noder/stmt.go

@@ -84,13 +84,13 @@ func (g *irgen) stmt(stmt syntax.Stmt) ir.Node {
 		// to know the types of the left and right sides in various cases.
 		delay := false
 		for _, e := range lhs {
-			if e.Typecheck() == 3 {
+			if e.Type().HasTParam() || e.Typecheck() == 3 {
 				delay = true
 				break
 			}
 		}
 		for _, e := range rhs {
-			if e.Typecheck() == 3 {
+			if e.Type().HasTParam() || e.Typecheck() == 3 {
 				delay = true
 				break
 			}
@@ -145,8 +145,20 @@ func (g *irgen) stmt(stmt syntax.Stmt) ir.Node {
 		return g.forStmt(stmt)
 	case *syntax.SelectStmt:
 		n := g.selectStmt(stmt)
+
+		delay := false
+		for _, ncase := range n.(*ir.SelectStmt).Cases {
+			if ncase.Comm != nil && ncase.Comm.Typecheck() == 3 {
+				delay = true
+				break
+			}
+		}
+		if delay {
+			n.SetTypecheck(3)
+		} else {
 			transformSelect(n.(*ir.SelectStmt))
 			n.SetTypecheck(1)
+		}
 		return n
 	case *syntax.SwitchStmt:
 		return g.switchStmt(stmt)

test/typeparam/issue48137.go

@@ -0,0 +1,25 @@
+// 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 Constraint[T any] interface {
+	~func() T
+}
+
+func Foo[T Constraint[T]]() T {
+	var t T
+
+	t = func() T {
+		return t
+	}
+	return t
+}
+
+func main() {
+	type Bar func() Bar
+	Foo[Bar]()
+}