internal/typeparams: add a helper to return the origin method

With instantiated types, method objects are no longer unique: they may
be instantiations of methods with generic receiver. However, some
use-cases require finding the canonical method representing the method
in the source. For these use-cases, provide an OriginMethod helper.

For golang/go#50447

Change-Id: I6f8af3fb5c5eeefb11f8f3bdba54cd6692ca389f
Reviewed-on: https://go-review.googlesource.com/c/tools/+/380554
Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
gopls-CI: kokoro <noreply+kokoro@google.com>
Reviewed-by: Tim King <taking@google.com>

internal/typeparams/common.go

@@ -77,3 +77,35 @@ func IsTypeParam(t types.Type) bool {
 	_, ok := t.(*TypeParam)
 	return ok
 }
+
+// OriginMethod returns the origin method associated with the method fn.
+// For methods on a non-generic receiver base type, this is just
+// fn. However, for methods with a generic receiver, OriginMethod returns the
+// corresponding method in the method set of the origin type.
+//
+// As a special case, if fn is not a method (has no receiver), OriginMethod
+// returns fn.
+func OriginMethod(fn *types.Func) *types.Func {
+	recv := fn.Type().(*types.Signature).Recv()
+	if recv == nil {
+
+		return fn
+	}
+	base := recv.Type()
+	p, isPtr := base.(*types.Pointer)
+	if isPtr {
+		base = p.Elem()
+	}
+	named, isNamed := base.(*types.Named)
+	if !isNamed {
+		// Receiver is a *types.Interface.
+		return fn
+	}
+	if ForNamed(named).Len() == 0 {
+		// Receiver base has no type parameters, so we can avoid the lookup below.
+		return fn
+	}
+	orig := NamedTypeOrigin(named)
+	gfn, _, _ := types.LookupFieldOrMethod(orig, true, fn.Pkg(), fn.Name())
+	return gfn.(*types.Func)
+}

internal/typeparams/common_test.go

@@ -6,16 +6,20 @@ package typeparams_test
 
 import (
 	"go/ast"
+	"go/parser"
+	"go/token"
+	"go/types"
 	"testing"
 
-	"golang.org/x/tools/internal/typeparams"
+	"golang.org/x/tools/internal/testenv"
+	. "golang.org/x/tools/internal/typeparams"
 )
 
 func TestGetIndexExprData(t *testing.T) {
 	x := &ast.Ident{}
 	i := &ast.Ident{}
 
-	want := &typeparams.IndexListExpr{X: x, Lbrack: 1, Indices: []ast.Expr{i}, Rbrack: 2}
+	want := &IndexListExpr{X: x, Lbrack: 1, Indices: []ast.Expr{i}, Rbrack: 2}
 	tests := map[ast.Node]bool{
 		&ast.IndexExpr{X: x, Lbrack: 1, Index: i, Rbrack: 2}: true,
 		want:         true,
@@ -23,7 +27,7 @@ func TestGetIndexExprData(t *testing.T) {
 	}
 
 	for n, isIndexExpr := range tests {
-		X, lbrack, indices, rbrack := typeparams.UnpackIndexExpr(n)
+		X, lbrack, indices, rbrack := UnpackIndexExpr(n)
 		if got := X != nil; got != isIndexExpr {
 			t.Errorf("UnpackIndexExpr(%v) = %v, _, _, _; want nil: %t", n, x, !isIndexExpr)
 		}
@@ -35,3 +39,121 @@ func TestGetIndexExprData(t *testing.T) {
 		}
 	}
 }
+
+func TestOriginMethodRecursive(t *testing.T) {
+	testenv.NeedsGo1Point(t, 18)
+	src := `package p
+
+type N[A any] int
+
+func (r N[B]) m() { r.m(); r.n() }
+
+func (r *N[C]) n() { }
+`
+	fset := token.NewFileSet()
+	f, err := parser.ParseFile(fset, "p.go", src, 0)
+	if err != nil {
+		t.Fatal(err)
+	}
+	info := types.Info{
+		Defs: make(map[*ast.Ident]types.Object),
+		Uses: make(map[*ast.Ident]types.Object),
+	}
+	var conf types.Config
+	if _, err := conf.Check("p", fset, []*ast.File{f}, &info); err != nil {
+		t.Fatal(err)
+	}
+
+	// Collect objects from types.Info.
+	var m, n *types.Func   // the 'origin' methods in Info.Defs
+	var mm, mn *types.Func // the methods used in the body of m
+
+	for _, decl := range f.Decls {
+		fdecl, ok := decl.(*ast.FuncDecl)
+		if !ok {
+			continue
+		}
+		def := info.Defs[fdecl.Name].(*types.Func)
+		switch fdecl.Name.Name {
+		case "m":
+			m = def
+			ast.Inspect(fdecl.Body, func(n ast.Node) bool {
+				if call, ok := n.(*ast.CallExpr); ok {
+					sel := call.Fun.(*ast.SelectorExpr)
+					use := info.Uses[sel.Sel].(*types.Func)
+					switch sel.Sel.Name {
+					case "m":
+						mm = use
+					case "n":
+						mn = use
+					}
+				}
+				return true
+			})
+		case "n":
+			n = def
+		}
+	}
+
+	tests := []struct {
+		name        string
+		input, want *types.Func
+	}{
+		{"declared m", m, m},
+		{"declared n", n, n},
+		{"used m", mm, m},
+		{"used n", mn, n},
+	}
+
+	for _, test := range tests {
+		if got := OriginMethod(test.input); got != test.want {
+			t.Errorf("OriginMethod(%q) = %v, want %v", test.name, test.input, test.want)
+		}
+	}
+}
+
+func TestOriginMethodUses(t *testing.T) {
+	testenv.NeedsGo1Point(t, 18)
+
+	tests := []string{
+		`type T interface { m() }; func _(t T) { t.m() }`,
+		`type T[P any] interface { m() P }; func _[A any](t T[A]) { t.m() }`,
+		`type T[P any] interface { m() P }; func _(t T[int]) { t.m() }`,
+		`type T[P any] int; func (r T[A]) m() { r.m() }`,
+		`type T[P any] int; func (r *T[A]) m() { r.m() }`,
+		`type T[P any] int; func (r *T[A]) m() {}; func _(t T[int]) { t.m() }`,
+		`type T[P any] int; func (r *T[A]) m() {}; func _[A any](t T[A]) { t.m() }`,
+	}
+
+	for _, src := range tests {
+		fset := token.NewFileSet()
+		f, err := parser.ParseFile(fset, "p.go", "package p; "+src, 0)
+		if err != nil {
+			t.Fatal(err)
+		}
+		info := types.Info{
+			Uses: make(map[*ast.Ident]types.Object),
+		}
+		var conf types.Config
+		pkg, err := conf.Check("p", fset, []*ast.File{f}, &info)
+		if err != nil {
+			t.Fatal(err)
+		}
+
+		T := pkg.Scope().Lookup("T").Type()
+		obj, _, _ := types.LookupFieldOrMethod(T, true, pkg, "m")
+		m := obj.(*types.Func)
+
+		ast.Inspect(f, func(n ast.Node) bool {
+			if call, ok := n.(*ast.CallExpr); ok {
+				sel := call.Fun.(*ast.SelectorExpr)
+				use := info.Uses[sel.Sel].(*types.Func)
+				orig := OriginMethod(use)
+				if orig != m {
+					t.Errorf("%s:\nUses[%v] = %v, want %v", src, types.ExprString(sel), use, m)
+				}
+			}
+			return true
+		})
+	}
+}