[FIX] Fixes #1867 - Alignment scores should be signed

Signed-off-by: Lydia Buntrock <lydia.buntrock@fu-berlin.de>

CHANGELOG.md

@@ -102,6 +102,8 @@ Note that 3.1.0 will be the first API stable release and interfaces in this rele
 * When invoking the alignment algorithm with a user defined thread count using the `seqan3::align_cfg::parallel`
   configuration element, `std::thread::hardware_concurrency()` many threads were always spawned. This is now fixed and
   only the specified number of threads will be spawned ([\#1854](https://github.com/seqan/seqan3/pull/1854)).
+* Using an unsigned `score_type` throws a static assert, since gaps and mismatches have negative scores and thus need a
+  signed score type ([\#...](https://github.com/seqan/seqan3/pull/...)).
 
 ### Argument Parser
 

include/seqan3/alignment/configuration/align_config_result.hpp

@@ -140,6 +140,10 @@ class result : public pipeable_config_element<result<alignment_result_tag_t, sco
 public:
     //!\brief The score type of the alignment result.
     using score_type = score_t;
+    static_assert(std::is_unsigned_v<score_type>,
+                  "The alignment algorithm cannot be computed with an unsigned type as the score type. If you "
+                  "explicitly want an unsigned type as the score type, please submit a feature request explaining the "
+                  "proposed use case on our github page.");
 
     /*!\name Constructors, destructor and assignment
      * \{

test/snippet/alignment/configuration/align_cfg_result_example.cpp

@@ -19,7 +19,7 @@ int main()
     // You can also change the score type:
 
     // Compute only the score given a specific score_type.
-    seqan3::align_cfg::result cfg_score_uint16{seqan3::with_score, seqan3::using_score_type<uint16_t>};
+    seqan3::align_cfg::result cfg_score_uint16{seqan3::with_score, seqan3::using_score_type<int16_t>};
 
     // Compute the score given a specific score_type and the back coordinate.
     seqan3::align_cfg::result cfg_end_double{seqan3::with_back_coordinate, seqan3::using_score_type<double>};