text/template: set variables correctly in range assignment

I unintentionally flipped them in CL 446795.

For #56490
Fixes #60801

Change-Id: I57586bec052e1b2cc61513870ce24dd6ce17e56b
Reviewed-on: https://go-review.googlesource.com/c/go/+/503575
Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Rob Pike <r@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Run-TryBot: Ian Lance Taylor <iant@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>

src/text/template/exec.go

@@ -361,19 +361,27 @@ func (s *state) walkRange(dot reflect.Value, r *parse.RangeNode) {
 	// mark top of stack before any variables in the body are pushed.
 	mark := s.mark()
 	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 {
 			if r.Pipe.IsAssign {
-				s.setVar(r.Pipe.Decl[0].Ident[0], elem)
+				// With two variables, index comes first.
+				// With one, we use the element.
+				if len(r.Pipe.Decl) > 1 {
+					s.setVar(r.Pipe.Decl[0].Ident[0], index)
+				} else {
+					s.setVar(r.Pipe.Decl[0].Ident[0], elem)
+				}
 			} else {
+				// Set top var (lexically the second if there
+				// are two) to the element.
 				s.setTopVar(1, elem)
 			}
 		}
-		// Set next var (lexically the first if there are two) to the index.
 		if len(r.Pipe.Decl) > 1 {
 			if r.Pipe.IsAssign {
-				s.setVar(r.Pipe.Decl[1].Ident[0], index)
+				s.setVar(r.Pipe.Decl[1].Ident[0], elem)
 			} else {
+				// Set next var (lexically the first if there
+				// are two) to the index.
 				s.setTopVar(2, index)
 			}
 		}

src/text/template/exec_test.go

@@ -694,6 +694,7 @@ var execTests = []execTest{
 	{"bug18c", "{{eq . 'P'}}", "true", 'P', true},
 
 	{"issue56490", "{{$i := 0}}{{$x := 0}}{{range $i = .AI}}{{end}}{{$i}}", "5", tVal, true},
+	{"issue60801", "{{$k := 0}}{{$v := 0}}{{range $k, $v = .AI}}{{$k}}={{$v}} {{end}}", "0=3 1=4 2=5 ", tVal, true},
 }
 
 func zeroArgs() string {