fix new type inference for noreturn [backport] (#22182)

fixes #22180

Backported since apparently the new type inference was backported

compiler/semstmts.nim

@@ -187,13 +187,15 @@ proc semIf(c: PContext, n: PNode; flags: TExprFlags; expectedType: PType = nil):
       it[0] = forceBool(c, semExprWithType(c, it[0], expectedType = getSysType(c.graph, n.info, tyBool)))
       it[1] = semExprBranch(c, it[1], flags, expectedType)
       typ = commonType(c, typ, it[1])
-      expectedType = typ
+      if not endsInNoReturn(it[1]):
+        expectedType = typ
       closeScope(c)
     elif it.len == 1:
       hasElse = true
       it[0] = semExprBranchScope(c, it[0], expectedType)
       typ = commonType(c, typ, it[0])
-      expectedType = typ
+      if not endsInNoReturn(it[0]):
+        expectedType = typ
     else: illFormedAst(it, c.config)
   if isEmptyType(typ) or typ.kind in {tyNil, tyUntyped} or
       (not hasElse and efInTypeof notin flags):
@@ -234,7 +236,8 @@ proc semTry(c: PContext, n: PNode; flags: TExprFlags; expectedType: PType = nil)
   var expectedType = expectedType
   n[0] = semExprBranchScope(c, n[0], expectedType)
   typ = commonType(c, typ, n[0].typ)
-  expectedType = typ
+  if not endsInNoReturn(n[0]):
+    expectedType = typ
 
   var last = n.len - 1
   var catchAllExcepts = 0
@@ -295,7 +298,8 @@ proc semTry(c: PContext, n: PNode; flags: TExprFlags; expectedType: PType = nil)
     if a.kind != nkFinally:
       a[^1] = semExprBranchScope(c, a[^1], expectedType)
       typ = commonType(c, typ, a[^1])
-      expectedType = typ
+      if not endsInNoReturn(a[^1]):
+        expectedType = typ
     else:
       a[^1] = semExprBranchScope(c, a[^1])
       dec last
@@ -1145,7 +1149,8 @@ proc semCase(c: PContext, n: PNode; flags: TExprFlags; expectedType: PType = nil
       var last = x.len-1
       x[last] = semExprBranchScope(c, x[last], expectedType)
       typ = commonType(c, typ, x[last])
-      expectedType = typ
+      if not endsInNoReturn(x[last]):
+        expectedType = typ
     of nkElifBranch:
       if hasElse: invalidOrderOfBranches(x)
       chckCovered = false
@@ -1154,13 +1159,15 @@ proc semCase(c: PContext, n: PNode; flags: TExprFlags; expectedType: PType = nil
       x[0] = forceBool(c, semExprWithType(c, x[0], expectedType = getSysType(c.graph, n.info, tyBool)))
       x[1] = semExprBranch(c, x[1], expectedType = expectedType)
       typ = commonType(c, typ, x[1])
-      expectedType = typ
+      if not endsInNoReturn(x[1]):
+        expectedType = typ
       closeScope(c)
     of nkElse:
       checkSonsLen(x, 1, c.config)
       x[0] = semExprBranchScope(c, x[0], expectedType)
       typ = commonType(c, typ, x[0])
-      expectedType = typ
+      if not endsInNoReturn(x[0]):
+        expectedType = typ
       if (chckCovered and covered == toCover(c, n[0].typ)) or hasElse:
         message(c.config, x.info, warnUnreachableElse)
       hasElse = true

tests/types/ttopdowninference.nim

@@ -290,3 +290,38 @@ block: # bug #21377
     {:}
 
   doAssert b(0) == {:}
+
+block: # bug #22180
+  type A = object
+  proc j() = discard
+
+  let x =
+    if false:
+      (ref A)(nil)
+    else:
+      if false:
+        quit 1
+      else:
+        if true:
+          j()
+          nil  # compiles with (ref A)(nil) here
+        else:
+          (ref A)(nil)
+  doAssert x.isNil
+  
+  let y =
+    case true
+    of false:
+      (ref A)(nil)
+    else:
+      case true
+      of false:
+        quit 1
+      else:
+        case true
+        of true:
+          j()
+          nil  # compiles with (ref A)(nil) here
+        else:
+          (ref A)(nil)
+  doAssert y.isNil