generate tyFromExpr for when in generics (nim-lang#24066)

fixes nim-lang#22342, fixes nim-lang#22607

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

tests/generics/tuninstantiatedgenericcalls.nim

@@ -333,10 +333,47 @@ block: # issue #24044
   type MyBuf[I] = ArrayBuf[maxLen(I)]
   var v: MyBuf[int]
 
-when false: # issue #22342, type section version of #22607
+block: # issue #22342, type section version of #22607
   type GenAlias[isInt: static bool] = (
     when isInt:
       int
     else:
       float
   )
+  doAssert GenAlias[true] is int
+  doAssert GenAlias[false] is float
+  proc foo(T: static bool): GenAlias[T] = discard
+  doAssert foo(true) is int
+  doAssert foo(false) is float
+  proc foo[T: static bool](v: var GenAlias[T]) =
+    v += 1
+  var x: int
+  foo[true](x)
+  doAssert not compiles(foo[false](x))
+  foo[true](x)
+  doAssert x == 2
+  var y: float
+  foo[false](y)
+  doAssert not compiles(foo[true](y))
+  foo[false](y)
+  doAssert y == 2
+
+block: # `when`, test no constant semchecks
+  type Foo[T] = (
+    when false:
+      {.error: "bad".}
+    elif defined(neverDefined):
+      {.error: "bad 2".}
+    else:
+      T
+  )
+  var x: Foo[int]
+  type Bar[T] = (
+    when true:
+      T
+    elif defined(js):
+      {.error: "bad".}
+    else:
+      {.error: "bad 2".}
+  )
+  var y: Bar[int]

tests/proc/tstaticsignature.nim

@@ -223,7 +223,7 @@ block: # issue #7547
   let z = initContainsFoo(5) # Error: undeclared identifier: 'N'
   doAssert z.Ffoo is int
 
-when false: # issue #22607, needs nkWhenStmt to be handled like nkRecWhen
+block: # issue #22607, needs nkWhenStmt to be handled like nkRecWhen
   proc test[x: static bool](
     t: (
       when x: