adapt generic default parameters to recent generics changes (#24065)

fixes #16700, fixes #20916, refs #24010

Fixes the instantiation issues for proc param default values encountered
in #24010 by:

1. semchecking generic default param values with `inGenericContext` for
#22029 and followups to apply (the bigger change in semtypes),
2. rejecting explicit generic instantiations with unresolved generic
types inside `inGenericContext` (sigmatch change),
3. instantiating the default param values using `prepareNode` rather
than an insufficient manual method (the bigger change in seminst).

This had an important side effect of references to other parameters not
working since they would be resolved as a symbol belonging to the
uninstantiated original generic proc rather than the later instantiated
proc. There is a more radical way to fix this which is generating ident
nodes with `tyFromExpr` in specifically this context, but instead we
just count them as belonging to the same proc in
`hoistParamsUsedInDefault`.

Other minor bugfixes:

* To make the error message in t20883 make sense, we now give a "cannot
instantiate" error when trying to instantiate a proc generic param with
`tyFromExpr`.
* Object constructors as default param values generated default values
of private fields going through `evalConstExpr` more than once, but the
VM doesn't mark the object fields as `nfSkipFieldChecking` which gives a
"cannot access field" error. So the VM now marks object fields it
generates as `nfSkipFieldChecking`. Not sure if this affects VM
performance, don't see why it would.
* The nkRecWhen changes in #24042 didn't cover the case where all
conditions are constantly false correctly, this is fixed with a minor
change. This isn't needed for this PR now but I encountered it after
forgetting to `dec c.inGenericContext`.

1 files changed, 5 insertions, 1 deletions

diff --git a/compiler/semexprs.nim b/compiler/semexprs.nim
index 9b255bdf4..bb82a7cd7 100644
--- a/compiler/semexprs.nim
+++ b/compiler/semexprs.nim
@@ -3084,7 +3084,11 @@ proc hoistParamsUsedInDefault(c: PContext, call, letSection, defExpr: var PNode)
   # duty is activated by returning a non-nil value. The caller is responsible
   # for replacing the input to the function with the returned non-nil value.
   # (which is the hoisted symbol)
-  if defExpr.kind == nkSym and defExpr.sym.kind == skParam and defExpr.sym.owner == call[0].sym:
+  if defExpr.kind == nkSym and defExpr.sym.kind == skParam and
+      (defExpr.sym.owner == call[0].sym or
+        # symbol was resolved before proc was instantiated:
+        (sfFromGeneric in call[0].sym.flags and
+          defExpr.sym.owner == call[0].sym.instantiatedFrom)):
     let paramPos = defExpr.sym.position + 1
 
     if call[paramPos].skipAddr.kind != nkSym and not (