fixes #844

compiler/semdata.nim

@@ -42,7 +42,7 @@ type
 
   TExprFlag* = enum 
     efLValue, efWantIterator, efInTypeof, efWantStmt, efDetermineType,
-    efAllowDestructor
+    efAllowDestructor, efWantValue
   TExprFlags* = set[TExprFlag]
 
   PContext* = ref TContext

compiler/semexprs.nim

@@ -35,7 +35,7 @@ proc semOperand(c: PContext, n: PNode, flags: TExprFlags = {}): PNode =
     result.typ = errorType(c)
 
 proc semExprWithType(c: PContext, n: PNode, flags: TExprFlags = {}): PNode =
-  result = semExpr(c, n, flags)
+  result = semExpr(c, n, flags+{efWantValue})
   if result.isNil or result.kind == nkEmpty: 
     # do not produce another redundant error message:
     #raiseRecoverableError("")
@@ -1993,7 +1993,7 @@ proc semExpr(c: PContext, n: PNode, flags: TExprFlags = {}): PNode =
     result = semStaticExpr(c, n)
   of nkAsgn: result = semAsgn(c, n)
   of nkBlockStmt, nkBlockExpr: result = semBlock(c, n)
-  of nkStmtList, nkStmtListExpr: result = semStmtList(c, n)
+  of nkStmtList, nkStmtListExpr: result = semStmtList(c, n, flags)
   of nkRaiseStmt: result = semRaise(c, n)
   of nkVarSection: result = semVarOrLet(c, n, skVar)
   of nkLetSection: result = semVarOrLet(c, n, skLet)

compiler/semstmts.nim

@@ -1206,7 +1206,7 @@ proc usesResult(n: PNode): bool =
       for c in n:
         if usesResult(c): return true
 
-proc semStmtList(c: PContext, n: PNode): PNode =
+proc semStmtList(c: PContext, n: PNode, flags: TExprFlags): PNode =
   # these must be last statements in a block:
   const
     LastBlockStmts = {nkRaiseStmt, nkReturnStmt, nkBreakStmt, nkContinueStmt}
@@ -1247,12 +1247,14 @@ proc semStmtList(c: PContext, n: PNode): PNode =
       if n.sons[i].typ == enforceVoidContext or usesResult(n.sons[i]):
         voidContext = true
         n.typ = enforceVoidContext
-      if i != last or voidContext or c.inTypeClass > 0:
+      if i == last and efWantValue in flags:
+        n.typ = n.sons[i].typ
+        if not isEmptyType(n.typ): n.kind = nkStmtListExpr
+      elif i != last or voidContext or c.inTypeClass > 0:
         discardCheck(c, n.sons[i])
       else:
         n.typ = n.sons[i].typ
-        if not isEmptyType(n.typ):
-          n.kind = nkStmtListExpr
+        if not isEmptyType(n.typ): n.kind = nkStmtListExpr
       case n.sons[i].kind
       of nkVarSection, nkLetSection:
         let (outer, inner) = insertDestructors(c, n.sons[i])

tests/exprs/tstmtexprs.nim

@@ -69,3 +69,15 @@ proc semiProblem() =
   if false: echo "aye"; echo "indeed"
 
 semiProblem()
+
+
+# bug #844
+
+import json 
+proc parseResponse(): PJsonNode =
+  result = % { "key1": % { "key2": % "value" } }
+  for key, val in result["key1"]:
+    var excMsg = key & "("
+    if (var n=result["key2"]; n != nil):
+      excMsg &= n.str
+    raise newException(ESynch, excMsg)