add test case to close nim-lang#7974 (nim-lang#22023)

close nim-lang#7974 by adding test case

tests/showoff/tgenericmacrotypes.nim

@@ -0,0 +1,55 @@
+# issue #7974
+
+import macros
+
+macro genTypeA(arg: typed): untyped =
+  if arg.typeKind != ntyTypeDesc:
+    error("expected typedesc", arg)
+
+  result = arg.getTypeInst[1]
+
+macro genTypeB(arg: typed): untyped =
+  if arg.typeKind != ntyTypeDesc:
+    error("expected typedesc", arg)
+
+
+  let typeSym = arg.getTypeInst[1]
+  result =
+    nnkTupleTy.newTree(
+      nnkIdentDefs.newTree(
+        ident"a", typeSym, newEmptyNode()
+      )
+    )
+
+type
+  # this is the trivial case, MyTypeA[T] is basically just T, nothing else. But it works.
+  MyTypeA[T] = genTypeA(T)
+  # in this case I generate `tuple[a: T]`. This this is something the compiler does not want
+  MyTypeB[T] = genTypeB(T)
+
+# these are just alias types for int32 and float32, nothing really happens, but it works
+var a1: MyTypeA[int32]
+doAssert a1 is MyTypeA[int32]
+doAssert a1 is int32
+a1 = 0'i32
+var a2: MyTypeA[float32]
+doAssert a2 is MyTypeA[float32]
+doAssert a2 is float32
+a2 = 0'f32
+var a3: MyTypeA[float32]
+doAssert a3 is MyTypeA[float32]
+doAssert a3 is float32
+a3 = 0'f32
+
+var b1: MyTypeB[int32]   # cannot generate VM code fur tuple[a: int32]
+doAssert b1 is MyTypeB[int32]
+doAssert b1 is tuple[a: int32]
+b1 = (a: 0'i32)
+var b2: MyTypeB[float32]
+doAssert b2 is MyTypeB[float32]
+doAssert b2 is tuple[a: float32]
+b2 = (a: 0'f32)
+var b3: MyTypeB[float32]
+doAssert b3 is MyTypeB[float32]
+doAssert b3 is tuple[a: float32]
+b3 = (a: 0'f32)