propagate efWantStmt in semWhen (#23400)

fixes #23399

The new case introduced in #21657 is triggered by `efWantStmt` but the
`when` statement doesn't normally propagate this flag, so propagate it
when the `semCheck` param in `semWhen` is true which happens when the
`when` statement is `efWhenStmt` anyway.

(cherry picked from commit fb6c805568)

compiler/semexprs.nim

@@ -2550,9 +2550,10 @@ proc semWhen(c: PContext, n: PNode, semCheck = true): PNode =
   # If semCheck is set to false, ``when`` will return the verbatim AST of
   # the correct branch. Otherwise the AST will be passed through semStmt.
   result = nil
+  let flags = if semCheck: {efWantStmt} else: {}
 
   template setResult(e: untyped) =
-    if semCheck: result = semExpr(c, e) # do not open a new scope!
+    if semCheck: result = semExpr(c, e, flags) # do not open a new scope!
     else: result = e
 
   # Check if the node is "when nimvm"
@@ -2578,7 +2579,7 @@ proc semWhen(c: PContext, n: PNode, semCheck = true): PNode =
       checkSonsLen(it, 2, c.config)
       if whenNimvm:
         if semCheck:
-          it[1] = semExpr(c, it[1])
+          it[1] = semExpr(c, it[1], flags)
           typ = commonType(c, typ, it[1].typ)
         result = n # when nimvm is not elimited until codegen
       else:
@@ -2594,7 +2595,7 @@ proc semWhen(c: PContext, n: PNode, semCheck = true): PNode =
       checkSonsLen(it, 1, c.config)
       if result == nil or whenNimvm:
         if semCheck:
-          it[0] = semExpr(c, it[0])
+          it[0] = semExpr(c, it[0], flags)
           typ = commonType(c, typ, it[0].typ)
         if result == nil:
           result = it[0]

tests/modules/tmodulesymtype.nim

@@ -13,3 +13,10 @@ proc foo() =
     sequtils
 
 foo()
+
+# issue #23399
+when isMainModule:
+  sequtils #[tt.Error
+  ^ expression has no type: sequtils]#
+
+discard