gopls/internal/golang/completion: fix crash adding receiver type params

Fix an inaccurate assumption that type names in receiver position must
be Named or Alias types. In the presence of invalid code, such objects
could also be have Basic type.

Fixes golang/go#71044

Change-Id: Iad6a3f09aa210e7eee2edf82cec6a990bc137e17
Reviewed-on: https://go-review.googlesource.com/c/tools/+/643016
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Alan Donovan <adonovan@google.com>

Author: findleyr

gopls/internal/golang/completion/format.go

@@ -20,7 +20,6 @@ import (
 	"golang.org/x/tools/gopls/internal/util/typesutil"
 	"golang.org/x/tools/internal/event"
 	"golang.org/x/tools/internal/imports"
-	"golang.org/x/tools/internal/typesinternal"
 )
 
 var (
@@ -60,9 +59,12 @@ func (c *completer) item(ctx context.Context, cand candidate) (CompletionItem, e
 	if obj.Type() == nil {
 		detail = ""
 	}
-	if isTypeName(obj) && c.wantTypeParams() {
-		// obj is a *types.TypeName, so its type must be Alias|Named.
-		tparams := typesinternal.TypeParams(obj.Type().(typesinternal.NamedOrAlias))
+
+	type hasTypeParams interface{ TypeParams() *types.TypeParamList }
+	if genericType, _ := obj.Type().(hasTypeParams); genericType != nil && isTypeName(obj) && c.wantTypeParams() {
+		// golang/go#71044: note that type names can be basic types, even in
+		// receiver position, for invalid code.
+		tparams := genericType.TypeParams()
 		label += typesutil.FormatTypeParams(tparams)
 		insert = label // maintain invariant above (label == insert)
 	}

gopls/internal/test/marker/testdata/fixedbugs/issue71044.txt

@@ -0,0 +1,18 @@
+This test checks that we don't crash while completing receivers that may happen
+to be builtin types (due to invalid code). This crash was reported by telemetry
+in golang/go#71044.
+
+-- flags --
+-ignore_extra_diags
+
+-- go.mod --
+module example.com/amap
+
+go 1.18
+
+-- a.go --
+package amap
+
+import "unsafe"
+
+func (unsafe.Pointer) _() {} //@ rank("unsafe")