l-values are preserved modulo type distinction

compiler/ccgexprs.nim

@@ -1395,6 +1395,10 @@ proc genRangeChck(p: BProc, n: PNode, d: var TLoc, magic: string) =
         toRope(magic)]))
 
 proc genConv(p: BProc, e: PNode, d: var TLoc) =
+  if equalOrDistinctOf(e.typ, e.sons[1].typ) or
+     equalOrDistinctOf(e.sons[1].typ, e.typ):
+    expr(p, e.sons[1], d)
+  else:
     genCast(p, e, d)
 
 proc convStrToCStr(p: BProc, n: PNode, d: var TLoc) =

compiler/ccgutils.nim

@@ -72,7 +72,7 @@ proc GetUniqueType*(key: PType): PType =
     if result == nil: 
       IdTablePut(gTypeTable[k], key, key)
       result = key
-  of tyGenericInst, tyDistinct, tyOrdinal: 
+  of tyGenericInst, tyDistinct, tyOrdinal, tyMutable, tyConst, tyIter: 
     result = GetUniqueType(lastSon(key))
   of tyProc: 
     nil
@@ -116,9 +116,8 @@ proc makeCString*(s: string): PRope =
   # further information.
   const 
     MaxLineLength = 64
-  var res: string
   result = nil
-  res = "\""
+  var res = "\""
   for i in countup(0, len(s) - 1):
     if (i + 1) mod MaxLineLength == 0:
       add(res, '\"')
@@ -132,9 +131,8 @@ proc makeCString*(s: string): PRope =
 
 proc makeLLVMString*(s: string): PRope = 
   const MaxLineLength = 64
-  var res: string
   result = nil
-  res = "c\""
+  var res = "c\""
   for i in countup(0, len(s) - 1): 
     if (i + 1) mod MaxLineLength == 0: 
       app(result, toRope(res))

compiler/parser.nim

@@ -1316,6 +1316,7 @@ proc complexOrSimpleStmt(p: var TParser): PNode =
   of tkConverter: result = parseRoutine(p, nkConverterDef)
   of tkType: result = parseSection(p, nkTypeSection, parseTypeDef)
   of tkConst: result = parseSection(p, nkConstSection, parseConstant)
+  of tkLet: result = parseSection(p, nkLetSection, parseConstant)
   of tkWhen: result = parseIfOrWhen(p, nkWhenStmt)
   of tkVar: result = parseSection(p, nkVarSection, parseVariable)
   else: result = simpleStmt(p)

compiler/semexprs.nim

@@ -378,6 +378,10 @@ proc isAssignable(c: PContext, n: PNode): TAssignableResult =
     # Object and tuple conversions are still addressable, so we skip them
     if skipTypes(n.typ, abstractPtrs).kind in {tyOpenArray, tyTuple, tyObject}: 
       result = isAssignable(c, n.sons[1])
+    elif equalOrDistinctOf(n.typ, n.sons[1].typ) or 
+         equalOrDistinctOf(n.sons[1].typ, n.typ):
+      # types that are equal modulo distinction preserve l-value:
+      result = isAssignable(c, n.sons[1])
   of nkHiddenDeref, nkDerefExpr: 
     result = arLValue
   of nkObjUpConv, nkObjDownConv, nkCheckedFieldExpr: