fix(cpp1): support empty and multi-argument indexing

regression-tests/pure2-bugfix-for-empty-index.cpp2

@@ -0,0 +1,10 @@
+t: type = {
+  operator[]: (inout this) -> i32 = 1;
+  operator[]: (inout this, x: _) -> i32 = 2;
+  operator[]: (inout this, x: _, y: _) -> i32 = 3;
+}
+main: () = {
+  [[assert: t()[] == 1]]
+  [[assert: t()[1] == 2]]
+  [[assert: t()[1, 2] == 3]]
+}

regression-tests/test-results/pure2-bugfix-for-empty-index.cpp

@@ -0,0 +1,42 @@
+
+#define CPP2_USE_MODULES         Yes
+
+//=== Cpp2 type declarations ====================================================
+
+
+#include "cpp2util.h"
+
+#line 1 "pure2-bugfix-for-empty-index.cpp2"
+class t;
+
+
+//=== Cpp2 type definitions and function declarations ===========================
+
+#line 1 "pure2-bugfix-for-empty-index.cpp2"
+class t {
+  public: [[nodiscard]] auto operator[]() -> cpp2::i32;
+  public: [[nodiscard]] auto operator[](auto const& x) -> cpp2::i32;
+  public: [[nodiscard]] auto operator[](auto const& x, auto const& y) -> cpp2::i32;
+
+  public: t() = default;
+  public: t(t const&) = delete; /* No 'that' constructor, suppress copy */
+  public: auto operator=(t const&) -> void = delete;
+#line 5 "pure2-bugfix-for-empty-index.cpp2"
+};
+auto main() -> int;
+
+
+//=== Cpp2 function definitions =================================================
+
+
+#line 2 "pure2-bugfix-for-empty-index.cpp2"
+  [[nodiscard]] auto t::operator[]() -> cpp2::i32 { return 1;  }
+  [[nodiscard]] auto t::operator[](auto const& x) -> cpp2::i32 { return 2;  }
+  [[nodiscard]] auto t::operator[](auto const& x, auto const& y) -> cpp2::i32 { return 3;  }
+
+auto main() -> int{
+  cpp2::Default.expects(t()[]==1, "");
+  cpp2::Default.expects(cpp2::assert_in_bounds(t(), 1)==2, "");
+  cpp2::Default.expects(t()[1, 2]==3, "");
+}
+

regression-tests/test-results/pure2-bugfix-for-empty-index.cpp2.output

@@ -0,0 +1,2 @@
+pure2-bugfix-for-empty-index.cpp2... ok (all Cpp2, passes safety checks)
+

source/cppfront.cpp

@@ -3029,6 +3029,7 @@ class cppfront
                 if (
                     flag_safe_subscripts
                     && i->op->type() == lexeme::LeftBracket
+                    && std::ssize(i->expr_list->expressions) == 1
                     )
                 {
                     suffix.emplace_back( ")", i->op->position() );
@@ -3065,6 +3066,7 @@ class cppfront
                 if (
                     flag_safe_subscripts
                     && i->op->type() == lexeme::LeftBracket
+                    && std::ssize(i->expr_list->expressions) == 1
                     )
                 {
                     prefix.emplace_back( "cpp2::assert_in_bounds(", i->op->position() );

source/parse.h

@@ -4084,7 +4084,7 @@ class parser
     //G postfix-expression:
     //G     primary-expression
     //G     postfix-expression postfix-operator     [Note: without whitespace before the operator]
-    //G     postfix-expression '[' expression-list ']'
+    //G     postfix-expression '[' expression-list? ']'
     //G     postfix-expression '(' expression-list? ')'
     //G     postfix-expression '.' id-expression
     //G
@@ -4161,13 +4161,9 @@ class parser
             if (term.op->type() == lexeme::LeftBracket)
             {
                 term.expr_list = expression_list(term.op);
-                if (
-                    !term.expr_list
-                    || term.expr_list->expressions.empty()
-                    )
+                if (!term.expr_list)
                 {
-                    error("subscript expression [ ] must not be empty (if you were trying to name a C-style array type, use 'std::array' instead)");
-                    next();
+                    error("[ is not followed by a valid expression list");
                     return {};
                 }
                 if (curr().type() != lexeme::RightBracket)