go/analysis/passes/nilness: handle type assertions too

This change causes nilness to detect patterns of the form:

	ptr, ok := x.(T)
	if !ok {
		println(*ptr) // nil dereference
	}

It found three bugs in k8s, e.g.

	// pkg/controller/volume/persistentvolume/pv_controller.go
	claim, ok = obj.(*v1.PersistentVolumeClaim)
	if !ok {
		return fmt.Errorf("cannot convert object from volume cache to volume %q!?: %#v", claim.Spec.VolumeName, obj)
	}

Fixes golang/go#47938

Change-Id: Idb4588ce037cb898ccc3feb460ea0b67f65803df
Reviewed-on: https://go-review.googlesource.com/c/tools/+/560075
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Tim King <taking@google.com>

go/analysis/passes/nilness/nilness.go

@@ -189,6 +189,42 @@ func runFunc(pass *analysis.Pass, fn *ssa.Function) {
 			}
 		}
 
+		// In code of the form:
+		//
+		// 	if ptr, ok := x.(*T); ok { ... } else { fsucc }
+		//
+		// the fsucc block learns that ptr == nil,
+		// since that's its zero value.
+		if If, ok := b.Instrs[len(b.Instrs)-1].(*ssa.If); ok {
+			// Handle "if ok" and "if !ok" variants.
+			cond, fsucc := If.Cond, b.Succs[1]
+			if unop, ok := cond.(*ssa.UnOp); ok && unop.Op == token.NOT {
+				cond, fsucc = unop.X, b.Succs[0]
+			}
+
+			// Match pattern:
+			//   t0 = typeassert (pointerlike)
+			//   t1 = extract t0 #0  // ptr
+			//   t2 = extract t0 #1  // ok
+			//   if t2 goto tsucc, fsucc
+			if extract1, ok := cond.(*ssa.Extract); ok && extract1.Index == 1 {
+				if assert, ok := extract1.Tuple.(*ssa.TypeAssert); ok &&
+					isNillable(assert.AssertedType) {
+					for _, pinstr := range *assert.Referrers() {
+						if extract0, ok := pinstr.(*ssa.Extract); ok &&
+							extract0.Index == 0 &&
+							extract0.Tuple == extract1.Tuple {
+							for _, d := range b.Dominees() {
+								if len(d.Preds) == 1 && d == fsucc {
+									visit(d, append(stack, fact{extract0, isnil}))
+								}
+							}
+						}
+					}
+				}
+			}
+		}
+
 		for _, d := range b.Dominees() {
 			visit(d, stack)
 		}
@@ -360,3 +396,23 @@ func (ff facts) negate() facts {
 	}
 	return nn
 }
+
+func is[T any](x any) bool {
+	_, ok := x.(T)
+	return ok
+}
+
+func isNillable(t types.Type) bool {
+	switch t := typeparams.CoreType(t).(type) {
+	case *types.Pointer,
+		*types.Map,
+		*types.Signature,
+		*types.Chan,
+		*types.Interface,
+		*types.Slice:
+		return true
+	case *types.Basic:
+		return t == types.Typ[types.UnsafePointer]
+	}
+	return false
+}

go/analysis/passes/nilness/testdata/src/a/a.go

@@ -216,3 +216,29 @@ func f14() {
 		print(x)
 	}
 }
+
+func f15(x any) {
+	ptr, ok := x.(*int)
+	if ok {
+		return
+	}
+	println(*ptr) // want "nil dereference in load"
+}
+
+func f16(x any) {
+	ptr, ok := x.(*int)
+	if !ok {
+		println(*ptr) // want "nil dereference in load"
+		return
+	}
+	println(*ptr)
+}
+
+func f18(x any) {
+	ptr, ok := x.(*int)
+	if ok {
+		println(ptr)
+		// falls through
+	}
+	println(*ptr)
+}