make `var`/pointer types not match if base type has to be converted (#24130)

split again from #24038, fixes
https://github.com/status-im/nimbus-eth2/pull/6554#issuecomment-2354977102

`var`/pointer types are no longer implicitly convertible to each other
if their element types either:

* require an int conversion or another conversion operation as long as
it's not to `openarray`,
* are subtypes with pointer indirection,

Previously any conversion below a subrange match would match if the
element type wasn't a pointer type, then it would error later in
`analyseIfAddressTaken`.

Different from #24038 in that the preview define that made subrange
matches also fail to match is removed for a simpler diff so that it can
be backported.

1 files changed, 14 insertions, 2 deletions

diff --git a/compiler/sigmatch.nim b/compiler/sigmatch.nim
index a48e79b7c..873eefd76 100644
--- a/compiler/sigmatch.nim
+++ b/compiler/sigmatch.nim
@@ -1125,9 +1125,21 @@ proc inferStaticsInRange(c: var TCandidate,
     doInferStatic(lowerBound, getInt(upperBound) + 1 - lengthOrd(c.c.config, concrete))
 
 template subtypeCheck() =
-  if result <= isSubrange and f.last.skipTypes(abstractInst).kind in {
-      tyRef, tyPtr, tyVar, tyLent, tyOwned}:
+  case result
+  of isIntConv:
     result = isNone
+  of isSubrange:
+    discard # XXX should be isNone with preview define, warnings
+  of isConvertible:
+    if f.last.skipTypes(abstractInst).kind != tyOpenArray:
+      # exclude var openarray which compiler supports
+      result = isNone
+  of isSubtype:
+    if f.last.skipTypes(abstractInst).kind in {
+        tyRef, tyPtr, tyVar, tyLent, tyOwned}:
+      # compiler can't handle subtype conversions with pointer indirection
+      result = isNone
+  else: discard
 
 proc isCovariantPtr(c: var TCandidate, f, a: PType): bool =
   # this proc is always called for a pair of matching types