Don't compile the arguments of special commands twice.

This fixes #1204.

pkg/eval/compile_effect.go

@@ -184,11 +184,13 @@ func (cp *compiler) formOp(n *parse.Form) effectOp {
 
 	if n.Head != nil {
 		headStr, ok := oneString(n.Head)
+		argOpsNeeded := true
 		if ok {
 			special, fnRef := resolveCmdHeadInternally(cp, headStr, n.Head)
 			switch {
 			case special != nil:
 				specialOp = special(cp, n)
+				argOpsNeeded = false
 			case fnRef != nil:
 				headOp = variableOp{n.Head.Range(), false, headStr + FnSuffix, fnRef}
 			default:
@@ -199,7 +201,9 @@ func (cp *compiler) formOp(n *parse.Form) effectOp {
 			// expression.
 			headOp = cp.compoundOp(n.Head)
 		}
+		if argOpsNeeded {
 			argOps = cp.compoundOps(n.Args)
+		}
 	} else {
 		// Assignment form.
 		lhs := cp.parseCompoundLValues(n.Vars)

pkg/eval/compile_effect_test.go

@@ -70,6 +70,10 @@ func TestCompileEffect(t *testing.T) {
 		// Command errors when any optional evaluation errors.
 		That("put &x=[][1]").Throws(ErrorWithType(errs.OutOfRange{}), "[][1]"),
 
+		// Regression test for #1204; ensures that the arguments of special
+		// forms are not accidentally compiled twice.
+		That("nop (and (use builtin)); nop $builtin:echo~").DoesNothing(),
+
 		// Assignments
 		// -----------