go/ssa: use ZeroString unconditionally

Follow up to https://go.dev/cl/626537

Change-Id: Ib8dee860c5ca65ea3dab8ef68564f9fe46c74832
Reviewed-on: https://go-review.googlesource.com/c/tools/+/628535
Reviewed-by: Robert Findley <rfindley@google.com>
Commit-Queue: Tim King <taking@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

go/callgraph/vta/testdata/src/callgraph_interfaces.go

@@ -49,11 +49,11 @@ func Baz(b bool) {
 
 // func Do(b bool) I:
 //    ...
-//   t1 = (C).Foo(struct{}{}:C)
+//   t1 = (C).Foo(C{}:C)
 //   t2 = NewB()
 //   t3 = make I <- B (t2)
 //   return t3
 
 // WANT:
 // Baz: Do(b) -> Do; invoke t0.Foo() -> A.Foo, B.Foo
-// Do: (C).Foo(struct{}{}:C) -> C.Foo; NewB() -> NewB
+// Do: (C).Foo(C{}:C) -> C.Foo; NewB() -> NewB

go/callgraph/vta/testdata/src/callgraph_static.go

@@ -22,7 +22,7 @@ func Baz(a A) {
 // func Baz(a A):
 //   t0 = (A).foo(a)
 //   t1 = Bar()
-//   t2 = Baz(struct{}{}:A)
+//   t2 = Baz(A{}:A)
 
 // WANT:
-// Baz: (A).foo(a) -> A.foo; Bar() -> Bar; Baz(struct{}{}:A) -> Baz
+// Baz: (A).foo(a) -> A.foo; Bar() -> Bar; Baz(A{}:A) -> Baz

go/callgraph/vta/testdata/src/callgraph_type_aliases.go

@@ -58,11 +58,11 @@ func Baz(b bool) {
 
 // func Do(b bool) I:
 //    ...
-//   t1 = (C).Foo(struct{}{}:Z)
+//   t1 = (C).Foo(Z{}:Z)
 //   t2 = NewY()
 //   t3 = make I <- B (t2)
 //   return t3
 
 // WANT:
 // Baz: Do(b) -> Do; invoke t0.Foo() -> A.Foo, B.Foo
-// Do: (C).Foo(struct{}{}:Z) -> C.Foo; NewY() -> NewY
+// Do: (C).Foo(Z{}:Z) -> C.Foo; NewY() -> NewY

go/callgraph/vta/vta_test.go

@@ -21,7 +21,7 @@ import (
 )
 
 func TestVTACallGraph(t *testing.T) {
-	errDiff := func(want, got, missing []string) {
+	errDiff := func(t *testing.T, want, got, missing []string) {
 		t.Errorf("got:\n%s\n\nwant:\n%s\n\nmissing:\n%s\n\ndiff:\n%s",
 			strings.Join(got, "\n"),
 			strings.Join(want, "\n"),
@@ -60,14 +60,14 @@ func TestVTACallGraph(t *testing.T) {
 			g := CallGraph(ssautil.AllFunctions(prog), nil)
 			got := callGraphStr(g)
 			if missing := setdiff(want, got); len(missing) > 0 {
-				errDiff(want, got, missing)
+				errDiff(t, want, got, missing)
 			}
 
 			// Repeat the test with explicit CHA initial call graph.
 			g = CallGraph(ssautil.AllFunctions(prog), cha.CallGraph(prog))
 			got = callGraphStr(g)
 			if missing := setdiff(want, got); len(missing) > 0 {
-				errDiff(want, got, missing)
+				errDiff(t, want, got, missing)
 			}
 		})
 	}

go/ssa/const.go

@@ -78,13 +78,7 @@ func zeroConst(t types.Type) *Const {
 func (c *Const) RelString(from *types.Package) string {
 	var s string
 	if c.Value == nil {
-		if _, ok := c.typ.(*types.TypeParam); ok {
-			// Type parameter's underlying type may be interface that is
-			// nillable. A better zero value of type parameter is *new(T).
-			s = typesinternal.ZeroString(c.typ, types.RelativeTo(from))
-		} else {
-			s = typesinternal.ZeroString(c.typ.Underlying(), types.RelativeTo(from))
-		}
+		s = typesinternal.ZeroString(c.typ, types.RelativeTo(from))
 	} else if c.Value.Kind() == constant.String {
 		s = constant.StringVal(c.Value)
 		const max = 20