fix: allow empty loop headers

compiler/noirc_evaluator/src/ssa/opt/unrolling.rs

@@ -302,6 +302,11 @@
     pub(super) fn get_const_upper_bound(&self, function: &Function) -> Option<FieldElement> {
         let block = &function.dfg[self.header];
         let instructions = block.instructions();
+        if instructions.is_empty() {
+            // If the loop condition is constant time, the loop header will be
+            // simplified to a simple jump.
+            return None;
+        }
         assert_eq!(
             instructions.len(),
             1,
@@ -1023,7 +1028,7 @@
     use super::{is_new_size_ok, BoilerplateStats, Loops};
 
     /// Tries to unroll all loops in each SSA function once, calling the `Function` directly,
     /// bypassing the iterative loop done by the SSA which does further optimisations.
     ///
     /// If any loop cannot be unrolled, it is left as-is or in a partially unrolled state.
     fn try_unroll_loops(mut ssa: Ssa) -> (Ssa, Vec<RuntimeError>) {

test_programs/execution_success/regression/src/main.nr

@@ -138,4 +138,31 @@ fn main(x: [u8; 5], z: Field, u: i16, v: i16) {
     assert(-u % -11 == -4);
     assert(u % -11 == u % (v + 2));
     assert(-u % -11 == -u % (v + 2));
+
+    // Issue 6736
+    unsafe { sub_array_extended_into_empty(); }
+}
+
+
+pub fn sub_array_extended<let SRC_LEN: u32, let DST_LEN: u32>(
+    src: [Field; SRC_LEN],
+    offset: u32,
+) -> [Field; DST_LEN] {
+    let available_elements_to_copy = SRC_LEN - offset;
+    let elements_to_copy = if DST_LEN > available_elements_to_copy {
+        available_elements_to_copy
+    } else {
+        DST_LEN
+    };
+
+    let mut dst: [Field; DST_LEN] = std::mem::zeroed();
+    for i in 0..elements_to_copy {
+        dst[i] = src[i + offset];
+    }
+
+    dst
+}
+
+unconstrained fn sub_array_extended_into_empty() {
+    assert_eq(sub_array_extended([], 0), []);
 }