Fix #8126 (#8712)

This is a temporary fix that will be reworked in a follow up commit
that aims to eliminate the tfExplicit flag from the compiler. The
complete and proper fix was considered too risky for inclusion just
before our 0.19 release.

tests/concepts/tmonoid.nim

@@ -11,3 +11,15 @@ type Monoid = concept x, y
 proc z(x: typedesc[int]): int = 0
 
 echo(int is Monoid)
+
+# https://github.com/nim-lang/Nim/issues/8126
+type AdditiveMonoid* = concept x, y, type T
+  x + y is T
+
+  # some redundant checks to test an alternative approaches:
+  type TT = type(x)
+  x + y is type(x)
+  x + y is TT
+
+doAssert(1 is AdditiveMonoid)
+