fix #5643; fix #5644

tests/generics/t5643.nim

@@ -0,0 +1,30 @@
+type
+  Matrix*[M, N: static[int], T: SomeReal] = object
+    data: ref array[N * M, T]
+
+  Matrix64*[M, N: static[int]] = Matrix[M, N, float64]
+
+proc zeros64(M,N: static[int]): Matrix64[M,N] =
+  new result.data
+  for i in 0 .. < (M * N):
+    result.data[i] = 0'f64
+
+proc bar*[M,N: static[int], T](a: Matrix[M,N,T], b: Matrix[M,N,T]) =
+  discard
+
+let a = zeros64(2,2)
+bar(a,a)
+  # https://github.com/nim-lang/Nim/issues/5643
+  #
+  # The test case was failing here, because the compiler failed to
+  # detect the two matrix instantiations as the same type.
+  #
+  # The root cause was that the `T` type variable is a different
+  # type after the first Matrix type has been matched.
+  #
+  # Sigmatch was failing to match the second version of `T`, but
+  # due to some complex interplay between tyOr, tyTypeDesc and
+  # tyGenericParam this was allowed to went through. The generic
+  # instantiation of the second matrix was incomplete and the
+  # generic cache lookup failed, producing two separate types.
+