fix #21251 Compiler SIGSEGV when using SharedTable (#21876)

fix #21251

(cherry picked from commit 76a98fee65)

compiler/semexprs.nim

@@ -2930,6 +2930,8 @@ proc semExpr(c: PContext, n: PNode, flags: TExprFlags = {}, expectedType: PType
         else:
           {checkUndeclared, checkModule, checkAmbiguity, checkPureEnumFields}
       s = qualifiedLookUp(c, n, checks)
+      if s == nil:
+        return
     if c.matchedConcept == nil: semCaptureSym(s, c.p.owner)
     case s.kind
     of skProc, skFunc, skMethod, skConverter, skIterator:

lib/pure/collections/deques.nim

@@ -71,9 +71,8 @@ template initImpl(result: typed, initialSize: int) =
   newSeq(result.data, correctSize)
 
 template checkIfInitialized(deq: typed) =
-  when compiles(defaultInitialSize):
-    if deq.mask == 0:
-      initImpl(deq, defaultInitialSize)
+  if deq.mask == 0:
+    initImpl(deq, defaultInitialSize)
 
 proc initDeque*[T](initialSize: int = defaultInitialSize): Deque[T] =
   ## Creates a new empty deque.

lib/pure/collections/tableimpl.nim

@@ -11,6 +11,9 @@
 
 include hashcommon
 
+const
+  defaultInitialSize* = 32
+
 template rawGetDeepImpl() {.dirty.} =   # Search algo for unconditional add
   genHashImpl(key, hc)
   var h: Hash = hc and maxHash(t)
@@ -31,9 +34,8 @@ proc rawInsert[X, A, B](t: var X, data: var KeyValuePairSeq[A, B],
   rawInsertImpl()
 
 template checkIfInitialized() =
-  when compiles(defaultInitialSize):
-    if t.dataLen == 0:
-      initImpl(t, defaultInitialSize)
+  if t.dataLen == 0:
+    initImpl(t, defaultInitialSize)
 
 template addImpl(enlarge) {.dirty.} =
   checkIfInitialized()

lib/pure/collections/tables.nim

@@ -217,8 +217,6 @@ type
     ## For creating a new empty TableRef, use `newTable proc
     ## <#newTable>`_.
 
-const
-  defaultInitialSize* = 32
 
 # ------------------------------ helpers ---------------------------------
 

tests/stdlib/t21251.nim

@@ -0,0 +1,6 @@
+import std / [tables, sets, sharedtables]
+
+var shared: SharedTable[int, int]
+shared.init
+
+shared[1] = 1