[SUGGESTION] Add possibility of using lambda in is-statements (inspect and if statements)

include/cpp2util.h

@@ -615,6 +615,119 @@ auto is( X const& x ) -> bool {
     return x == X();
 }
 
+//-------------------------------------------------------------------------------------------------------------
+//  Built-in is (literals)
+//
+
+template< auto value, typename X, typename V = CPP2_TYPEOF(value)>
+    // This requires are needed for gcc compiler to avoid ambiguity when value is double
+    requires (
+        (std::floating_point<V> && !std::floating_point<X>) ||
+        (std::integral<V> && !std::integral<X>) ||
+        (std::is_enum_v<V> && !std::is_same_v<V, X> )
+    )
+constexpr auto is( X const& x ) -> bool {
+    return false;
+}
+
+template< auto value, typename X >
+    requires (std::is_enum_v<CPP2_TYPEOF(value)> && std::is_same_v<X,CPP2_TYPEOF(value)>)
+constexpr auto is( X const& x ) -> bool {
+    return x == value;
+}
+
+template< auto value, typename X >
+    requires std::integral<CPP2_TYPEOF(value)> && std::integral<X>
+constexpr auto is( X const& x ) -> bool {
+    return x == value;
+}
+
+// Workaroud for lack of support for floating point template argument by clang and MSVC
+struct double_wrapper {
+    double value = {};
+
+    template <typename T>
+        requires std::is_floating_point_v<T>
+    constexpr double_wrapper(T d) : value(d) {}
+
+    constexpr operator double() const {
+        return value;
+    }
+
+    template <typename T>
+    constexpr bool operator==(T rhs) const {
+        return value == rhs;
+    }
+};
+
+// This is needed to solve some issue with clang - probably clang bug.
+// Without it clang is not able to match with default specialisation:
+// template< auto value, typename X > auto is( X const& x ) -> bool;
+#if defined(__clang__)
+
+template< double_wrapper value, typename X >
+constexpr auto is( X const& x ) -> bool {
+    if constexpr (std::is_floating_point_v<X>)
+        return x == value;
+    else
+        return false;
+}
+
+#else
+
+template< double_wrapper value, typename X >
+    requires std::is_floating_point_v<X>
+constexpr auto is( X const& x ) -> bool {
+        return x == value;
+}
+
+#endif
+
+template <auto N>
+struct cstring_wrapper {
+    char cs[N];
+
+    constexpr cstring_wrapper(const char (&s)[N]) noexcept {
+        std::copy(s, s+N, cs);
+    }
+
+    constexpr bool operator==(const std::string_view& sv) const noexcept {
+        return std::equal(cs, cs+N-1, sv.begin(), sv.end()); // N-1 as sv is not null-terminated
+    }
+};
+
+template< cstring_wrapper value, typename X >
+constexpr auto is( X const& x ) -> bool {
+    if constexpr (std::is_convertible_v<X, std::string_view>)
+        return value == x;
+    else
+        return false;
+}
+
+template <typename F, typename Capture = std::monostate>
+    requires (
+        requires { &F::operator(); }
+    )
+struct lambda_wrapper {
+    F f;
+    Capture capture;
+
+    constexpr lambda_wrapper(F f, Capture capture = {})
+    : f{f}, capture{capture} {}
+
+    template <typename X>
+    auto operator()(X&& x) const {
+        if constexpr (std::is_same_v<Capture, std::monostate>)
+            return f(std::forward<X>(x));
+        else
+            return f(std::forward<X>(x), capture);
+    }
+};
+
+template< lambda_wrapper value, typename X >
+constexpr auto is( X const& x ) -> bool {
+    return value(x);
+}
 
 //-------------------------------------------------------------------------------------------------------------
 //  Built-in as (partial)

source/cppfront.cpp

@@ -1282,8 +1282,17 @@ class cppfront
 
             auto id = std::string{};
             printer.emit_to_string(&id);
-            assert(alt->id_expression);
-            emit(*alt->id_expression);
+            assert(alt->id_expression || alt->literal);
+            if (alt->id_expression) {
+                emit(*alt->id_expression);
+                if (alt->expr) {
+                    push_need_expression_list_parens(true);
+                    emit(*alt->expr);
+                    pop_need_expression_list_parens();
+                }
+            } else if (alt->literal) {
+                emit(*alt->literal);
+            }
             printer.emit_to_string();
 
             assert (*alt->is_as_keyword == "is" || *alt->is_as_keyword == "as");

source/parse.h

@@ -667,6 +667,8 @@ struct alternative_node
     std::unique_ptr<id_expression_node>  id_expression;
     source_position                      equal_sign;
     std::unique_ptr<statement_node>      statement;
+    token const*                         literal;
+    std::unique_ptr<expression_list_node> expr;
 
     auto position() const -> source_position
     {
@@ -776,8 +778,11 @@ auto alternative_node::visit(auto& v, int depth) -> void
     }
     assert (is_as_keyword);
     v.start(*is_as_keyword, depth+1);
-    assert (id_expression);
-    id_expression->visit(v, depth+1);
+    assert (id_expression || literal);
+    if (id_expression)
+        id_expression->visit(v, depth+1);
+    else if (literal)
+        literal->visit(v, depth+1);
     assert (statement);
     statement->visit(v, depth+1);
     v.end(*this, depth);
@@ -2346,11 +2351,34 @@ class parser
         if (auto id = id_expression()) {
             n->id_expression = std::move(id);
         }
+        else if (is_literal(curr().type())) {
+            n->literal = &curr();
+            next();
+        }
         else {
-            error("expected id-expression after 'is' in inspect alternative");
+            error("expected id-expression or literal after 'is' in inspect alternative");
             return {};
         }
 
+        if (curr().type() == lexeme::LeftParen) {
+            auto open_paren = curr().position();
+            next();
+            auto expr_list = expression_list(open_paren);
+            if (!expr_list) {
+                error("unexpected text - ( is not followed by an expression-list");
+                next();
+                return {};
+            }
+            if (curr().type() != lexeme::RightParen) {
+                error("unexpected text - expression-list is not terminated by )");
+                next();
+                return {};
+            }
+            expr_list->close_paren = curr().position();
+            next();
+            n->expr = std::move(expr_list);
+        }
+
         if (curr().type() != lexeme::Assignment) {
             error("expected = at start of inspect alternative body");
             return {};