text/template: correct assignment, not declaration, in range

We were mishandling {{range $i = .}}, treating it as though it were
{{range $i := .}}. That happened to work if $i were the most recently
declared variable, but not otherwise.

Fixes #56490

Change-Id: I222a009d671d86c06a980a54388e05f12101c00b
Reviewed-on: https://go-review.googlesource.com/c/go/+/446795
Run-TryBot: Ian Lance Taylor <iant@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Rob Pike <r@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>

src/text/template/exec.go

@@ -363,11 +363,19 @@ func (s *state) walkRange(dot reflect.Value, r *parse.RangeNode) {
 	oneIteration := func(index, elem reflect.Value) {
 		// Set top var (lexically the second if there are two) to the element.
 		if len(r.Pipe.Decl) > 0 {
-			s.setTopVar(1, elem)
+			if r.Pipe.IsAssign {
+				s.setVar(r.Pipe.Decl[0].Ident[0], elem)
+			} else {
+				s.setTopVar(1, elem)
+			}
 		}
 		// Set next var (lexically the first if there are two) to the index.
 		if len(r.Pipe.Decl) > 1 {
-			s.setTopVar(2, index)
+			if r.Pipe.IsAssign {
+				s.setVar(r.Pipe.Decl[1].Ident[0], index)
+			} else {
+				s.setTopVar(2, index)
+			}
 		}
 		defer s.pop(mark)
 		defer func() {

src/text/template/exec_test.go

@@ -692,6 +692,8 @@ var execTests = []execTest{
 	{"bug18a", "{{eq . '.'}}", "true", '.', true},
 	{"bug18b", "{{eq . 'e'}}", "true", 'e', true},
 	{"bug18c", "{{eq . 'P'}}", "true", 'P', true},
+
+	{"issue56490", "{{$i := 0}}{{$x := 0}}{{range $i = .AI}}{{end}}{{$i}}", "5", tVal, true},
 }
 
 func zeroArgs() string {