text/template: compare reflect.Value instances differently

To avoid false positives from the reflectvaluecompare checker #43993

Use v.IsValid() instead of

var zero reflect.Value
v != zero

Also avoid comparing directly with the singleton reflect.Value
representing a missing value. Detect the missing value by type instead.

Change-Id: I3a00d63cf61c077e7c7ae816474aa1f032be325b
Reviewed-on: https://go-review.googlesource.com/c/go/+/308769
Reviewed-by: Cherry Mui <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Run-TryBot: Keith Randall <[email protected]>
Reviewed-by: Daniel Martí <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>

src/text/template/exec.go

@@ -94,6 +94,12 @@ type missingValType struct{}
 
 var missingVal = reflect.ValueOf(missingValType{})
 
+var missingValReflectType = reflect.TypeOf(missingValType{})
+
+func isMissing(v reflect.Value) bool {
+ return v.IsValid() && v.Type() == missingValReflectType
+}
+
 // at marks the state to be on node n, for error reporting.
 func (s *state) at(node parse.Node) {
  s.node = node
@@ -471,7 +477,7 @@ func (s *state) evalPipeline(dot reflect.Value, pipe *parse.PipeNode) (value ref
 }
 
 func (s *state) notAFunction(args []parse.Node, final reflect.Value) {
- if len(args) > 1 || final != missingVal {
+ if len(args) > 1 || !isMissing(final) {
  s.errorf("can't give argument to non-function %s", args[0])
  }
 }
@@ -629,7 +635,7 @@ func (s *state) evalField(dot reflect.Value, fieldName string, node parse.Node,
  if method := ptr.MethodByName(fieldName); method.IsValid() {
  return s.evalCall(dot, method, false, node, fieldName, args, final)
  }
- hasArgs := len(args) > 1 || final != missingVal
+ hasArgs := len(args) > 1 || !isMissing(final)
  // It's not a method; must be a field of a struct or an element of a map.
  switch receiver.Kind() {
  case reflect.Struct:
@@ -700,7 +706,7 @@ func (s *state) evalCall(dot, fun reflect.Value, isBuiltin bool, node parse.Node
  }
  typ := fun.Type()
  numIn := len(args)
- if final != missingVal {
+ if !isMissing(final) {
  numIn++
  }
  numFixed := len(args)
@@ -763,7 +769,7 @@ func (s *state) evalCall(dot, fun reflect.Value, isBuiltin bool, node parse.Node
  }
  }
  // Add final value if necessary.
- if final != missingVal {
+ if !isMissing(final) {
  t := typ.In(typ.NumIn() - 1)
  if typ.IsVariadic() {
  if numIn-1 < numFixed {

src/text/template/funcs.go

@@ -438,7 +438,7 @@ func basicKind(v reflect.Value) (kind, error) {
 
 // isNil returns true if v is the zero reflect.Value, or nil of its type.
 func isNil(v reflect.Value) bool {
- if v == zero {
+ if !v.IsValid() {
  return true
  }
  switch v.Kind() {