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.

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: