fixes #25945; cannot map the empty seq type to a C type (#25954)

fixes #25945

When `@[]` appears inside a nested `if` expression that also contains
statements, the AST wraps it in `nkStmtListExpr` nodes. The empty
container's `tyEmpty` element type was never resolved to a concrete
type, causing the C codegen to ICE with "cannot map the empty seq type
to a C type".

Walk through nested statement-list/block expressions in
`fitNodePostMatch` to find the innermost value node and propagate the
formal type to empty containers.

compiler/sem.nim

@@ -89,6 +89,18 @@ proc fitNodePostMatch(c: PContext, formal: PType, arg: PNode): PNode =
     changeType(c, x, formal, check=true)
   result = arg
   result = skipHiddenSubConv(result, c.graph, c.idgen)
+  # Walk through nested statement-list/block expressions to find the innermost
+  # value node. Empty containers (e.g. `@[]`) inside `nkStmtListExpr` wrappers
+  # need their type resolved to match the formal type, otherwise the C codegen
+  # cannot map `tyEmpty` to a concrete type (fixes #25945).
+  var tail = result
+  while tail.kind in {nkStmtList, nkStmtListExpr, nkBlockStmt, nkBlockExpr, nkPragmaBlock} and tail.len > 0:
+    tail = tail.lastSon
+
+  if tail.typ != nil and tail.typ.isEmptyContainer and
+        formal.kind notin {tyUntyped, tyBuiltInTypeClass, tyAnything}:
+    changeType(c, tail, formal, check=true)
+
   # mark inserted converter as used:
   var a = result
   if a.kind == nkHiddenDeref: a = a[0]

tests/ccgbugs2/tcodegen.nim

@@ -75,3 +75,15 @@ block: # importc type inheritance
   doAssert(cast[cint](b) == 123)
   var c = foo(b)
   doAssert(cast[cint](c) == 123)
+
+block: # bug #25945
+  var stateRefund = 0
+  let authCode =
+    if true:
+      if false:
+        stateRefund += 0
+      @[]
+    else:
+      @([1.byte])
+
+discard (if true: (discard; @[]) else: @[0])