Make unreachable else in case statements a warning instead of an error (#14190)

* Fix #14019 by making trailing else a warning

* Rename to UnreachableElse

compiler/lineinfos.nim

@@ -39,7 +39,7 @@ type
     warnEachIdentIsTuple,
     warnUnsafeSetLen,
     warnUnsafeDefault,
-    warnProveInit, warnProveField, warnProveIndex,
+    warnProveInit, warnProveField, warnProveIndex, warnUnreachableElse,
     warnStaticIndexCheck, warnGcUnsafe, warnGcUnsafe2,
     warnUninit, warnGcMem, warnDestructor, warnLockLevel, warnResultShadowed,
     warnInconsistentSpacing, warnCaseTransition, warnCycleCreated, warnUser,
@@ -96,6 +96,7 @@ const
     warnProveInit: "Cannot prove that '$1' is initialized. This will become a compile time error in the future.",
     warnProveField: "cannot prove that field '$1' is accessible",
     warnProveIndex: "cannot prove index '$1' is valid",
+    warnUnreachableElse: "unreachable else, all cases are already covered",
     warnStaticIndexCheck: "$1",
     warnGcUnsafe: "not GC-safe: '$1'",
     warnGcUnsafe2: "$1",
@@ -153,7 +154,7 @@ const
     "UnsafeCode", "UnusedImport", "InheritFromException",
     "EachIdentIsTuple",
     "UnsafeSetLen", "UnsafeDefault",
-    "ProveInit", "ProveField", "ProveIndex",
+    "ProveInit", "ProveField", "ProveIndex", "UnreachableElse",
     "IndexCheck", "GcUnsafe", "GcUnsafe2", "Uninit",
     "GcMem", "Destructor", "LockLevel", "ResultShadowed",
     "Spacing", "CaseTransition", "CycleCreated", "User"]

compiler/semstmts.nim

@@ -998,7 +998,7 @@ proc semCase(c: PContext, n: PNode; flags: TExprFlags): PNode =
       x[0] = semExprBranchScope(c, x[0])
       typ = commonType(typ, x[0])
       if (chckCovered and covered == toCover(c, n[0].typ)) or hasElse:
-        localError(c.config, x.info, "invalid else, all cases are already covered")
+        message(c.config, x.info, warnUnreachableElse)
       hasElse = true
       chckCovered = false
     else:

compiler/semtypes.nim

@@ -695,7 +695,7 @@ proc semRecordCase(c: PContext, n: PNode, check: var IntSet, pos: var int,
     of nkElse:
       checkSonsLen(b, 1, c.config)
       if chckCovered and covered == toCover(c, a[0].typ):
-        localError(c.config, b.info, "invalid else, all cases are already covered")
+        message(c.config, b.info, warnUnreachableElse)
       chckCovered = false
     else: illFormedAst(n, c.config)
     delSon(b, b.len - 1)