From 9a9aad449fddb1d3f66e6200d2c6c63a0eed63a4 Mon Sep 17 00:00:00 2001
From: Keith Randall <khr@golang.org>
Date: Thu, 8 Apr 2021 17:06:29 -0700
Subject: [PATCH] text/template: compare reflect.Value instances differently
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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 <cherryyz@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Keith Randall <khr@golang.org>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Reviewed-by: Ian Lance Taylor <iant@google.com>
---
 src/text/template/exec.go  | 14 ++++++++++----
 src/text/template/funcs.go |  2 +-
 2 files changed, 11 insertions(+), 5 deletions(-)

diff --git a/src/text/template/exec.go b/src/text/template/exec.go
index 37984cf91a0..66cb535c47f 100644
--- a/src/text/template/exec.go
+++ b/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 {
diff --git a/src/text/template/funcs.go b/src/text/template/funcs.go
index 390d47ebbb6..42bb529e509 100644
--- a/src/text/template/funcs.go
+++ b/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() {