[flang] Fixed regression with CDEFINED linkage (llvm#164616)

llvm#162722 introduced a regression
that started creating initializers for CDEFINED variables. CDEFINED
variables cannot have initializers, because their storage is expected
come from elsewhere, likely outside of Fortran. Fixed the regression and
improved the regression test to catch the incorrect initialization case.

Also, based on the code review feedback, made CDEFINED variable
initialization a hard error and updated tests accordingly.

Author: eugeneepshteyn

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

@@ -73,7 +73,7 @@ ENUM_CLASS(UsageWarning, Portability, PointerToUndefinable,
     ZeroDoStep, UnusedForallIndex, OpenMPUsage, DataLength, IgnoredDirective,
     HomonymousSpecific, HomonymousResult, IgnoredIntrinsicFunctionType,
     PreviousScalarUse, RedeclaredInaccessibleComponent, ImplicitShared,
-    IndexVarRedefinition, IncompatibleImplicitInterfaces, CdefinedInit,
+    IndexVarRedefinition, IncompatibleImplicitInterfaces,
     VectorSubscriptFinalization, UndefinedFunctionResult, UselessIomsg,
     MismatchingDummyProcedure, SubscriptedEmptyArray, UnsignedLiteralTruncation,
     CompatibleDeclarationsFromDistinctModules, ConstantIsContiguous,

flang/lib/Semantics/resolve-names.cpp

@@ -9194,11 +9194,11 @@ bool DeclarationVisitor::CheckNonPointerInitialization(
               "'%s' has already been initialized"_err_en_US);
         } else if (IsAllocatable(ultimate)) {
           Say(name, "Allocatable object '%s' cannot be initialized"_err_en_US);
+        } else if (details->isCDefined()) {
+          // CDEFINED variables cannot have initializer, because their storage
+          // may come outside of Fortran.
+          Say(name, "CDEFINED variable cannot be initialized"_err_en_US);
         } else {
-          if (details->isCDefined()) {
-            context().Warn(common::UsageWarning::CdefinedInit, name.source,
-                "CDEFINED variable should not have an initializer"_warn_en_US);
-          }
           return true;
         }
       } else {

flang/test/Lower/cdefined.f90

@@ -1,9 +1,10 @@
 ! RUN: bbc -emit-hlfir -o - %s | FileCheck %s
 ! Ensure that CDEFINED variable has external (default) linkage and that
-! it doesn't have an initializer
+! it doesn't have either zero or constant initializer.
 module m
   use iso_c_binding
-  integer(c_int), bind(C, name='c_global', CDEFINED) :: c  = 42
+  integer(c_int), bind(C, name='c_global', CDEFINED) :: c
   ! CHECK: fir.global @c_global : i32
-  ! CHECK-NOT: fir.zero_bits 
+  ! CHECK-NOT: fir.zero_bits
+  ! CHECK-NOT: arith.constant
 end

flang/test/Semantics/cdefined.f90

@@ -1,6 +1,6 @@
-! RUN: %python %S/test_errors.py %s %flang_fc1 -pedantic -Werror
+! RUN: %python %S/test_errors.py %s %flang_fc1
 module m
   use iso_c_binding
-  !WARNING: CDEFINED variable should not have an initializer [-Wcdefined-init]
+  !ERROR: CDEFINED variable cannot be initialized
   integer(c_int), bind(C, name='c_global', CDEFINED) :: c  = 42
 end