cmd/api: include type arguments in API

T[A] and T[B] are different types, but we were writing them both as
just "T". Reported by Russ.

Change-Id: I27db35946b6d3a1fad6c56c785fa6d850ad71d00
Reviewed-on: https://go-review.googlesource.com/c/go/+/558716
Reviewed-by: Robert Findley <[email protected]>
Auto-Submit: Matthew Dempsky <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>

Author: mdempsky

src/cmd/api/main_test.go

@@ -851,6 +851,16 @@ func (w *Walker) writeType(buf *bytes.Buffer, typ types.Type) {
 			buf.WriteByte('.')
 		}
 		buf.WriteString(typ.Obj().Name())
+		if targs := typ.TypeArgs(); targs.Len() > 0 {
+			buf.WriteByte('[')
+			for i := 0; i < targs.Len(); i++ {
+				if i > 0 {
+					buf.WriteString(", ")
+				}
+				w.writeType(buf, targs.At(i))
+			}
+			buf.WriteByte(']')
+		}
 
 	case *types.TypeParam:
 		// Type parameter names may change, so use a placeholder instead.

src/cmd/api/testdata/src/pkg/p4/golden.txt

@@ -1,4 +1,4 @@
-pkg p4, func NewPair[$0 interface{ M }, $1 interface{ ~int }]($0, $1) Pair
+pkg p4, func NewPair[$0 interface{ M }, $1 interface{ ~int }]($0, $1) Pair[$0, $1]
 pkg p4, method (Pair[$0, $1]) Second() $1
 pkg p4, method (Pair[$0, $1]) First() $0
 pkg p4, type Pair[$0 interface{ M }, $1 interface{ ~int }] struct