[flang] Fix purity checking for internal subprograms (llvm#91759)

ELEMENTAL internal subprograms are pure unless explicitly IMPURE.

Author: klausler

flang/lib/Semantics/check-purity.cpp

@@ -39,12 +39,16 @@ bool PurityChecker::InPureSubprogram() const {
 
 bool PurityChecker::HasPurePrefix(
     const std::list<parser::PrefixSpec> &prefixes) const {
+  bool result{false};
   for (const parser::PrefixSpec &prefix : prefixes) {
-    if (std::holds_alternative<parser::PrefixSpec::Pure>(prefix.u)) {
-      return true;
+    if (std::holds_alternative<parser::PrefixSpec::Impure>(prefix.u)) {
+      return false;
+    } else if (std::holds_alternative<parser::PrefixSpec::Pure>(prefix.u) ||
+        std::holds_alternative<parser::PrefixSpec::Elemental>(prefix.u)) {
+      result = true;
     }
   }
-  return false;
+  return result;
 }
 
 void PurityChecker::Entered(

flang/test/Semantics/pure02.f90

@@ -0,0 +1,59 @@
+! RUN: %python %S/test_errors.py %s %flang_fc1
+pure subroutine s1
+ contains
+  !ERROR: An internal subprogram of a pure subprogram must also be pure
+  subroutine t1
+  end
+  pure subroutine t2 ! ok
+  end
+  elemental subroutine t3(k) ! ok
+    integer, intent(in) :: k
+  end
+  !ERROR: An internal subprogram of a pure subprogram must also be pure
+  impure elemental subroutine t4(k)
+    integer, intent(in) :: k
+  end
+  !ERROR: An internal subprogram of a pure subprogram must also be pure
+  elemental impure subroutine t5(k)
+    integer, intent(in) :: k
+  end
+end
+
+elemental subroutine s2(j)
+  integer, intent(in) :: j
+ contains
+  !ERROR: An internal subprogram of a pure subprogram must also be pure
+  subroutine t1
+  end
+  pure subroutine t2 ! ok
+  end
+  elemental subroutine t3(k) ! ok
+    integer, intent(in) :: k
+  end
+  !ERROR: An internal subprogram of a pure subprogram must also be pure
+  impure elemental subroutine t4(k)
+    integer, intent(in) :: k
+  end
+  !ERROR: An internal subprogram of a pure subprogram must also be pure
+  elemental impure subroutine t5(k)
+    integer, intent(in) :: k
+  end
+end
+
+impure elemental subroutine s3(j)
+  integer, intent(in) :: j
+ contains
+  subroutine t1
+  end
+  pure subroutine t2
+  end
+  elemental subroutine t3(k)
+    integer, intent(in) :: k
+  end
+  impure elemental subroutine t4(k)
+    integer, intent(in) :: k
+  end
+  elemental impure subroutine t5(k)
+    integer, intent(in) :: k
+  end
+end