improve diagnostic for inferred uninhabited

Author: Azoy

include/swift/AST/DiagnosticsSema.def

@@ -3303,6 +3303,12 @@ ERROR(ambiguous_enum_pattern_type,none,
 WARNING(type_inferred_to_undesirable_type,none,
         "%select{variable|constant}2 %0 inferred to have type %1, "
         "which may be unexpected", (Identifier, Type, bool))
+WARNING(type_inferred_to_uninhabited_type,none,
+        "%select{variable|constant}2 %0 inferred to have type %1, "
+        "which is an enum with no cases", (Identifier, Type, bool))
+WARNING(type_inferred_to_uninhabited_tuple_type,none,
+        "%select{variable|constant}2 %0 inferred to have type %1, "
+        "which contains an enum with no cases", (Identifier, Type, bool))
 NOTE(add_explicit_type_annotation_to_silence,none,
      "add an explicit type annotation to silence this warning", ())
 

lib/Sema/TypeCheckPattern.cpp

@@ -1100,6 +1100,7 @@ bool TypeChecker::coercePatternToType(Pattern *&P, TypeResolution resolution,
     Type diagTy = type->getOptionalObjectType();
     if (!diagTy) diagTy = type;
 
+    auto diag = diag::type_inferred_to_undesirable_type;
     bool shouldRequireType = false;
     if (NP->isImplicit()) {
       // If the whole pattern is implicit, the user didn't write it.
@@ -1111,14 +1112,22 @@ bool TypeChecker::coercePatternToType(Pattern *&P, TypeResolution resolution,
         shouldRequireType = true;
     } else if (diagTy->isStructurallyUninhabited()) {
       shouldRequireType = true;
+      diag = diag::type_inferred_to_uninhabited_type;
+      
+      if (diagTy->is<TupleType>()) {
+        diag = diag::type_inferred_to_uninhabited_tuple_type;
+      } else {
+        assert((diagTy->is<EnumType>() || diagTy->is<BoundGenericEnumType>()) &&
+          "unknown structurally uninhabited type");
+      }
     }
 
     if (shouldRequireType &&
         !options.is(TypeResolverContext::ForEachStmt) &&
         !options.is(TypeResolverContext::EditorPlaceholderExpr) &&
         !(options & TypeResolutionFlags::FromNonInferredPattern)) {
-      diagnose(NP->getLoc(), diag::type_inferred_to_undesirable_type,
-               NP->getDecl()->getName(), type, NP->getDecl()->isLet());
+      diagnose(NP->getLoc(), diag, NP->getDecl()->getName(), type,
+               NP->getDecl()->isLet());
       diagnose(NP->getLoc(), diag::add_explicit_type_annotation_to_silence);
     }