[flang] Relax constraints on PURE/ELEMENTAL dummy arguments

The standard requires that dummy arguments to PURE functions
be INTENT(IN) or VALUE, but PURE subroutines are allowed to
have modifiable dummy arguments.  This makes it impossible to
declare atomic operations as PURE functions, which consequently
makes such atomic operations ineligible for use in parallel
constructs and DO CONCURRENT.

This patch downgrades this error to a warning by default, which
can be seen with -pedantic & al. and remain an error with -Werror.

Author: klausler

flang/docs/Extensions.md

@@ -118,6 +118,12 @@ end
   procedure interface.  This compiler accepts it, since there is otherwise
   no way to declare an interoperable dummy procedure with an arbitrary
   interface like `void (*)()`.
+* `PURE` functions are allowed to have dummy arguments that are
+  neither `INTENT(IN)` nor `VALUE`, similar to `PURE` subroutines,
+  with a warning.
+  This enables atomic memory operations to be naturally represented
+  as `PURE` functions, which allows their use in parallel constructs
+  and `DO CONCURRENT`.
 
 ## Extensions, deletions, and legacy features supported by default
 

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

@@ -50,7 +50,7 @@ ENUM_CLASS(LanguageFeature, BackslashEscapes, OldDebugLines,
     EmptySequenceType, NonSequenceCrayPointee, BranchIntoConstruct,
     BadBranchTarget, ConvertedArgument, HollerithPolymorphic, ListDirectedSize,
     NonBindCInteroperability, CudaManaged, CudaUnified,
-    PolymorphicActualAllocatableOrPointerToMonomorphicDummy)
+    PolymorphicActualAllocatableOrPointerToMonomorphicDummy, RelaxedPureDummy)
 
 // Portability and suspicious usage warnings
 ENUM_CLASS(UsageWarning, Portability, PointerToUndefinable,

flang/lib/Semantics/check-declarations.cpp

@@ -704,29 +704,58 @@ void CheckHelper::CheckObjectEntity(
     if (InPure() && !IsStmtFunction(DEREF(innermostSymbol_)) &&
         !IsPointer(symbol) && !IsIntentIn(symbol) &&
         !symbol.attrs().test(Attr::VALUE)) {
-      if (InFunction()) { // C1583
-        messages_.Say(
-            "non-POINTER dummy argument of pure function must be INTENT(IN) or VALUE"_err_en_US);
-      } else if (IsIntentOut(symbol)) {
+      const char *what{InFunction() ? "function" : "subroutine"};
+      bool ok{true};
+      if (IsIntentOut(symbol)) {
         if (type && type->IsPolymorphic()) { // C1588
           messages_.Say(
-              "An INTENT(OUT) dummy argument of a pure subroutine may not be polymorphic"_err_en_US);
+              "An INTENT(OUT) dummy argument of a pure %s may not be polymorphic"_err_en_US,
+              what);
+          ok = false;
         } else if (derived) {
           if (FindUltimateComponent(*derived, [](const Symbol &x) {
                 const DeclTypeSpec *type{x.GetType()};
                 return type && type->IsPolymorphic();
               })) { // C1588
             messages_.Say(
-                "An INTENT(OUT) dummy argument of a pure subroutine may not have a polymorphic ultimate component"_err_en_US);
+                "An INTENT(OUT) dummy argument of a pure %s may not have a polymorphic ultimate component"_err_en_US,
+                what);
+            ok = false;
           }
           if (HasImpureFinal(symbol)) { // C1587
             messages_.Say(
-                "An INTENT(OUT) dummy argument of a pure subroutine may not have an impure FINAL subroutine"_err_en_US);
+                "An INTENT(OUT) dummy argument of a pure %s may not have an impure FINAL subroutine"_err_en_US,
+                what);
+            ok = false;
           }
         }
       } else if (!IsIntentInOut(symbol)) { // C1586
-        messages_.Say(
-            "non-POINTER dummy argument of pure subroutine must have INTENT() or VALUE attribute"_err_en_US);
+        if (context_.IsEnabled(common::LanguageFeature::RelaxedPureDummy)) {
+          if (context_.ShouldWarn(common::LanguageFeature::RelaxedPureDummy) &&
+              !InModuleFile() && !InElemental()) {
+            messages_.Say(
+                "non-POINTER dummy argument of pure %s should have INTENT() or VALUE attribute"_warn_en_US,
+                what);
+            ok = false;
+          }
+        } else {
+          messages_.Say(
+              "non-POINTER dummy argument of pure %s must have INTENT() or VALUE attribute"_warn_en_US,
+              what);
+          ok = false;
+        }
+      }
+      if (ok && InFunction()) {
+        if (context_.IsEnabled(common::LanguageFeature::RelaxedPureDummy)) {
+          if (context_.ShouldWarn(common::LanguageFeature::RelaxedPureDummy) &&
+              !InModuleFile() && !InElemental()) {
+            messages_.Say(
+                "non-POINTER dummy argument of pure function should be INTENT(IN) or VALUE"_warn_en_US);
+          }
+        } else {
+          messages_.Say(
+              "non-POINTER dummy argument of pure function must be INTENT(IN) or VALUE"_err_en_US);
+        }
       }
     }
     if (auto ignoreTKR{GetIgnoreTKR(symbol)}; !ignoreTKR.empty()) {
@@ -798,9 +827,17 @@ void CheckHelper::CheckObjectEntity(
             "A dummy argument of an ELEMENTAL procedure may not be a POINTER"_err_en_US);
       }
       if (!symbol.attrs().HasAny(Attrs{Attr::VALUE, Attr::INTENT_IN,
-              Attr::INTENT_INOUT, Attr::INTENT_OUT})) { // C15102
-        messages_.Say(
-            "A dummy argument of an ELEMENTAL procedure must have an INTENT() or VALUE attribute"_err_en_US);
+              Attr::INTENT_INOUT, Attr::INTENT_OUT})) { // F'2023 C15120
+        if (context_.IsEnabled(common::LanguageFeature::RelaxedPureDummy)) {
+          if (context_.ShouldWarn(common::LanguageFeature::RelaxedPureDummy) &&
+              !InModuleFile()) {
+            messages_.Say(
+                "A dummy argument of an ELEMENTAL procedure should have an INTENT() or VALUE attribute"_warn_en_US);
+          }
+        } else {
+          messages_.Say(
+              "A dummy argument of an ELEMENTAL procedure must have an INTENT() or VALUE attribute"_err_en_US);
+        }
       }
     } else if (IsFunctionResult(symbol)) { // C15101
       if (details.shape().Rank() > 0) {

