fixes #7601, array construction of ptr generics (#7671)

* fixes #7601, array construction of ptr generics

* add more test

compiler/sem.nim

@@ -153,8 +153,9 @@ proc commonType*(x, y: PType): PType =
     if a.kind in {tyRef, tyPtr}:
       k = a.kind
       if b.kind != a.kind: return x
-      a = a.lastSon
+      # bug #7601, array construction of ptr generic
-      b = b.lastSon
+      a = a.lastSon.skipTypes({tyGenericInst})
+      b = b.lastSon.skipTypes({tyGenericInst})
     if a.kind == tyObject and b.kind == tyObject:
       result = commonSuperclass(a, b)
       # this will trigger an error later:

tests/array/tarraycons_ptr_generic.nim

@@ -0,0 +1,51 @@
+discard """
+  output: '''apple
+banana
+Fruit
+2
+4
+3
+none
+skin
+paper
+'''
+"""
+type
+  Fruit = object of RootObj
+    name: string
+  Apple = object of Fruit
+  Banana = object of Fruit
+
+var
+  ir = Fruit(name: "Fruit")
+  ia = Apple(name: "apple")
+  ib = Banana(name: "banana")
+
+let x = [ia.addr, ib.addr, ir.addr]
+for c in x: echo c.name
+
+type
+  Vehicle[T] = object of RootObj
+    tire: T
+  Car[T] = object of Vehicle[T]
+  Bike[T] = object of Vehicle[T]
+
+var v = Vehicle[int](tire: 3)
+var c = Car[int](tire: 4)
+var b = Bike[int](tire: 2)
+
+let y = [b.addr, c.addr, v.addr]
+for c in y: echo c.tire
+
+type
+  Book[T] = ref object of RootObj
+    cover: T
+  Hard[T] = ref object of Book[T]
+  Soft[T] = ref object of Book[T]
+
+var bn = Book[string](cover: "none")
+var hs = Hard[string](cover: "skin")
+var bp = Soft[string](cover: "paper")
+
+let z = [bn, hs, bp]
+for c in z: echo c.cover

tests/array/tarraycons_ptr_generic2.nim

@@ -0,0 +1,17 @@
+discard """
+  file: "tarraycons_ptr_generic2.nim"
+  line: 17
+  errormsg: "type mismatch: got <ptr Hard[system.string]> but expected 'Book[system.string]'"
+"""
+
+type
+  Book[T] = ref object of RootObj
+    cover: T
+  Hard[T] = ref object of Book[T]
+  Soft[T] = ref object of Book[T]
+
+var bn = Book[string](cover: "none")
+var hs = Hard[string](cover: "skin")
+var bp = Soft[string](cover: "paper")
+
+let z = [bn, hs.addr, bp]