generate tyFromExpr for when in generics (#24066)

fixes #22342, fixes #22607

Another followup of #22029, `when` expressions in general in generic
type bodies now behave like `nkRecWhen` does since #24042, leaving them
as `tyFromExpr` if a condition is uncertain. The tests for the issues
were originally added but left disabled in #24005.

compiler/semexprs.nim

@@ -2685,6 +2685,7 @@ proc semWhen(c: PContext, n: PNode, semCheck = true): PNode =
       whenNimvm = exprNode.sym.magic == mNimvm
     if whenNimvm: n.flags.incl nfLL
 
+  var cannotResolve = false
   for i in 0..<n.len:
     var it = n[i]
     case it.kind
@@ -2695,6 +2696,20 @@ proc semWhen(c: PContext, n: PNode, semCheck = true): PNode =
           it[1] = semExpr(c, it[1], flags)
           typ = commonType(c, typ, it[1].typ)
         result = n # when nimvm is not elimited until codegen
+      elif c.inGenericContext > 0:
+        let e = semExprWithType(c, it[0])
+        if e.typ.kind == tyFromExpr:
+          it[0] = makeStaticExpr(c, e)
+          cannotResolve = true
+        else:
+          it[0] = forceBool(c, e)
+          let val = getConstExpr(c.module, it[0], c.idgen, c.graph)
+          if val == nil or val.kind != nkIntLit:
+            cannotResolve = true
+          elif not cannotResolve and val.intVal != 0 and result == nil:
+            setResult(it[1])
+            return # we're not in nimvm and we already have a result
+        it[1] = semGenericStmt(c, it[1])
       else:
         let e = forceBool(c, semConstExpr(c, it[0]))
         if e.kind != nkIntLit:
@@ -2706,7 +2721,9 @@ proc semWhen(c: PContext, n: PNode, semCheck = true): PNode =
           return # we're not in nimvm and we already have a result
     of nkElse, nkElseExpr:
       checkSonsLen(it, 1, c.config)
-      if result == nil or whenNimvm:
+      if cannotResolve:
+        it[0] = semGenericStmt(c, it[0])
+      elif result == nil or whenNimvm:
         if semCheck:
           it[0] = semExpr(c, it[0], flags)
           typ = commonType(c, typ, it[0].typ)
@@ -2715,6 +2732,10 @@ proc semWhen(c: PContext, n: PNode, semCheck = true): PNode =
         if result == nil:
           result = it[0]
     else: illFormedAst(n, c.config)
+  if cannotResolve:
+    result = n
+    result.typ = makeTypeFromExpr(c, result.copyTree)
+    return
   if result == nil:
     result = newNodeI(nkEmpty, n.info)
   if whenNimvm:

compiler/semtypes.nim

@@ -2068,7 +2068,10 @@ proc semTypeNode(c: PContext, n: PNode, prev: PType): PType =
   of nkWhenStmt:
     var whenResult = semWhen(c, n, false)
     if whenResult.kind == nkStmtList: whenResult.transitionSonsKind(nkStmtListType)
-    result = semTypeNode(c, whenResult, prev)
+    if whenResult.kind == nkWhenStmt:
+      result = whenResult.typ
+    else:
+      result = semTypeNode(c, whenResult, prev)
   of nkBracketExpr:
     checkMinSonsLen(n, 2, c.config)
     var head = n[0]