go/types: ensure that we always get a new signature in expandNamed

CL 349412 introduced a bug when Checker.subst does not return a new
signature: we were still setting the receiver to the instantiated type.

I'm not sure how this could manifest in practice (other than confusing
object strings). It's possible that I could generate a testdata-driven
test for this, but in the interest of time I just added a test to verify
object strings.

Change-Id: I29bc8e1419ddc4574755c3def52d18cb71c738eb
Reviewed-on: https://go-review.googlesource.com/c/go/+/350143
Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

src/go/types/instantiate_test.go

@@ -6,6 +6,7 @@ package types_test
 
 import (
 	. "go/types"
+	"strings"
 	"testing"
 )
 
@@ -109,3 +110,45 @@ var X T[int]
 		}
 	}
 }
+
+func TestImmutableSignatures(t *testing.T) {
+	const src = genericPkg + `p
+
+type T[P any] struct{}
+
+func (T[P]) m() {}
+
+var _ T[int]
+`
+	pkg, err := pkgFor(".", src, nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+	typ := pkg.Scope().Lookup("T").Type().(*Named)
+	obj, _, _ := LookupFieldOrMethod(typ, false, pkg, "m")
+	if obj == nil {
+		t.Fatalf(`LookupFieldOrMethod(%s, "m") = %v, want func m`, typ, obj)
+	}
+
+	// Verify that the original method is not mutated by instantiating T (this
+	// bug manifested when subst did not return a new signature).
+	want := "func (T[P]).m()"
+	if got := stripAnnotations(ObjectString(obj, RelativeTo(pkg))); got != want {
+		t.Errorf("instantiated %q, want %q", got, want)
+	}
+}
+
+// Copied from errors.go.
+func stripAnnotations(s string) string {
+	var b strings.Builder
+	for _, r := range s {
+		// strip #'s and subscript digits
+		if r < '₀' || '₀'+10 <= r { // '₀' == U+2080
+			b.WriteRune(r)
+		}
+	}
+	if b.Len() < len(s) {
+		return b.String()
+	}
+	return s
+}

src/go/types/named.go

@@ -309,6 +309,12 @@ func (check *Checker) completeMethod(env *Environment, m *Func) {
 
 	smap := makeSubstMap(origSig.RecvTypeParams().list(), rtyp.targs.list())
 	sig := check.subst(orig.pos, origSig, smap, env).(*Signature)
+	if sig == origSig {
+		// No substitution occurred, but we still need to create a copy to hold the
+		// instantiated receiver.
+		copy := *origSig
+		sig = &copy
+	}
 	sig.recv = NewParam(origSig.recv.pos, origSig.recv.pkg, origSig.recv.name, rtyp)
 
 	m.typ = sig