always lookup pure enum symbols if expected type is enum (#23976)

fixes #23689

Normally pure enum symbols only "exist" in lookup if nothing else with
the same name is in scope. But if an expression is expected to be an
enum type, we know that ambiguity can be resolved between different
symbols based on their type, so we can include the normally inaccessible
pure enum fields in the ambiguity resolution in the case that the
expected enum type is actually a pure enum. This handles the use case in
the issue of the type inference for enums reverted in #23588.

I know pure enums are supposed to be on their way out so this might seem
excessive, but the `pure` pragma can't be removed in the code in the
issue due to a redefinition error, they have to be separated into
different modules. Normal enums can still resolve the ambiguity here
though. I always think about making a list of all the remaining use
cases for pure enums and I always forget.

Will close #23694 if CI passes

compiler/lookups.nim

@@ -632,9 +632,10 @@ type
 
 const allExceptModule = {low(TSymKind)..high(TSymKind)} - {skModule, skPackage}
 
-proc lookUpCandidates*(c: PContext, ident: PIdent, filter: set[TSymKind]): seq[PSym] =
+proc lookUpCandidates*(c: PContext, ident: PIdent, filter: set[TSymKind],
+                       includePureEnum = false): seq[PSym] =
   result = searchInScopesFilterBy(c, ident, filter)
-  if result.len == 0:
+  if skEnumField in filter and (result.len == 0 or includePureEnum):
     result.add allPureEnumFields(c, ident)
 
 proc qualifiedLookUp*(c: PContext, n: PNode, flags: set[TLookupFlag]): PSym =

compiler/semexprs.nim

@@ -3107,7 +3107,10 @@ proc resolveIdentToSym(c: PContext, n: PNode, resultNode: var PNode,
   if efNoEvaluateGeneric in flags or expectedType != nil:
     # `a[...]` where `a` is a module or package is not possible
     filter.excl {skModule, skPackage}
-  let candidates = lookUpCandidates(c, ident, filter)
+  let includePureEnum = expectedType != nil and
+    expectedType.skipTypes(abstractRange-{tyDistinct}).kind == tyEnum
+  let candidates = lookUpCandidates(c, ident, filter,
+    includePureEnum = includePureEnum)
   if candidates.len == 0:
     result = errorUndeclaredIdentifierHint(c, ident, n.info)
   elif candidates.len == 1 or {efNoEvaluateGeneric, efInCall} * flags != {}:

tests/enum/ttypenameconflict.nim

@@ -0,0 +1,13 @@
+# issue #23689
+
+type
+  MyEnum {.pure.} = enum
+    A, B, C, D
+
+  B = object
+    field: int
+
+let x: MyEnum = B
+doAssert $x == "B"
+doAssert typeof(x) is MyEnum
+doAssert x in {A, B}