[flang] Add support for -fimplicit-none-ext option (#125248)

When -fimplicit-none-ext is passed, flang behaves as if "implicit
none(external)" was specified for all relevant constructs in Fortran
source file.

Note: implicit17.f90 was based on implicit07.f90 with `implicit
none(external)` removed and `-fimplicit-none-ext` added.

Author: eugeneepshteyn

clang/include/clang/Driver/Options.td

@@ -6876,6 +6876,7 @@ defm backslash : OptInFC1FFlag<"backslash", "Specify that backslash in string in
 defm xor_operator : OptInFC1FFlag<"xor-operator", "Enable .XOR. as a synonym of .NEQV.">;
 defm logical_abbreviations : OptInFC1FFlag<"logical-abbreviations", "Enable logical abbreviations">;
 defm implicit_none : OptInFC1FFlag<"implicit-none", "No implicit typing allowed unless overridden by IMPLICIT statements">;
+defm implicit_none_ext : OptInFC1FFlag<"implicit-none-ext", "No implicit externals allowed">;
 defm underscoring : OptInFC1FFlag<"underscoring", "Appends one trailing underscore to external names">;
 defm ppc_native_vec_elem_order: BoolOptionWithoutMarshalling<"f", "ppc-native-vector-element-order",
   PosFlag<SetTrue, [], [ClangOption], "Specifies PowerPC native vector element order (default)">,

clang/lib/Driver/ToolChains/Flang.cpp

@@ -42,6 +42,7 @@ void Flang::addFortranDialectOptions(const ArgList &Args,
                             options::OPT_fopenacc,
                             options::OPT_finput_charset_EQ,
                             options::OPT_fimplicit_none,
+                            options::OPT_fimplicit_none_ext,
                             options::OPT_fno_implicit_none,
                             options::OPT_fbackslash,
                             options::OPT_fno_backslash,

flang/include/flang/Common/Fortran-features.h

@@ -34,13 +34,13 @@ ENUM_CLASS(LanguageFeature, BackslashEscapes, OldDebugLines,
     EquivalenceSameNonSequence, AdditionalIntrinsics, AnonymousParents,
     OldLabelDoEndStatements, LogicalIntegerAssignment, EmptySourceFile,
     ProgramReturn, ImplicitNoneTypeNever, ImplicitNoneTypeAlways,
-    ForwardRefImplicitNone, OpenAccessAppend, BOZAsDefaultInteger,
-    DistinguishableSpecifics, DefaultSave, PointerInSeqType, NonCharacterFormat,
-    SaveMainProgram, SaveBigMainProgramVariables,
-    DistinctArrayConstructorLengths, PPCVector, RelaxedIntentInChecking,
-    ForwardRefImplicitNoneData, NullActualForAllocatable,
-    ActualIntegerConvertedToSmallerKind, HollerithOrCharacterAsBOZ,
-    BindingAsProcedure, StatementFunctionExtensions,
+    ImplicitNoneExternal, ForwardRefImplicitNone, OpenAccessAppend,
+    BOZAsDefaultInteger, DistinguishableSpecifics, DefaultSave,
+    PointerInSeqType, NonCharacterFormat, SaveMainProgram,
+    SaveBigMainProgramVariables, DistinctArrayConstructorLengths, PPCVector,
+    RelaxedIntentInChecking, ForwardRefImplicitNoneData,
+    NullActualForAllocatable, ActualIntegerConvertedToSmallerKind,
+    HollerithOrCharacterAsBOZ, BindingAsProcedure, StatementFunctionExtensions,
     UseGenericIntrinsicWhenSpecificDoesntMatch, DataStmtExtensions,
     RedundantContiguous, RedundantAttribute, InitBlankCommon,
     EmptyBindCDerivedType, MiscSourceExtensions, AllocateToOtherLength,

flang/lib/Common/Fortran-features.cpp

@@ -22,6 +22,7 @@ LanguageFeatureControl::LanguageFeatureControl() {
   disable_.set(LanguageFeature::CudaUnified);
   disable_.set(LanguageFeature::ImplicitNoneTypeNever);
   disable_.set(LanguageFeature::ImplicitNoneTypeAlways);
+  disable_.set(LanguageFeature::ImplicitNoneExternal);
   disable_.set(LanguageFeature::DefaultSave);
   disable_.set(LanguageFeature::SaveMainProgram);
   // These features, if enabled, conflict with valid standard usage,

flang/lib/Frontend/CompilerInvocation.cpp

@@ -740,6 +740,12 @@ static bool parseFrontendArgs(FrontendOptions &opts, llvm::opt::ArgList &args,
       args.hasFlag(clang::driver::options::OPT_fimplicit_none,
                    clang::driver::options::OPT_fno_implicit_none, false));
 
+  // -f{no-}implicit-none-ext
+  opts.features.Enable(
+      Fortran::common::LanguageFeature::ImplicitNoneExternal,
+      args.hasFlag(clang::driver::options::OPT_fimplicit_none_ext,
+                   clang::driver::options::OPT_fno_implicit_none_ext, false));
+
   // -f{no-}backslash
   opts.features.Enable(Fortran::common::LanguageFeature::BackslashEscapes,
                        args.hasFlag(clang::driver::options::OPT_fbackslash,

flang/lib/Semantics/resolve-names.cpp

@@ -87,7 +87,8 @@ class ImplicitRules {
   bool inheritFromParent_{false}; // look in parent if not specified here
   bool isImplicitNoneType_{
       context_.IsEnabled(common::LanguageFeature::ImplicitNoneTypeAlways)};
-  bool isImplicitNoneExternal_{false};
+  bool isImplicitNoneExternal_{
+      context_.IsEnabled(common::LanguageFeature::ImplicitNoneExternal)};
   // map_ contains the mapping between letters and types that were defined
   // by the IMPLICIT statements of the related scope. It does not contain
   // the default Fortran mappings nor the mapping defined in parents.

flang/test/Semantics/implicit17.f90

@@ -0,0 +1,14 @@
+! RUN: %python %S/test_errors.py %s %flang_fc1 -fimplicit-none-ext
+external x
+integer :: f, i, arr(1) = [0]
+call x
+!ERROR: 'y' is an external procedure without the EXTERNAL attribute in a scope with IMPLICIT NONE(EXTERNAL)
+call y
+!ERROR: 'f' is an external procedure without the EXTERNAL attribute in a scope with IMPLICIT NONE(EXTERNAL)
+i = f()
+block
+  !ERROR: 'z' is an external procedure without the EXTERNAL attribute in a scope with IMPLICIT NONE(EXTERNAL)
+  call z
+end block
+print *, arr(1) ! no error
+end