sem: use proper error propagation in semWhen

Replace `semConstExpr` with `semRealConstExpr`, the latter which
returns `nkError` nodes.

`semRealConstExpr` was also missing the execution context handling that
`semConstExpr` uses.

Author: zerbina

compiler/sem/sem.nim

@@ -682,7 +682,9 @@ proc semRealConstExpr(c: PContext, n: PNode): PNode =
   addInNimDebugUtils(c.config, "semRealConstExpr", n, result)
   assert not n.isError
 
+  pushExecCon(c, {})
   result = semExprWithType(c, n)
+  popExecCon(c)
   if result.kind != nkError:
     result = evalConstExpr(c, result)
 

compiler/sem/semexprs.nim

@@ -2945,7 +2945,7 @@ proc semWhen(c: PContext, n: PNode, flags: TExprFlags): PNode =
       case it.kind
       of nkElifBranch, nkElifExpr:
         checkSonsLen(it, 2, c.config)
-        let e = forceBool(c, semConstExpr(c, it[0]))
+        let e = forceBool(c, semRealConstExpr(c, it[0]))
         if e.kind == nkError:
           # error in the condition expression; wrap and return
           result = copyNodeWithKids(n)