[flang] Allow empty SEQUENCE types (#66252)

The Fortran standards require (F'2023 C745) that a derived type with the
SEQUENCE attribute have at least one component. No Fortran compiler
actually enforces this constraint. Accept this usage with a warning.

Author: klausler

flang/docs/Extensions.md

@@ -428,6 +428,9 @@ end
 * Since Fortran 90, INCLUDE lines have been allowed to have
   a numeric kind parameter prefix on the file name.  No other
   Fortran compiler supports them that I can find.
+* A `SEQUENCE` derived type is required (F'2023 C745) to have
+  at least one component.  No compiler enforces this constraint;
+  this compiler emits a warning.
 
 ## Behavior in cases where the standard is ambiguous or indefinite
 

flang/lib/Semantics/resolve-names.cpp

@@ -5226,9 +5226,10 @@ bool DeclarationVisitor::Pre(const parser::DerivedTypeDef &x) {
   Walk(componentDefs);
   if (derivedTypeInfo_.sequence) {
     details.set_sequence(true);
-    if (componentDefs.empty()) { // C740
+    if (componentDefs.empty()) {
+      // F'2023 C745 - not enforced by any compiler
       Say(stmt.source,
-          "A sequence type must have at least one component"_err_en_US);
+          "A sequence type should have at least one component"_warn_en_US);
     }
     if (!details.paramNames().empty()) { // C740
       Say(stmt.source,

flang/test/Semantics/resolve31.f90

@@ -68,7 +68,7 @@ module m4
   !ERROR: A sequence type may not have a CONTAINS statement
   contains
   end type
-  !ERROR: A sequence type must have at least one component
+  !WARNING: A sequence type should have at least one component
   type :: emptyType
     sequence
   end type emptyType