Disallow nil dereference at compile time (#16032)

* bring back the semfold of nil

* remove space

* fix test

* proc type can't be dereferenced

* disallow nil dereference at compile time

* changelog

changelog.md

@@ -51,7 +51,7 @@
 
 - The `cstring` doesn't support `[]=` operator in JS backend.
 
-
+- nil dereference is not allowed at compile time. `cast[ptr int](nil)[]` is rejected at compile time.
 
 ## Compiler changes
 

compiler/semexprs.nim

@@ -1440,6 +1440,11 @@ proc buildOverloadedSubscripts(n: PNode, ident: PIdent): PNode =
 proc semDeref(c: PContext, n: PNode): PNode =
   checkSonsLen(n, 1, c.config)
   n[0] = semExprWithType(c, n[0])
+  let a = getConstExpr(c.module, n[0], c.idgen, c.graph)
+  if a != nil:
+    if a.kind == nkNilLit:
+      localError(c.config, n.info, "nil dereference is not allowed")
+    n[0] = a
   result = n
   var t = skipTypes(n[0].typ, {tyGenericInst, tyVar, tyLent, tyAlias, tySink, tyOwned})
   case t.kind

compiler/semfold.nim

@@ -671,6 +671,10 @@ proc getConstExpr(m: PSym, n: PNode; idgen: IdGenerator; g: ModuleGraph): PNode
     var a = getConstExpr(m, n[1], idgen, g)
     if a == nil: return
     result = foldConv(n, a, g, check=true)
+  of nkDerefExpr, nkHiddenDeref:
+    let a = getConstExpr(m, n[0], idgen, g)
+    if a != nil and a.kind == nkNilLit:
+       localError(g.config, n.info, "nil dereference is not allowed")
   of nkCast:
     var a = getConstExpr(m, n[1], idgen, g)
     if a == nil: return

tests/misc/tcast.nim

@@ -33,7 +33,8 @@ reject: discard cast[ptr](a)
 # bug #15623
 block:
   if false:
-    echo cast[ptr int](nil)[]
+    let x = cast[ptr int](nil)
+    echo x[]
 
 block:
   if false: