[dev.go2go] go/go2go, go/types: enable constraint type inference

Constraint type inference was not enabled for go/go2go and gotype.

Fixes #40859.

Change-Id: Idcab4d0a2eae6f42f938604fb05aabe4ad94c934
Reviewed-on: https://go-review.googlesource.com/c/go/+/249737
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/go/go2go/go2go.go

@@ -83,8 +83,9 @@ func rewriteFilesInPath(importer *Importer, importPath, dir string, go2files []s
 
 		var merr multiErr
 		conf := types.Config{
-			Importer: importer,
-			Error:    merr.add,
+			InferFromConstraints: true,
+			Importer:             importer,
+			Error:                merr.add,
 		}
 		path := importPath
 		if path == "" {
@@ -131,8 +132,9 @@ func RewriteBuffer(importer *Importer, filename string, file []byte) ([]byte, er
 	}
 	var merr multiErr
 	conf := types.Config{
-		Importer: importer,
-		Error:    merr.add,
+		InferFromConstraints: true,
+		Importer:             importer,
+		Error:                merr.add,
 	}
 	tpkg, err := conf.Check(pf.Name.Name, fset, []*ast.File{pf}, importer.info)
 	if err != nil {

src/go/types/gotype.go

@@ -290,7 +290,8 @@ func checkPkgFiles(files []*ast.File) {
 
 	// if checkPkgFiles is called multiple times, set up conf only once
 	conf := types.Config{
-		FakeImportC: true,
+		InferFromConstraints: true,
+		FakeImportC:          true,
 		Error: func(err error) {
 			if !*allErrors && errorCount >= 10 {
 				panic(bailout{})

test/gen/g043.go2

@@ -0,0 +1,38 @@
+// compile
+
+// Copyright 2020 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.
+
+// Issue 40859.
+package main
+
+import (
+	"fmt"
+)
+
+type Number interface {
+	type int, int8, int16, int32, int64, uint, uint8, uint16, uint32, uint64, uintptr, float32, float64
+}
+
+type MySlice []int
+
+type SC[type E] interface {
+	type []E
+}
+
+func DoubleDefined[type S SC[E], E Number](s S) S {
+	r := make(S, len(s))
+	for i, v := range s {
+		r[i] = v + v
+	}
+	return r
+}
+
+func main() {
+	// Expicit types work fine
+	fmt.Println(DoubleDefined[MySlice, int](MySlice{1}))
+
+	// Constraint type inference does not work?
+	fmt.Println(DoubleDefined(MySlice{1}))
+}