Importc codegen fix (#25511)

This fixes two issues with impotc'ed types.
1. Passing an importc'ed inherited object to where superclass is
expected emitted `v.Sup` previously. Now it emits `v`, similar to cpp
codegen.
2. Casting between different nim types that resolve to the same C type
previously was done like `*(T*)&v`, now it is just `v`.

compiler/ccgexprs.nim

@@ -3271,7 +3271,11 @@ proc upConv(p: BProc, n: PNode, d: var TLoc) =
         p.s(cpsStmts).addCallStmt(cgsymValue(p.module, "raiseObjectConversionError"))
         raiseInstr(p, p.s(cpsStmts))
 
-  if n[0].typ.kind != tyObject:
+  # skip cast when types map to the same C type
+  # this avoids invalid C code like `*(T*)&x` for types that can't have their address taken (e.g., WASM __externref_t)
+  if getTypeDesc(p.module, n.typ) == getTypeDesc(p.module, n[0].typ):
+    expr(p, n[0], d)
+  elif n[0].typ.kind != tyObject:
     let destTyp = getTypeDesc(p.module, n.typ)
     let val = rdLoc(a)
     if n.isLValue:
@@ -3317,7 +3321,7 @@ proc downConv(p: BProc, n: PNode, d: var TLoc) =
         cCast(ptrType(destType),
           wrapPar(cAddr(wrapPar(val))))),
       a.storage)
-  elif p.module.compileToCpp:
+  elif p.module.compileToCpp or isImportedType(src):
     # C++ implicitly downcasts for us
     expr(p, arg, d)
   else:

tests/ccgbugs2/tcodegen.nim

@@ -56,3 +56,22 @@ proc main = # bug #24677
   for NDEBUG in 0..2:
     doAssert NDEBUG == NDEBUG
 main()
+
+block: # importc type inheritance
+  type
+    A {.inheritable, pure, bycopy, importc: "int".} = object
+    B {.importc: "int", bycopy.} = object of A
+
+  {.emit: """
+  int foo(int a) {
+    return 123;
+  }
+  """.}
+
+  proc foo(a: A): B {.importc, nodecl.}
+
+  var a: A
+  var b = foo(a)
+  doAssert(cast[cint](b) == 123)
+  var c = foo(b)
+  doAssert(cast[cint](c) == 123)