implement template default values using other params (#24073)

fixes #23506

#24065 broke compilation of template parameter default values that
depended on other template parameters. But this was never implemented
anyway, actually attempting to use those default values breaks the
compiler as in #23506. So these are now implemented as well as fixing
the regression.

First, if a default value expression uses any unresolved arguments
(generic or normal template parameters) in a template header, we leave
it untyped, instead of applying the generic typechecking (fixing the
regression). Then, just before the body of the template is about to be
explored, the default value expressions are handled in the same manner
as the body as well. This captures symbols including the parameters, so
the expression is checked again if it contains a parameter symbol, and
marked with `nfDefaultRefsParam` if it does (as an optimization to not
check it later).

Then when the template is being evaluated, when substituting a
parameter, if we try to substitute with a node marked
`nfDefaultRefsParam`, we also evaluate it as we would the template body
instead of passing it as just a copy (the reason why it never worked
before). This way we save time if a default value doesn't refer to
another parameter and could just be copied regardless.

compiler/evaltempl.nim

@@ -33,6 +33,19 @@ proc evalTemplateAux(templ, actual: PNode, c: var TemplCtx, result: PNode) =
     let x = param
     if x.kind == nkArgList:
       for y in items(x): result.add(y)
+    elif nfDefaultRefsParam in x.flags:
+      # value of default param needs to be evaluated like template body
+      # if it contains other template params
+      var res: PNode
+      if isAtom(x):
+        res = newNodeI(nkPar, x.info)
+        evalTemplateAux(x, actual, c, res)
+        if res.len == 1: res = res[0]
+      else:
+        res = copyNode(x)
+        for i in 0..<x.safeLen:
+          evalTemplateAux(x[i], actual, c, res)
+      result.add res
     else:
       result.add copyTree(x)
 

compiler/semexprs.nim

@@ -813,7 +813,7 @@ proc isUnresolvedSym(s: PSym): bool =
   result = s.kind == skGenericParam
   if not result and s.typ != nil:
     result = tfInferrableStatic in s.typ.flags or
-        (s.kind == skParam and s.typ.isMetaType) or
+        (s.kind == skParam and (s.typ.isMetaType or sfTemplateParam in s.flags)) or
         (s.kind == skType and
         s.typ.flags * {tfGenericTypeParam, tfImplicitTypeParam} != {})
 

compiler/semtempl.nim

@@ -743,6 +743,17 @@ proc semTemplateDef(c: PContext, n: PNode): PNode =
     c: c,
     owner: s
   )
+  # handle default params:
+  for i in 1..<s.typ.n.len:
+    let param = s.typ.n[i].sym
+    if param.ast != nil:
+      # param default values need to be treated like template body:
+      if sfDirty in s.flags:
+        param.ast = semTemplBodyDirty(ctx, param.ast)
+      else:
+        param.ast = semTemplBody(ctx, param.ast)
+      if param.ast.referencesAnotherParam(s):
+        param.ast.flags.incl nfDefaultRefsParam
   if sfDirty in s.flags:
     n[bodyPos] = semTemplBodyDirty(ctx, n[bodyPos])
   else:

compiler/semtypes.nim

@@ -1365,6 +1365,11 @@ proc semProcTypeNode(c: PContext, n, genericParams: PNode,
           "either use ';' (semicolon) or explicitly write each default value")
         message(c.config, a.info, warnImplicitDefaultValue, msg)
       block determineType:
+        if kind == skTemplate and hasUnresolvedArgs(c, def):
+          # template default value depends on other parameter
+          # don't do any typechecking
+          def.typ = makeTypeFromExpr(c, def.copyTree)
+          break determineType
         let isGeneric = isCurrentlyGeneric()
         inc c.inGenericContext, ord(isGeneric)
         def = semExprWithType(c, def, {efDetermineType, efAllowSymChoice}, typ)

tests/template/tdefaultparam.nim

@@ -0,0 +1,15 @@
+block:
+  template foo(a: untyped, b: untyped = a(0)): untyped =
+    let x = a(0)
+    let y = b
+    (x, y)
+  proc bar(x: int): int = x + 1
+  doAssert foo(bar, b = bar(0)) == (1, 1)
+  doAssert foo(bar) == (1, 1)
+
+block: # issue #23506
+  var a: string
+  template foo(x: int; y = x) =
+    a = $($x, $y)
+  foo(1)
+  doAssert a == "(\"1\", \"1\")"