fixes #12612 [backport] (#12681)

compiler/vmgen.nim

@@ -220,9 +220,9 @@ proc getFreeRegister(cc: PCtx; k: TSlotKind; start: int): TRegister =
         return TRegister(i)
   if c.maxSlots >= high(TRegister):
     globalError(cc.config, cc.bestEffort, "VM problem: too many registers required")
-  result = TRegister(c.maxSlots)
-  c.slots[c.maxSlots] = (inUse: true, kind: k)
-  inc c.maxSlots
+  result = TRegister(max(c.maxSlots, start))
+  c.slots[result] = (inUse: true, kind: k)
+  c.maxSlots = result + 1
 
 proc getTemp(cc: PCtx; tt: PType): TRegister =
   let typ = tt.skipTypesOrNil({tyStatic})
@@ -1483,7 +1483,10 @@ proc checkCanEval(c: PCtx; n: PNode) =
   if {sfCompileTime, sfGlobal} <= s.flags: return
   if s.kind in {skVar, skTemp, skLet, skParam, skResult} and
       not s.isOwnedBy(c.prc.sym) and s.owner != c.module and c.mode != emRepl:
-    cannotEval(c, n)
+    # little hack ahead for bug #12612: assume gensym'ed variables
+    # are in the right scope:
+    if sfGenSym in s.flags and c.prc.sym == nil: discard
+    else: cannotEval(c, n)
   elif s.kind in {skProc, skFunc, skConverter, skMethod,
                   skIterator} and sfForward in s.flags:
     cannotEval(c, n)

tests/stdlib/tstrformat.nim

@@ -134,6 +134,14 @@ doAssert fmt"{nat:3o}" == "100"
 doAssert fmt"{nat:3x}" == " 40"
 doAssert fmt"{nat:3X}" == " 40"
 
+# bug #12612
+proc my_proc =
+  const value = "value"
+  const a = &"{value}"
+  assert a == value
+
+my_proc()
+
 block:
   template fmt(pattern: string; openCloseChar: char): untyped =
     fmt(pattern, openCloseChar, openCloseChar)