[dev.typeparams] Add optional sub-dict entry for typeparam bound calls

In the case that a generic function/method f does a method call on a
type param allowed by its bound, an instantiation of f may do a direct
method call of a concrete type or a method call defined on a generic
type, depending on whether the passed type in a concrete type or an
instantiated type with the appropriate method defined. See the test case
boundmethod.go added to this change.

In order to keep the dictionary format the same for all instantiations
of a generic function/method, I decided to have an optional
sub-dictionary entry for "bounds" calls. At the point that we are
creating the actual dictionary, we can then fill in the needed
sub-dictionary, if the type arg is an instantiated type, or a zeroed
dictionary entry, if type arg is not instantiated and the method will be
on a concrete type.

In order to implement this, I now fill in n.Selection for "bounds"
method calls in generic functions as well. Also, I need to calculate
n.Selection correctly during import for the case where it is now set -
method calls on generic types, and bounds calls on typeparams.

With this change, the dictionaries/sub-dictionaries are correct for
absdiff.go. The new test boundmethod.go illustrates the case where the
bound sub-dict entry is not used for a dictionary for stringify[myint],
but is used for a dictionary for stringify[StringInt[myint]].

Change-Id: Ie2bcb971b7019a9f1da68c97eb03da2333327457
Reviewed-on: https://go-review.googlesource.com/c/go/+/333456
Run-TryBot: Dan Scales <danscales@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>
Trust: Dan Scales <danscales@google.com>

1 files changed, 60 insertions, 0 deletions

diff --git a/test/typeparam/boundmethod.go b/test/typeparam/boundmethod.go
new file mode 100644
index 0000000000..c150f9d85a
--- /dev/null
+++ b/test/typeparam/boundmethod.go
@@ -0,0 +1,60 @@
+// 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.
+
+// This test illustrates how a type bound method (String below) can be implemented
+// either by a concrete type (myint below) or a instantiated generic type
+// (StringInt[myint] below).
+
+package main
+
+import (
+        "fmt"
+        "reflect"
+        "strconv"
+)
+
+type myint int
+
+//go:noinline
+func (m myint) String() string {
+        return strconv.Itoa(int(m))
+}
+
+type Stringer interface {
+        String() string
+}
+
+func stringify[T Stringer](s []T) (ret []string) {
+        for _, v := range s {
+                ret = append(ret, v.String())
+        }
+        return ret
+}
+
+type StringInt[T any] T
+
+//go:noinline
+func (m StringInt[T]) String() string {
+        return "aa"
+}
+
+func main() {
+        x := []myint{myint(1), myint(2), myint(3)}
+
+        got := stringify(x)
+        want := []string{"1", "2", "3"}
+        if !reflect.DeepEqual(got, want) {
+                panic(fmt.Sprintf("got %s, want %s", got, want))
+        }
+
+        x2 := []StringInt[myint]{StringInt[myint](1), StringInt[myint](2), StringInt[myint](3)}
+
+        got2 := stringify(x2)
+        want2 := []string{"aa", "aa", "aa"}
+        if !reflect.DeepEqual(got2, want2) {
+                panic(fmt.Sprintf("got %s, want %s", got2, want2))
+        }
+}