sagemathgh-38105: Fixing the index set of a rank 1 Cartan matrix

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Fixes sagemath#37979 by handling passing along the index set for the special
cased rank 1 Cartan matrix.

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### ⌛ Dependencies

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URL: sagemath#38105
Reported by: Travis Scrimshaw
Reviewer(s): Frédéric Chapoton

build/pkgs/configure/checksums.ini

@@ -1,3 +1,3 @@
 tarball=configure-VERSION.tar.gz
-sha1=8164bb387998beb2257d3e09f10697195e958860
-sha256=981eb6316a4ff9179b0eb7242e23f4cb39e00372d2aa855efde6885e599e26e8
+sha1=ee7627e65e85f8f74e5bdff5201f6de13106050e
+sha256=8c6746170ea46928047268bc0f8bbc378ed1af8c62fb5e51b575e0a7bbf4dbea

build/pkgs/configure/package-version.txt

@@ -1 +1 @@
-fb2b8c823bcc3f168c1e6e467f254557fa714db9
+b72c3bed388e4e57ff7fca3151a57fda13eff35a

src/sage/combinat/root_system/cartan_matrix.py

@@ -388,6 +388,14 @@ def _CM_init(self, cartan_type, index_set, cartan_type_check):
 
             sage: C = CartanMatrix(['A',1,1])  # indirect doctest                       # needs sage.graphs
             sage: TestSuite(C).run(skip=["_test_category", "_test_change_ring"])        # needs sage.graphs
+
+        Check that :issue:`37979` is fixed::
+
+            sage: C = CartanMatrix([[2]], index_set=(4,))
+            sage: C.index_set()
+            (4,)
+            sage: CartanType("A3").subtype((2,)) is CartanType("A1").relabel({1:2})
+            True
         """
         self._index_set = index_set
         self.set_immutable()
@@ -396,6 +404,8 @@ def _CM_init(self, cartan_type, index_set, cartan_type_check):
             cartan_type = CartanType(cartan_type)
         elif self.nrows() == 1:
             cartan_type = CartanType(['A', 1])
+            if index_set != (1,):
+                cartan_type = cartan_type.relabel({1: index_set[0]})
         elif cartan_type_check:
             # Placeholder so we don't have to reimplement creating a
             #   Dynkin diagram from a Cartan matrix