go/internal/gcimporter: rewrite interface receiver parameters

For a type definition like `type I interface{ M() }`, the go/types API
traditionally sets `M`'s receiver parameter type to `I`, whereas
Unified IR was (intentionally) leaving it as `interface{ M() }`.

I still think `interface{ M() }` is the more consistent and
semantically correct type to use in this scenario, but I concede that
users want `I` instead, as evidenced by existing tooling and tests.

Updates #49906.

Change-Id: I74ba5e8b08e4e98ed9dc49f72b7834d5b552058b
Reviewed-on: https://go-review.googlesource.com/c/go/+/421355
Reviewed-by: David Chase <drchase@google.com>
Auto-Submit: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>

3 files changed, 25 insertions, 23 deletions

diff --git a/src/go/internal/gcimporter/gcimporter_test.go b/src/go/internal/gcimporter/gcimporter_test.go
index 68a077c190..dd41c2550c 100644
--- a/src/go/internal/gcimporter/gcimporter_test.go
+++ b/src/go/internal/gcimporter/gcimporter_test.go
@@ -461,14 +461,6 @@ func verifyInterfaceMethodRecvs(t *testing.T, named *types.Named, level int) {
 		return // not an interface
 	}
 
-	// The unified IR importer always sets interface method receiver
-	// parameters to point to the Interface type, rather than the Named.
-	// See #49906.
-	var want types.Type = named
-	if goexperiment.Unified {
-		want = iface
-	}
-
 	// check explicitly declared methods
 	for i := 0; i < iface.NumExplicitMethods(); i++ {
 		m := iface.ExplicitMethod(i)
@@ -477,8 +469,8 @@ func verifyInterfaceMethodRecvs(t *testing.T, named *types.Named, level int) {
 			t.Errorf("%s: missing receiver type", m)
 			continue
 		}
-		if recv.Type() != want {
-			t.Errorf("%s: got recv type %s; want %s", m, recv.Type(), want)
+		if recv.Type() != named {
+			t.Errorf("%s: got recv type %s; want %s", m, recv.Type(), named)
 		}
 	}
 
diff --git a/src/go/internal/gcimporter/ureader.go b/src/go/internal/gcimporter/ureader.go
index 97f0664fe3..d45ea80ecc 100644
--- a/src/go/internal/gcimporter/ureader.go
+++ b/src/go/internal/gcimporter/ureader.go
@@ -493,10 +493,6 @@ func (pr *pkgReader) objIdx(idx pkgbits.Index) (*types.Package, string) {
 
 			named.SetTypeParams(r.typeParamNames())
 
-			// TODO(mdempsky): Rewrite receiver types to underlying is an
-			// Interface? The go/types importer does this (I think because
-			// unit tests expected that), but cmd/compile doesn't care
-			// about it, so maybe we can avoid worrying about that here.
 			rhs := r.typ()
 			pk := r.p
 			pk.laterFor(named, func() {
@@ -508,6 +504,28 @@ func (pr *pkgReader) objIdx(idx pkgbits.Index) (*types.Package, string) {
 					f()                        // initialize RHS
 				}
 				underlying := rhs.Underlying()
+
+				// If the underlying type is an interface, we need to
+				// duplicate its methods so we can replace the receiver
+				// parameter's type (#49906).
+				if iface, ok := underlying.(*types.Interface); ok && iface.NumExplicitMethods() != 0 {
+					methods := make([]*types.Func, iface.NumExplicitMethods())
+					for i := range methods {
+						fn := iface.ExplicitMethod(i)
+						sig := fn.Type().(*types.Signature)
+
+						recv := types.NewVar(fn.Pos(), fn.Pkg(), "", named)
+						methods[i] = types.NewFunc(fn.Pos(), fn.Pkg(), fn.Name(), types.NewSignature(recv, sig.Params(), sig.Results(), sig.Variadic()))
+					}
+
+					embeds := make([]types.Type, iface.NumEmbeddeds())
+					for i := range embeds {
+						embeds[i] = iface.EmbeddedType(i)
+					}
+
+					underlying = types.NewInterfaceType(methods, embeds)
+				}
+
 				named.SetUnderlying(underlying)
 			})
 
diff --git a/src/go/types/eval_test.go b/src/go/types/eval_test.go
index 6f5b548eb2..b0745c16d9 100644
--- a/src/go/types/eval_test.go
+++ b/src/go/types/eval_test.go
@@ -12,7 +12,6 @@ import (
 	"go/importer"
 	"go/parser"
 	"go/token"
-	"internal/goexperiment"
 	"internal/testenv"
 	"strings"
 	"testing"
@@ -209,7 +208,7 @@ func TestCheckExpr(t *testing.T) {
 	// expr is an identifier or selector expression that is passed
 	// to CheckExpr at the position of the comment, and object is
 	// the string form of the object it denotes.
-	src := `
+	const src = `
 package p
 
 import "fmt"
@@ -236,13 +235,6 @@ func f(a int, s string) S {
 	return S{}
 }`
 
-	// The unified IR importer always sets interface method receiver
-	// parameters to point to the Interface type, rather than the Named.
-	// See #49906.
-	if goexperiment.Unified {
-		src = strings.ReplaceAll(src, "func (fmt.Stringer).", "func (interface).")
-	}
-
 	fset := token.NewFileSet()
 	f, err := parser.ParseFile(fset, "p", src, parser.ParseComments)
 	if err != nil {