[flang] Allow initialization in blank COMMON

[klausler]

Summary

This is nonconformant usage, but widely accepted as an extension.
Downgrade the error message to a warning.

[PeteSteinfeld]

All builds, tests, and looks good.

flang/docs/Extensions.md

* We support some combinations of specific procedures in generic
   interfaces that a strict reading of the standard would preclude
   when their calls must nonetheless be distinguishable.
   Specifically, `ALLOCATABLE` dummy arguments are distinguishing
   if an actual argument acceptable to one could not be passed to
   the other & vice versa because exactly one is polymorphic or
   exactly one is unlimited polymorphic).
 * External unit 0 is predefined and connected to the standard error output,
   and defined as `ERROR_UNIT` in the intrinsic `ISO_FORTRAN_ENV` module.
+* Objects in blank COMMON may be initialized.
 
 ### Extensions supported when enabled by options
 
 * C-style backslash escape sequences in quoted CHARACTER literals
   (but not Hollerith) [-fbackslash]
 * Logical abbreviations `.T.`, `.F.`, `.N.`, `.A.`, `.O.`, and `.X.`
   [-flogical-abbreviations]
 * `.XOR.` as a synonym for `.NEQV.` [-fxor-operator]

flang/lib/Semantics/check-data.cpp

bool operator()(const Symbol &symbol) { // C876
     const Scope &scope{context_.FindScope(source_)};
     bool isFirstSymbol{isFirstSymbol_};
     isFirstSymbol_ = false;
     if (const char *whyNot{IsAutomatic(symbol) ? "Automatic variable"
                 : IsDummy(symbol)              ? "Dummy argument"
                 : IsFunctionResult(symbol)     ? "Function result"
                 : IsAllocatable(symbol)        ? "Allocatable"
                 : IsInitialized(symbol, true)  ? "Default-initialized"
-                : IsInBlankCommon(symbol)      ? "Blank COMMON object"
                 : IsProcedure(symbol) && !IsPointer(symbol) ? "Procedure"
                 // remaining checks don't apply to components
                 : !isFirstSymbol                   ? nullptr
                 : IsHostAssociated(symbol, scope)  ? "Host-associated object"
                 : IsUseAssociated(symbol, scope)   ? "USE-associated object"
                 : symbol.has<AssocEntityDetails>() ? "Construct association"
                 : IsPointer(symbol) && (hasComponent_ || hasSubscript_)
                 ? "Target of pointer"
                 : nullptr}) {
       context_.Say(source_,
           "%s '%s' must not be initialized in a DATA statement"_err_en_US,
           whyNot, symbol.name());
       return false;
-    } else if (IsProcedurePointer(symbol)) {
+    }
+    if (IsProcedurePointer(symbol)) {
       context_.Say(source_,
           "Procedure pointer '%s' in a DATA statement is not standard"_en_US,
           symbol.name());
     }
+    if (IsInBlankCommon(symbol)) {
+      context_.Say(source_,
+          "Blank COMMON object '%s' in a DATA statement is not standard"_en_US,
+          symbol.name());
+    }
     return true;
   }
   bool operator()(const evaluate::Component &component) {
     hasComponent_ = true;
     const Symbol &lastSymbol{component.GetLastSymbol()};
     if (isPointerAllowed_) {
       if (IsPointer(lastSymbol) && hasSubscript_) { // C877
         context_.Say(source_,

flang/test/Semantics/data04.f90

subroutine checkDerivedType(m2_number)
       !C876
       !OK: m2_number3 is not associated through use association
       DATA m2_number3%number /1/
     end
   end
 
   program new
     use m2
-    integer a
-    real    b,c
-    type seqType
-      sequence
-      integer number
-    end type
-    type(SeqType) num
-    COMMON b,a,c,num
     type(newType) m2_number2
     !C876
-    !ERROR: Blank COMMON object 'b' must not be initialized in a DATA statement
-    DATA b /1/
-    !C876
     !ERROR: USE-associated object 'm2_i' must not be initialized in a DATA statement
     DATA m2_i /1/
     !C876
     !ERROR: USE-associated object 'm2_number1' must not be initialized in a DATA statement
     DATA m2_number1%number /1/
     !C876
     !OK: m2_number2 is not associated through use association
     DATA m2_number2%number /1/
-    !C876
-    !ERROR: Blank COMMON object 'num' must not be initialized in a DATA statement
-    DATA num%number /1/
   end program

flang/test/Semantics/data14.f90

+! RUN: %flang_fc1 -fsyntax-only %s 2>&1 | FileCheck %s
+! Verify varnings on nonconforming DATA statements
+! As a common extension, C876 violations are not errors.
+program main
+  type :: seqType
+    sequence
+    integer :: number
+  end type
+  type(seqType) :: x
+  integer :: j
+  common j, x, y
+  !CHECK: Blank COMMON object 'j' in a DATA statement is not standard
+  data j/1/
+  !CHECK: Blank COMMON object 'x' in a DATA statement is not standard
+  data x%number/2/
+end