Don't skip closure captures when fixing them

crates/compiler/can/src/module.rs

@@ -1051,14 +1051,13 @@ fn fix_values_captured_in_closure_expr(
                     debug_assert!(!captures.is_empty());
                     captured_symbols.extend(captures);
                     captured_symbols.swap_remove(i);
-                    // Jump two, because the next element is now one of the newly-added captures,
-                    // which we don't need to check.
-                    i += 2;
 
                     added_captures = true;
-                } else {
-                    i += 1;
                 }
+
+                // Always jump one, because the current element either does not have captures or
+                // is now one of the newly-added captures, which we don't need to check.
+                i += 1;
             }
             if added_captures {
                 // Re-sort, since we've added new captures.

crates/compiler/test_mono/generated/issue_6174.txt

@@ -0,0 +1,37 @@
+procedure Bool.1 ():
+    let Bool.23 : Int1 = false;
+    ret Bool.23;
+
+procedure Test.2 (Test.11, Test.1):
+    if Test.1 then
+        let Test.29 : I64 = 0i64;
+        let Test.28 : [C {}, C I64] = TagId(1) Test.29;
+        ret Test.28;
+    else
+        let Test.27 : {} = Struct {};
+        let Test.26 : [C {}, C I64] = TagId(0) Test.27;
+        ret Test.26;
+
+procedure Test.3 (Test.12, Test.1):
+    if Test.1 then
+        let Test.23 : I64 = 0i64;
+        let Test.22 : [C {}, C I64] = TagId(1) Test.23;
+        ret Test.22;
+    else
+        let Test.21 : {} = Struct {};
+        let Test.20 : [C {}, C I64] = TagId(0) Test.21;
+        ret Test.20;
+
+procedure Test.4 (Test.13, Test.1):
+    let Test.25 : {} = Struct {};
+    let Test.17 : [C {}, C I64] = CallByName Test.2 Test.25 Test.1;
+    let Test.19 : {} = Struct {};
+    let Test.18 : [C {}, C I64] = CallByName Test.3 Test.19 Test.1;
+    let Test.16 : List [C {}, C I64] = Array [Test.17, Test.18];
+    ret Test.16;
+
+procedure Test.0 ():
+    let Test.1 : Int1 = CallByName Bool.1;
+    let Test.15 : {} = Struct {};
+    let Test.14 : List [C {}, C I64] = CallByName Test.4 Test.15 Test.1;
+    ret Test.14;

crates/compiler/test_mono/src/tests.rs

@@ -3474,3 +3474,29 @@ fn issue_5513() {
         "
     )
 }
+
+#[mono_test]
+fn issue_6174() {
+    indoc!(
+        r"
+        g = Bool.false
+
+        a = \_ ->
+            if g then
+                Ok 0
+            else
+                Err NoNumber
+
+        b = \_ ->
+            if g then
+                Ok 0
+            else
+                Err NoNumber
+
+        c = \_ ->
+            [a {}, b {}]
+
+        c {}
+        "
+    )
+}