Bugfix: The compiler were not inserting proper downcasts for generic types

This resulted in a codegen error in C++ mode, because the generic
types were not defined in modules where calls requiring downcasts
were used (generating a downcast forces the inclusion of the full
definition of the involved types).

compiler/sigmatch.nim

@@ -1962,7 +1962,8 @@ proc paramTypesMatchAux(m: var TCandidate, f, a: PType,
     inc(m.genericMatches)
     if arg.typ == nil:
       result = arg
-    elif skipTypes(arg.typ, abstractVar-{tyTypeDesc}).kind == tyTuple:
+    elif skipTypes(arg.typ, abstractVar-{tyTypeDesc}).kind == tyTuple or
+         m.inheritancePenalty > 0:
       result = implicitConv(nkHiddenSubConv, f, arg, m, c)
     elif arg.typ.isEmptyContainer:
       result = arg.copyTree

compiler/types.nim

@@ -612,7 +612,7 @@ proc firstOrd*(t: PType): BiggestInt =
     else:
       assert(t.n.sons[0].kind == nkSym)
       result = t.n.sons[0].sym.position
-  of tyGenericInst, tyDistinct, tyTypeDesc, tyAlias, tyStatic:
+  of tyGenericInst, tyDistinct, tyTypeDesc, tyAlias, tyStatic, tyInferred:
     result = firstOrd(lastSon(t))
   of tyOrdinal:
     if t.len > 0: result = firstOrd(lastSon(t))
@@ -651,7 +651,7 @@ proc lastOrd*(t: PType; fixedUnsigned = false): BiggestInt =
   of tyEnum:
     assert(t.n.sons[sonsLen(t.n) - 1].kind == nkSym)
     result = t.n.sons[sonsLen(t.n) - 1].sym.position
-  of tyGenericInst, tyDistinct, tyTypeDesc, tyAlias, tyStatic:
+  of tyGenericInst, tyDistinct, tyTypeDesc, tyAlias, tyStatic, tyInferred:
     result = lastOrd(lastSon(t))
   of tyProxy: result = 0
   of tyOrdinal:

tests/generics/module_with_generics.nim

@@ -0,0 +1,14 @@
+type
+  Base[T] = ref object {.inheritable.}
+    value*: T
+
+  Derived[T] = ref object of Base[T]
+    derivedValue*: T
+
+proc makeDerived*[T](v: T): Derived[T] =
+  new result
+  result.value = v
+
+proc setBaseValue*[T](a: Base[T], value: T) =
+  a.value = value
+

tests/generics/t5602_inheritence.nim

@@ -1,10 +1,11 @@
 discard """
   output: "seq[float]\n0"
+  targets: "c cpp"
 """
 
 # https://github.com/nim-lang/Nim/issues/5602
 
-import typetraits
+import typetraits, module_with_generics
 
 type
   Foo[T] = object of RootObj
@@ -16,3 +17,8 @@ proc p[T](f: Foo[T]): T =
 var s: Bar[float]
 echo p(s).len # the bug was: p(s) should return seq[float], but returns float instead
 
+# Test overloading and code generation when
+# downcasting is required for generic types:
+var d = makeDerived(10)
+setBaseValue(d, 20)
+