flang/test/Semantics/call10.f90

@@ -1,4 +1,4 @@
-! RUN: %python %S/test_errors.py %s %flang_fc1
+! RUN: %python %S/test_errors.py %s %flang_fc1 -pedantic
 ! Test 15.7 (C1583-C1590, C1592-C1599) constraints and restrictions
 ! for pure procedures.
 ! (C1591 is tested in call11.f90; C1594 in call12.f90.)
@@ -53,14 +53,14 @@ pure real function f02(a)
     real, value :: a ! ok
   end function
   pure real function f03(a) ! C1583
-    !ERROR: non-POINTER dummy argument of pure function must be INTENT(IN) or VALUE
+    !ERROR: non-POINTER dummy argument of pure function should have INTENT() or VALUE attribute
     real :: a
   end function
   pure real function f03a(a)
     real, pointer :: a ! ok
   end function
   pure real function f04(a) ! C1583
-    !ERROR: non-POINTER dummy argument of pure function must be INTENT(IN) or VALUE
+    !WARNING: non-POINTER dummy argument of pure function should be INTENT(IN) or VALUE
     real, intent(out) :: a
   end function
   pure real function f04a(a)
@@ -83,7 +83,7 @@ pure function f08() ! C1585
   end function
 
   pure subroutine s01(a) ! C1586
-    !ERROR: non-POINTER dummy argument of pure subroutine must have INTENT() or VALUE attribute
+    !WARNING: non-POINTER dummy argument of pure subroutine should have INTENT() or VALUE attribute
     real :: a
   end subroutine
   pure subroutine s01a(a)

flang/test/Semantics/elemental01.f90

@@ -1,4 +1,4 @@
-! RUN: %python %S/test_errors.py %s %flang_fc1
+! RUN: %python %S/test_errors.py %s %flang_fc1 -pedantic
 ! Tests ELEMENTAL subprogram constraints C15100-15102
 
 !ERROR: An ELEMENTAL subroutine may not have an alternate return dummy argument
@@ -26,7 +26,7 @@ elemental subroutine ptrarg(a)
 end subroutine
 
 impure elemental subroutine barearg(a)
-  !ERROR: A dummy argument of an ELEMENTAL procedure must have an INTENT() or VALUE attribute
+  !WARNING: A dummy argument of an ELEMENTAL procedure should have an INTENT() or VALUE attribute
   real :: a
 end subroutine