fix #12282 distinct now does not create erroneous copy in VM (#17594)

compiler/vmgen.nim

@@ -833,10 +833,20 @@ proc genAddSubInt(c: PCtx; n: PNode; dest: var TDest; opc: TOpcode) =
   c.genNarrow(n, dest)
 
 proc genConv(c: PCtx; n, arg: PNode; dest: var TDest; opc=opcConv) =
-  if n.typ.kind == arg.typ.kind and arg.typ.kind == tyProc:
-    # don't do anything for lambda lifting conversions:
+  let t2 = n.typ.skipTypes({tyDistinct})
+  let targ2 = arg.typ.skipTypes({tyDistinct})
+
+  proc implicitConv(): bool =
+    if sameType(t2, targ2): return true
+    # xxx consider whether to use t2 and targ2 here
+    if n.typ.kind == arg.typ.kind and arg.typ.kind == tyProc:
+      # don't do anything for lambda lifting conversions:
+      return true
+
+  if implicitConv():
     gen(c, arg, dest)
     return
+
   let tmp = c.genx(arg)
   if dest < 0: dest = c.getTemp(n.typ)
   c.gABC(n, opc, dest, tmp)

tests/distinct/tdistinct.nim

@@ -1,4 +1,5 @@
 discard """
+  targets: "c js"
   output: '''
 tdistinct
 25
@@ -138,3 +139,34 @@ block tRequiresInit:
   accept:
     let s = "test"
     doAssert s == "test"
+
+type Foo = distinct string
+
+template main() =
+  # xxx put everything here to test under RT + VM
+  block: # bug #12282
+    block:
+      proc test() =
+        var s: Foo
+        s.string.add('c')
+        doAssert s.string == "c" # was failing
+      test()
+
+    block:
+      proc add(a: var Foo, b: char) {.borrow.}
+      proc test() =
+        var s: Foo
+        s.add('c')
+        doAssert s.string == "c" # was ok
+      test()
+
+    block:
+      proc add(a: var Foo, b: char) {.borrow.}
+      proc test() =
+        var s: string
+        s.Foo.add('c')
+        doAssert s.string == "c" # was failing
+      test()
+
+static: main()
+main()