Add hasDefaultValue type trait (#22636)

Needed for #21842.

changelog.md

@@ -11,7 +11,8 @@
 
 [//]: # "Additions:"
 
-- Adds `newStringUninit` to system, which creates a new string of length `len` like `newString` but with uninitialized content.
+- Added `newStringUninit` to system, which creates a new string of length `len` like `newString` but with uninitialized content.
+- Added `hasDefaultValue` to `std/typetraits` to check if a type has a valid default value.
 
 [//]: # "Deprecations:"
 

compiler/semmagic.nim

@@ -197,6 +197,8 @@ proc evalTypeTrait(c: PContext; traitCall: PNode, operand: PType, context: PSym)
     let complexObj = containsGarbageCollectedRef(t) or
                      hasDestructor(t)
     result = newIntNodeT(toInt128(ord(not complexObj)), traitCall, c.idgen, c.graph)
+  of "hasDefaultValue":
+    result = newIntNodeT(toInt128(ord(not operand.requiresInit)), traitCall, c.idgen, c.graph)
   of "isNamedTuple":
     var operand = operand.skipTypes({tyGenericInst})
     let cond = operand.kind == tyTuple and operand.n != nil

lib/pure/typetraits.nim

@@ -96,6 +96,23 @@ proc supportsCopyMem*(t: typedesc): bool {.magic: "TypeTrait".}
   ##
   ## Other languages name a type like these `blob`:idx:.
 
+proc hasDefaultValue*(t: typedesc): bool {.magic: "TypeTrait".} =
+  ## Returns true if `t` has a valid default value.
+  runnableExamples:
+    {.experimental: "strictNotNil".}
+    type
+      NilableObject = ref object
+        a: int
+      Object = NilableObject not nil
+      RequiresInit[T] = object
+        a {.requiresInit.}: T
+
+    assert hasDefaultValue(NilableObject)
+    assert not hasDefaultValue(Object)
+    assert hasDefaultValue(string)
+    assert not hasDefaultValue(var string)
+    assert not hasDefaultValue(RequiresInit[int])
+
 proc isNamedTuple*(T: typedesc): bool {.magic: "TypeTrait".} =
   ## Returns true for named tuples, false for any other type.
   runnableExamples: