restore proper handling of no formals vs. 0 formals

e.g. (foo@{}: 1) { a = 3; } should error, but wasn't with the previous
commit

src/libexpr/eval.cc

@@ -1496,13 +1496,13 @@ void EvalState::callFunction(Value & fun, std::span<Value *> args, Value & vRes,
 
             ExprLambda & lambda(*vCur.lambda().fun);
 
-            auto size = (!lambda.arg ? 0 : 1) + lambda.nFormals;
+            auto size = (!lambda.arg ? 0 : 1) + (lambda.hasFormals ? lambda.getFormals().size() : 0);
             Env & env2(mem.allocEnv(size));
             env2.up = vCur.lambda().env;
 
             Displacement displ = 0;
 
-            if (!lambda.hasFormals())
+            if (!lambda.hasFormals)
                 env2.values[displ++] = args[0];
             else {
                 try {
@@ -1747,7 +1747,7 @@ void EvalState::autoCallFunction(const Bindings & args, Value & fun, Value & res
         }
     }
 
-    if (!fun.isLambda() || !fun.lambda().fun->hasFormals()) {
+    if (!fun.isLambda() || !fun.lambda().fun->hasFormals) {
         res = fun;
         return;
     }

src/libexpr/include/nix/expr/nixexpr.hh

@@ -490,6 +490,7 @@ struct ExprLambda : Expr
     Symbol arg;
 
     bool ellipsis;
+    bool hasFormals;
     uint16_t nFormals;
     Formal * formalsStart;
 
@@ -501,6 +502,7 @@ struct ExprLambda : Expr
         : pos(pos)
         , arg(arg)
         , ellipsis(formals.ellipsis)
+        , hasFormals(true)
         , nFormals(formals.formals.size())
         , formalsStart(alloc.allocate_object<Formal>(nFormals))
         , body(body)
@@ -511,7 +513,7 @@ struct ExprLambda : Expr
     ExprLambda(std::pmr::polymorphic_allocator<char> & alloc, PosIdx pos, Symbol arg, Expr * body)
         : pos(pos)
         , arg(arg)
-        , nFormals(0)
+        , hasFormals(false)
         , formalsStart(nullptr)
         , body(body) {};
 
@@ -529,11 +531,6 @@ struct ExprLambda : Expr
     void setName(Symbol name) override;
     std::string showNamePos(const EvalState & state) const;
 
-    inline bool hasFormals() const
-    {
-        return nFormals > 0;
-    }
-
     std::vector<Formal> getFormalsLexicographic(const SymbolTable & symbols) const
     {
         std::vector<Formal> result(getFormals().begin(), getFormals().end());
@@ -551,6 +548,7 @@ struct ExprLambda : Expr
 
     std::span<Formal> getFormals() const
     {
+        assert(hasFormals);
         return {formalsStart, nFormals};
     }
 

src/libexpr/nixexpr.cc

@@ -154,7 +154,7 @@ void ExprList::show(const SymbolTable & symbols, std::ostream & str) const
 void ExprLambda::show(const SymbolTable & symbols, std::ostream & str) const
 {
     str << "(";
-    if (hasFormals()) {
+    if (hasFormals) {
         str << "{ ";
         bool first = true;
         // the natural Symbol ordering is by creation time, which can lead to the
@@ -451,14 +451,14 @@ void ExprLambda::bindVars(EvalState & es, const std::shared_ptr<const StaticEnv>
     if (es.debugRepl)
         es.exprEnvs.insert(std::make_pair(this, env));
 
-    auto newEnv = std::make_shared<StaticEnv>(nullptr, env, nFormals + (!arg ? 0 : 1));
+    auto newEnv = std::make_shared<StaticEnv>(nullptr, env, (hasFormals ? getFormals().size() : 0) + (!arg ? 0 : 1));
 
     Displacement displ = 0;
 
     if (arg)
         newEnv->vars.emplace_back(arg, displ++);
 
-    if (hasFormals()) {
+    if (hasFormals) {
         for (auto & i : getFormals())
             newEnv->vars.emplace_back(i.name, displ++);
 

src/libexpr/primops.cc

@@ -3363,7 +3363,7 @@ static void prim_functionArgs(EvalState & state, const PosIdx pos, Value ** args
     if (!args[0]->isLambda())
         state.error<TypeError>("'functionArgs' requires a function").atPos(pos).debugThrow();
 
-    if (!args[0]->lambda().fun->hasFormals()) {
+    if (!args[0]->lambda().fun->hasFormals) {
         v.mkAttrs(&Bindings::emptyBindings);
         return;
     }

src/libexpr/value-to-xml.cc

@@ -145,7 +145,7 @@ static void printValueAsXML(
             posToXML(state, xmlAttrs, state.positions[v.lambda().fun->pos]);
         XMLOpenElement _(doc, "function", xmlAttrs);
 
-        if (v.lambda().fun->hasFormals()) {
+        if (v.lambda().fun->hasFormals) {
             XMLAttrs attrs;
             if (v.lambda().fun->arg)
                 attrs["name"] = state.symbols[v.lambda().fun->arg];