[flang] handle intrinsic interfaces in FunctionRef::GetType (#89583)

User functions may be declared with an interface that is a specific
intrinsic. In such case, there is no result type available from the
procedure symbol (at least without using evaluate::Probe), and
FunctionRef::GetType() returned nullopt. This caused lowering to crash.
The result type of specific intrinsic procedures is always a lengthless
intrinsic type, so it is fully defined in the template argument of
FunctionRef. Use it.

flang/include/flang/Evaluate/call.h

@@ -287,15 +287,18 @@ public:
       : ProcedureRef{std::move(p), std::move(a)} {}
 
   std::optional<DynamicType> GetType() const {
-    if (auto type{proc_.GetType()}) {
+    if constexpr (IsLengthlessIntrinsicType<A>) {
+      return A::GetType();
+    } else if (auto type{proc_.GetType()}) {
       // TODO: Non constant explicit length parameters of PDTs result should
       // likely be dropped too. This is not as easy as for characters since some
       // long lived DerivedTypeSpec pointer would need to be created here. It is
       // not clear if this is causing any issue so far since the storage size of
       // PDTs is independent of length parameters.
       return type->DropNonConstantCharacterLength();
+    } else {
+      return std::nullopt;
     }
-    return std::nullopt;
   }
 };
 } // namespace Fortran::evaluate

flang/test/Lower/HLFIR/calls-f77.f90

@@ -186,3 +186,16 @@ subroutine alternate_return_call(n1, n2, k)
   ! CHECK: ^[[block2]]: // pred: ^bb0
 7 k =  1; return
 end
+
+! -----------------------------------------------------------------------------
+!     Test calls to user procedures with intrinsic interfaces
+! -----------------------------------------------------------------------------
+
+! CHECK-NAME: func.func @_QPintrinsic_iface()
+subroutine intrinsic_iface()
+  intrinsic acos
+  real :: x
+  procedure(acos) :: proc
+  x = proc(1.0)
+end subroutine
+! CHECK" fir.call @_QPproc(%{{.*}}) {{.*}}: (!fir.ref<f32>) -> f32