pkg/eval: Restore the compilation order of var forms.

This fixes #1829.

pkg/eval/builtin_special.go

@@ -206,13 +206,15 @@ func compileLHSRHS(cp *compiler, args []*parse.Compound, end int, lf lvalueFlag)
 func compileLHSOptionalRHS(cp *compiler, args []*parse.Compound, end int, lf lvalueFlag) (lvaluesGroup, valuesOp) {
 	for i, cn := range args {
 		if parse.SourceText(cn) == "=" {
-			lhs := cp.compileCompoundLValues(args[:i], lf)
+			var rhs valuesOp
 			if i == len(args)-1 {
-				return lhs, nopValuesOp{diag.PointRanging(end)}
+				rhs = nopValuesOp{diag.PointRanging(end)}
+			} else {
+				rhs = seqValuesOp{
+					diag.MixedRanging(args[i+1], args[len(args)-1]),
+					cp.compoundOps(args[i+1:])}
 			}
-			return lhs, seqValuesOp{
-				diag.MixedRanging(args[i+1], args[len(args)-1]),
-				cp.compoundOps(args[i+1:])}
+			return cp.compileCompoundLValues(args[:i], lf), rhs
 		}
 	}
 	return cp.compileCompoundLValues(args, lf), nil

pkg/eval/builtin_special_test.elvts

@@ -76,6 +76,12 @@ Compilation error: invalid value for unknown-command: bad
 ▶ new
 ▶ old
 
+## RHS sees old variable when shadowing (https://b.elv.sh/1829) ##
+~> var x = foo
+~> var x = [$x]
+~> put $x
+▶ [foo]
+
 ## concurrent creation and access ##
 // Ensure that there is no race with "go test -race"
 ~> var x = 1

website/ref/language.md

@@ -1699,7 +1699,7 @@ function as a rest variable.
 
 When declaring a variable that already exists, the existing variable is
 shadowed. The shadowed variable may still be accessed indirectly if it is
-referenced by a function. Example:
+previously referenced by a function. Example:
 
 ```elvish-transcript
 ~> var x = old
@@ -1711,6 +1711,16 @@ referenced by a function. Example:
 ▶ old
 ```
 
+If the right-hand-side of the `var` command references the variable being
+shadowed, it sees the old variable:
+
+```elvish-transcript
+~> var x = foo
+~> var x = [$x] # $x in RHS refers to old $x
+~> put $x
+▶ [foo]
+```
+
 ## Assigning variables or elements: `set` {#set}
 
 The `set` special command sets the value of variables or elements.