From 6487b5b5301f6cb37c95c3571578d5a42adb2a1a Mon Sep 17 00:00:00 2001
From: hachem <im@hachem.wtf>
Date: Sat, 19 Sep 2026 20:50:28 +0200
Subject: feat: declval

---
 ast.h                     |  1 +
 eval.c                    |  9 +++++++
 examples/fibonacci.sfinae | 68 +++++++++++++++++++++++++++++++++++++++++++++++
 sfinae.l                  |  1 +
 sfinae.y                  |  3 +++
 5 files changed, 82 insertions(+)
 create mode 100644 examples/fibonacci.sfinae

diff --git a/ast.h b/ast.h
index e8d4c45..2da24d8 100644
--- a/ast.h
+++ b/ast.h
@@ -33,6 +33,7 @@ enum NodeKind
     VALUE_INTEGRAL_CONSTANT,
     VALUE_CONDITIONAL,
     VALUE_VARIABLE,
+    VALUE_DECLVAL,
 
     COND_TRUE,
     COND_FALSE,
diff --git a/eval.c b/eval.c
index c872c90..aeae982 100644
--- a/eval.c
+++ b/eval.c
@@ -121,6 +121,15 @@ static int evaluate_value(struct Machine* machine, const struct Node* node, int6
 
             *out = variable->value;
             return 0;
+        case VALUE_DECLVAL:
+            if (evaluate_type(machine, node->first, &type))
+                return 1;
+
+            if (type.kind != KIND_VALUE)
+                return complain(node, "declval of a type with no value in it, what did you expect");
+
+            *out = type.number;
+            return 0;
         case VALUE_CONDITIONAL:
             if (evaluate_condition(machine, node->first, &truth))
                 return 1;
diff --git a/examples/fibonacci.sfinae b/examples/fibonacci.sfinae
new file mode 100644
index 0000000..844aa69
--- /dev/null
+++ b/examples/fibonacci.sfinae
@@ -0,0 +1,68 @@
+// fib(0) to fib(10), each line a tuple: some characters, the counter, and the call
+// ::declval<T>() pulls the value back out of a call result so it can go into ::sum
+
+::define<
+    ::name<'f', 'i', 'b'>,
+    ::parameters<::name<'n'>>,
+    ::sequence<
+        ::enable_if_t<
+            ::less<::value_of<::name<'n'>>, ::difference<'C', 'A'>>::value,
+            ::sequence<::return<::decltype(::value_of<::name<'n'>>)>::commit>
+        >::commit,
+        ::return<
+            ::decltype(
+                ::sum<
+                    ::declval<
+                        ::invoke_result_t<
+                            ::name<'f', 'i', 'b'>,
+                            ::arguments<
+                                ::decltype(::sum<::value_of<::name<'n'>>, ::difference<'A', 'B'>>)
+                            >
+                        >
+                    >(),
+                    ::declval<
+                        ::invoke_result_t<
+                            ::name<'f', 'i', 'b'>,
+                            ::arguments<
+                                ::decltype(::sum<::value_of<::name<'n'>>, ::difference<'A', 'C'>>)
+                            >
+                        >
+                    >()
+                >
+            )
+        >::commit
+    >
+>::commit;
+
+::declare<::name<'i'>, ::decltype(::difference<'A', 'A'>)>::commit;
+
+::repeat<
+    ::difference<'L', 'A'>,
+    ::sequence<
+        ::emit<
+            ::tuple<
+            ::integral_constant<::char, ::difference<'f', '\0'>>,
+            ::integral_constant<::char, ::difference<'i', '\0'>>,
+            ::integral_constant<::char, ::difference<'b', '\0'>>,
+            ::integral_constant<::char, ::difference<'(', '\0'>>,
+                ::decltype(::value_of<::name<'i'>>),
+            ::integral_constant<::char, ::difference<')', '\0'>>,
+            ::integral_constant<::char, ::difference<' ', '\0'>>,
+            ::integral_constant<::char, ::difference<'-', '\0'>>,
+            ::integral_constant<::char, ::difference<'>', '\0'>>,
+            ::integral_constant<::char, ::difference<' ', '\0'>>,
+                ::invoke_result_t<
+                    ::name<'f', 'i', 'b'>,
+                    ::arguments<::decltype(::value_of<::name<'i'>>)>
+                >
+            >
+        >::commit,
+        ::declare<
+            ::name<'n', 'e', 'x', 't'>,
+            ::decltype(::sum<::value_of<::name<'i'>>, ::difference<'B', 'A'>>)
+        >::commit,
+        ::undeclare<::name<'i'>>::commit,
+        ::declare<::name<'i'>, ::decltype(::value_of<::name<'n', 'e', 'x', 't'>>)>::commit,
+        ::undeclare<::name<'n', 'e', 'x', 't'>>::commit
+    >
+>::commit;
diff --git a/sfinae.l b/sfinae.l
index 63314c9..7157b7f 100644
--- a/sfinae.l
+++ b/sfinae.l
@@ -47,6 +47,7 @@ static int fail(const char* message)
 "sequence"              return TOK_SEQUENCE;
 "name"                  return TOK_NAME;
 "value_of"              return TOK_VALUE_OF;
+"declval"               return TOK_DECLVAL;
 "void"                  return TOK_VOID;
 "int32_t"               return TOK_INT32;
 "byte"                  return TOK_BYTE;
diff --git a/sfinae.y b/sfinae.y
index 4e366a8..14eb5f8 100644
--- a/sfinae.y
+++ b/sfinae.y
@@ -33,6 +33,7 @@ void yyerror(const char* message);
 %token TOK_SEQUENCE
 %token TOK_NAME
 %token TOK_VALUE_OF
+%token TOK_DECLVAL
 %token TOK_VOID
 %token TOK_INT32
 %token TOK_BYTE
@@ -197,6 +198,8 @@ value
         { $$ = node_binary(VALUE_QUOTIENT, $4, $6); }
     | TOK_COLONCOLON TOK_VALUE_OF '<' name '>'
         { $$ = node_named(VALUE_VARIABLE, &$4, NULL); }
+    | TOK_COLONCOLON TOK_DECLVAL '<' type '>' '(' ')'
+        { $$ = node_unary(VALUE_DECLVAL, $4); }
     | TOK_COLONCOLON TOK_EXTENT '<' type '>' TOK_COLONCOLON TOK_VALUE
         { $$ = node_unary(VALUE_EXTENT, $4); }
     | TOK_COLONCOLON TOK_TUPLE_SIZE '<' type '>' TOK_COLONCOLON TOK_VALUE
-- 
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