implement generic default values for object fields (#24384)

fixes #21941, fixes #23594

compiler/sem.nim

@@ -733,6 +733,7 @@ proc preparePContext*(graph: ModuleGraph; module: PSym; idgen: IdGenerator): PCo
   result.semInferredLambda = semInferredLambda
   result.semGenerateInstance = generateInstance
   result.instantiateOnlyProcType = instantiateOnlyProcType
+  result.fitDefaultNode = fitDefaultNode
   result.semTypeNode = semTypeNode
   result.instTypeBoundOp = sigmatch.instTypeBoundOp
   result.hasUnresolvedArgs = hasUnresolvedArgs

compiler/semdata.nim

@@ -142,6 +142,7 @@ type
     instantiateOnlyProcType*: proc (c: PContext, pt: LayeredIdTable,
                                     prc: PSym, info: TLineInfo): PType
       # used by sigmatch for explicit generic instantiations
+    fitDefaultNode*: proc (c: PContext, n: var PNode, expectedType: PType)
     includedFiles*: IntSet    # used to detect recursive include files
     pureEnumFields*: TStrTable   # pure enum fields that can be used unambiguously
     userPragmas*: TStrTable

compiler/semtypes.nim

@@ -273,28 +273,23 @@ proc annotateClosureConv(n: PNode) =
     for i in 0..<n.len:
       annotateClosureConv(n[i])
 
-proc fitDefaultNode(c: PContext, n: PNode): PType =
+proc fitDefaultNode(c: PContext, n: var PNode, expectedType: PType) =
   inc c.inStaticContext
-  let expectedType = if n[^2].kind != nkEmpty: semTypeNode(c, n[^2], nil) else: nil
-  n[^1] = semConstExpr(c, n[^1], expectedType = expectedType)
-  let oldType = n[^1].typ
-  n[^1].flags.incl nfSem
-  if n[^2].kind != nkEmpty:
-    if expectedType != nil and oldType != expectedType:
-      n[^1] = fitNodeConsiderViewType(c, expectedType, n[^1], n[^1].info)
-      changeType(c, n[^1], expectedType, true) # infer types for default fields value
-        # bug #22926; be cautious that it uses `semConstExpr` to
-        # evaulate the default fields; it's only natural to use
-        # `changeType` to infer types for constant values
-        # that's also the reason why we don't use `semExpr` to check
-        # the type since two overlapping error messages might be produced
-    annotateClosureConv(n)
-    result = n[^1].typ
-  else:
-    result = n[^1].typ
+  n = semConstExpr(c, n, expectedType = expectedType)
+  let oldType = n.typ
+  n.flags.incl nfSem
+  if expectedType != nil and oldType != expectedType:
+    n = fitNodeConsiderViewType(c, expectedType, n, n.info)
+    changeType(c, n, expectedType, true) # infer types for default fields value
+      # bug #22926; be cautious that it uses `semConstExpr` to
+      # evaulate the default fields; it's only natural to use
+      # `changeType` to infer types for constant values
+      # that's also the reason why we don't use `semExpr` to check
+      # the type since two overlapping error messages might be produced
+  annotateClosureConv(n)
   # xxx any troubles related to defaults fields, consult `semConst` for a potential answer
-  if n[^1].kind != nkNilLit:
-    typeAllowedCheck(c, n.info, result, skConst, {taProcContextIsNotMacro, taIsDefaultField})
+  if n.kind != nkNilLit:
+    typeAllowedCheck(c, n.info, n.typ, skConst, {taProcContextIsNotMacro, taIsDefaultField})
   dec c.inStaticContext
 
 proc isRecursiveType*(t: PType): bool =
@@ -521,7 +516,14 @@ proc semTuple(c: PContext, n: PNode, prev: PType): PType =
     checkMinSonsLen(a, 3, c.config)
     var hasDefaultField = a[^1].kind != nkEmpty
     if hasDefaultField:
-      typ = fitDefaultNode(c, a)
+      typ = if a[^2].kind != nkEmpty: semTypeNode(c, a[^2], nil) else: nil
+      if c.inGenericContext > 0:
+        a[^1] = semExprWithType(c, a[^1], {efDetermineType, efAllowSymChoice}, typ)
+        if typ == nil:
+          typ = a[^1].typ
+      else:
+        fitDefaultNode(c, a[^1], typ)
+        typ = a[^1].typ
     elif a[^2].kind != nkEmpty:
       typ = semTypeNode(c, a[^2], nil)
       if c.graph.config.isDefined("nimPreviewRangeDefault") and typ.skipTypes(abstractInst).kind == tyRange:
@@ -885,8 +887,15 @@ proc semRecordNodeAux(c: PContext, n: PNode, check: var IntSet, pos: var int,
     var typ: PType
     var hasDefaultField = n[^1].kind != nkEmpty
     if hasDefaultField:
-      typ = fitDefaultNode(c, n)
-      propagateToOwner(rectype, typ)
+      typ = if n[^2].kind != nkEmpty: semTypeNode(c, n[^2], nil) else: nil
+      if c.inGenericContext > 0:
+        n[^1] = semExprWithType(c, n[^1], {efDetermineType, efAllowSymChoice}, typ)
+        if typ == nil:
+          typ = n[^1].typ
+      else:
+        fitDefaultNode(c, n[^1], typ)
+        typ = n[^1].typ
+        propagateToOwner(rectype, typ)
     elif n[^2].kind == nkEmpty:
       localError(c.config, n.info, errTypeExpected)
       typ = errorType(c)

compiler/semtypinst.nim

@@ -276,6 +276,11 @@ proc replaceTypeVarsN(cl: var TReplTypeVars, n: PNode; start=0; expectedType: PT
         replaceTypeVarsS(cl, n.sym, result.typ)
       else:
         replaceTypeVarsS(cl, n.sym, replaceTypeVarsT(cl, n.sym.typ))
+    if result.sym.kind == skField and result.sym.ast != nil and
+        (cl.owner == nil or result.sym.owner == cl.owner):
+      # instantiate default value of object/tuple field
+      cl.c.fitDefaultNode(cl.c, result.sym.ast, result.sym.typ)
+      result.sym.typ = result.sym.ast.typ
     # sym type can be nil if was gensym created by macro, see #24048
     if result.sym.typ != nil and result.sym.typ.kind == tyVoid:
       # don't add the 'void' field

tests/objects/tgenericdefaultvalue.nim

@@ -0,0 +1,26 @@
+block: # issue #23594
+  type
+    Gen[T] = object
+      a: T = 1.0
+
+    Spec32 = Gen[float32]
+    Spec64 = Gen[float64]
+
+  var
+    a: Spec32
+    b: Spec64
+  doAssert sizeof(a) == 4
+  doAssert sizeof(b) == 8
+  doAssert a.a is float32
+  doAssert b.a is float64
+
+block: # issue #21941
+  func what[T](): T =
+    123
+
+  type MyObject[T] = object
+      f: T = what[T]()
+
+  var m: MyObject[float] = MyObject[float]()
+  doAssert m.f is float
+  doAssert m.f == 123.0