[flang] Implemented a warning about contiguity of compile time constant values (llvm#161084)

Implemented `common::UsageWarning::ConstantIsContiguous` to warn about
the
following case:
```
integer, parameter :: num = 3
integer, parameter :: arr(num)=[(i, i=1,num)]
logical, parameter :: result=is_contiguous(arr(num:1:-1))
end
```
Here, while array section is discontiguous, `arr` is a compile time
constant,
so array section created at compile time will end up being contiguous
and
`result` will be "true". If `arr` wasn't a constant, the result at
runtime
would have been "false".

Author: eugeneepshteyn

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

@@ -76,7 +76,7 @@ ENUM_CLASS(UsageWarning, Portability, PointerToUndefinable,
     IndexVarRedefinition, IncompatibleImplicitInterfaces, CdefinedInit,
     VectorSubscriptFinalization, UndefinedFunctionResult, UselessIomsg,
     MismatchingDummyProcedure, SubscriptedEmptyArray, UnsignedLiteralTruncation,
-    CompatibleDeclarationsFromDistinctModules,
+    CompatibleDeclarationsFromDistinctModules, ConstantIsContiguous,
     NullActualForDefaultIntentAllocatable, UseAssociationIntoSameNameSubprogram,
     HostAssociatedIntentOutInSpecExpr, NonVolatilePointerToVolatile,
     RealConstantWidening, VolatileOrAsynchronousTemporary)

flang/lib/Evaluate/fold-logical.cpp

@@ -799,12 +799,20 @@ Expr<Type<TypeCategory::Logical, KIND>> FoldIntrinsicFunction(
     }
   } else if (name == "is_contiguous") {
     if (args.at(0)) {
+      auto warnContiguous{[&]() {
+        if (auto source{args[0]->sourceLocation()}) {
+          context.Warn(common::UsageWarning::ConstantIsContiguous, *source,
+              "is_contiguous() is always true for named constants and subobjects of named constants"_warn_en_US);
+        }
+      }};
       if (auto *expr{args[0]->UnwrapExpr()}) {
         if (auto contiguous{IsContiguous(*expr, context)}) {
+          warnContiguous();
           return Expr<T>{*contiguous};
         }
       } else if (auto *assumedType{args[0]->GetAssumedTypeDummy()}) {
         if (auto contiguous{IsContiguous(*assumedType, context)}) {
+          warnContiguous();
           return Expr<T>{*contiguous};
         }
       }

flang/test/Semantics/contiguous-warn.f90

@@ -0,0 +1,6 @@
+! RUN: %python %S/test_errors.py %s %flang_fc1 -pedantic -Werror
+integer, parameter :: num = 3
+integer, parameter :: arr(num)=[(i, i=1,num)]
+!WARNING: is_contiguous() is always true for named constants and subobjects of named constants [-Wconstant-is-contiguous]
+logical, parameter :: result=is_contiguous(arr(num:1:-1))
+end