[flang] Retain no-op default: cases when compiling with gcc

Original-commit: flang-compiler/f18@8ebfd8d2e9
Reviewed-on: https://github.com/flang-compiler/f18/pull/774
Tree-same-pre-rewrite: false

flang/lib/common/idioms.h

@@ -74,6 +74,16 @@ template<typename... LAMBDAS> visitors(LAMBDAS... x)->visitors<LAMBDAS...>;
 // For switch statements without default: labels.
 #define CRASH_NO_CASE DIE("no case")
 
+// For switch statements whose cases have return statements for
+// all possibilities.  Clang emits warnings if the default: is
+// present, gcc emits warings if it is absent.
+#if __clang__
+#define SWITCH_COVERS_ALL_CASES
+#else
+#define SWITCH_COVERS_ALL_CASES \
+  default: CRASH_NO_CASE; \
+           #endif
+
 // For cheap assertions that should be applied in production.
 // To disable, compile with '-DCHECK=(void)'
 #ifndef CHECK

flang/lib/parser/characters.cc

@@ -122,7 +122,9 @@ template<> EncodedCharacter EncodeCharacter<Encoding::UTF_8>(char32_t ucs) {
 EncodedCharacter EncodeCharacter(Encoding encoding, char32_t ucs) {
   switch (encoding) {
   case Encoding::LATIN_1: return EncodeCharacter<Encoding::LATIN_1>(ucs);
-  case Encoding::UTF_8: return EncodeCharacter<Encoding::UTF_8>(ucs);
+  case Encoding::UTF_8:
+    return EncodeCharacter<Encoding::UTF_8>(ucs);
+    SWITCH_COVERS_ALL_CASES
   }
 }
 
@@ -254,6 +256,7 @@ DecodedCharacter DecodeCharacter(Encoding encoding, const char *cp,
     return DecodeCharacter<Encoding::LATIN_1>(cp, bytes, backslashEscapes);
   case Encoding::UTF_8:
     return DecodeCharacter<Encoding::UTF_8>(cp, bytes, backslashEscapes);
+    SWITCH_COVERS_ALL_CASES
   }
 }
 

flang/lib/semantics/expression.cc

@@ -128,6 +128,7 @@ common::IfNoLvalue<MaybeExpr, WRAPPED> TypedWrapper(
         dyType.kind(), std::move(x));
   case TypeCategory::Derived:
     return AsGenericExpr(Expr<SomeDerived>{WRAPPER<SomeDerived>{std::move(x)}});
+    SWITCH_COVERS_ALL_CASES
   }
 }
 

flang/lib/semantics/resolve-names-utils.cc

@@ -193,7 +193,9 @@ static GenericKind MapIntrinsicOperator(IntrinsicOperator op) {
   case IntrinsicOperator::OR: return GenericKind::OpOR;
   case IntrinsicOperator::XOR: return GenericKind::OpXOR;
   case IntrinsicOperator::EQV: return GenericKind::OpEQV;
-  case IntrinsicOperator::NEQV: return GenericKind::OpNEQV;
+  case IntrinsicOperator::NEQV:
+    return GenericKind::OpNEQV;
+    SWITCH_COVERS_ALL_CASES
   }
 }
 

flang/lib/semantics/resolve-names.cc

@@ -4824,7 +4824,7 @@ const DeclTypeSpec &ConstructVisitor::ToDeclTypeSpec(
 
       );
     }
-  case common::TypeCategory::Character: CRASH_NO_CASE;
+  case common::TypeCategory::Character: CRASH_NO_CASE; SWITCH_COVERS_ALL_CASES
   }
 }
 
@@ -5730,6 +5730,7 @@ bool ResolveNamesVisitor::BeginScope(const ProgramTree &node) {
   case ProgramTree::Kind::Module: BeginModule(node.name(), false); return true;
   case ProgramTree::Kind::Submodule:
     return BeginSubmodule(node.name(), node.GetParentId());
+    SWITCH_COVERS_ALL_CASES
   }
 }
 

flang/lib/semantics/scope.cc

@@ -215,7 +215,9 @@ bool Scope::CanImport(const SourceName &name) const {
   case ImportKind::None: return false;
   case ImportKind::All:
   case ImportKind::Default: return true;
-  case ImportKind::Only: return importNames_.count(name) > 0;
+  case ImportKind::Only:
+    return importNames_.count(name) > 0;
+    SWITCH_COVERS_ALL_CASES
   }
 }
 

flang/lib/semantics/type.cc

@@ -166,6 +166,7 @@ std::string ParamValue::AsFortran() const {
     } else {
       return "";
     }
+    SWITCH_COVERS_ALL_CASES
   }
 }
 
@@ -251,7 +252,7 @@ std::string DeclTypeSpec::AsFortran() const {
   case TypeDerived: return "TYPE(" + derivedTypeSpec().AsFortran() + ')';
   case ClassDerived: return "CLASS(" + derivedTypeSpec().AsFortran() + ')';
   case TypeStar: return "TYPE(*)";
-  case ClassStar: return "CLASS(*)";
+  case ClassStar: return "CLASS(*)"; SWITCH_COVERS_ALL_CASES
   }
 }