Make unreachable code a warning instead of an error (#14816)

* Make unreachable code a warning

* Adapt test

* Trigger CI

compiler/lineinfos.nim

@@ -49,7 +49,8 @@ type
     warnEachIdentIsTuple,
     warnUnsafeSetLen,
     warnUnsafeDefault,
-    warnProveInit, warnProveField, warnProveIndex, warnUnreachableElse,
+    warnProveInit, warnProveField, warnProveIndex,
+    warnUnreachableElse, warnUnreachableCode,
     warnStaticIndexCheck, warnGcUnsafe, warnGcUnsafe2,
     warnUninit, warnGcMem, warnDestructor, warnLockLevel, warnResultShadowed,
     warnInconsistentSpacing, warnCaseTransition, warnCycleCreated,
@@ -110,6 +111,7 @@ const
     warnProveField: "cannot prove that field '$1' is accessible",
     warnProveIndex: "cannot prove index '$1' is valid",
     warnUnreachableElse: "unreachable else, all cases are already covered",
+    warnUnreachableCode: "unreachable code after 'return' statement or '{.noReturn.}' proc",
     warnStaticIndexCheck: "$1",
     warnGcUnsafe: "not GC-safe: '$1'",
     warnGcUnsafe2: "$1",
@@ -169,7 +171,7 @@ const
     "UnsafeCode", "UnusedImport", "InheritFromException",
     "EachIdentIsTuple",
     "UnsafeSetLen", "UnsafeDefault",
-    "ProveInit", "ProveField", "ProveIndex", "UnreachableElse",
+    "ProveInit", "ProveField", "ProveIndex", "UnreachableElse", "UnreachableCode",
     "IndexCheck", "GcUnsafe", "GcUnsafe2", "Uninit",
     "GcMem", "Destructor", "LockLevel", "ResultShadowed",
     "Spacing", "CaseTransition", "CycleCreated",

compiler/semstmts.nim

@@ -2269,10 +2269,8 @@ proc semStmtList(c: PContext, n: PNode, flags: TExprFlags): PNode =
         sfNoReturn in n[i][0].sym.flags:
       for j in i + 1..<n.len:
         case n[j].kind
-        of nkPragma, nkCommentStmt, nkNilLit, nkEmpty, nkBlockExpr,
-            nkBlockStmt, nkState: discard
-        else: localError(c.config, n[j].info,
-          "unreachable statement after 'return' statement or '{.noReturn.}' proc")
+        of nkPragma, nkCommentStmt, nkNilLit, nkEmpty, nkState: discard
+        else: message(c.config, n[j].info, warnUnreachableCode)
     else: discard
 
   if result.len == 1 and

tests/controlflow/tstatret.nim

@@ -1,7 +1,7 @@
 discard """
-  errormsg: "unreachable statement after 'return' statement or '{.noReturn.}' proc"
-  file: "tstatret.nim"
-  line: 9
+  nimout: '''
+tstatret.nim(9, 7) Warning: unreachable code after 'return' statement or '{.noReturn.}' proc [UnreachableCode]
+'''
 """
 # no statement after return
 proc main() =