feat(mps_parser): reject Lazy Constraints LP sections

Previously 'Lazy Constraints' was accepted and parsed as a regular
constraints block. This quietly pulled in content that's meant to be
solver-side metadata, not model structure. Treat it the same as 'User
Cuts': recognize the header as a section boundary so the prior section
ends cleanly, then throw with a clear "not supported (scope is
LP/MIP/QP only)" message.

Removes the LazyConstraints enumerator and its dispatch arm, adds the
display-name branch so reject_unsupported_section() prints "Lazy
Constraints", and adds two tests (unsupported_lazy_constraints_section_throws
and unsupported_user_cuts_section_throws). PARSER_TEST: 95/95 pass.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Signed-off-by: Miles Lubin <mlubin@nvidia.com>

Author: mlubin

cpp/libmps_parser/src/lp_parser.cpp

@@ -223,7 +223,6 @@ class LpParseEngine {
     None,
     Objective,
     Constraints,
-    LazyConstraints,
     Bounds,
     Generals,
     Binaries,
@@ -529,16 +528,16 @@ bool LpParseEngine<i_t, f_t>::at_section_boundary() const
   if (is_binaries_keyword(lower)) return true;
   if (is_end_keyword(lower)) return true;
 
-  // Multi-word section headers: "Subject To", "Such That", "Lazy Constraints",
-  // and the unsupported "User Cuts" / "General Constraints".
+  // Multi-word section headers: "Subject To" / "Such That" are supported;
+  // "Lazy Constraints", "User Cuts", and "General Constraints" are
+  // recognized as boundaries so the prior section ends cleanly, but
+  // reject_unsupported_section() throws once dispatch reaches them.
   const LpToken& t2 = peek(1);
   if (lower == "subject" && name_equals_ci(t2, "to")) return true;
   if (lower == "such" && name_equals_ci(t2, "that")) return true;
   if (lower == "st" || lower == "st." || lower == "s.t.") return true;
   if (lower == "lazy" && name_equals_ci(t2, "constraints")) return true;
   if (lower == "user" && name_equals_ci(t2, "cuts")) return true;
-  // "General Constraints" is unsupported but still a section boundary;
-  // reject_unsupported_section() throws before we'd keep parsing.
   if (lower == "general" && name_equals_ci(t2, "constraints")) return true;
 
   // Unsupported single-token sections.
@@ -564,6 +563,8 @@ void LpParseEngine<i_t, f_t>::reject_unsupported_section()
     name = "Semi-continuous";
   } else if (lower == "user" && name_equals_ci(peek(1), "cuts")) {
     name = "User Cuts";
+  } else if (lower == "lazy" && name_equals_ci(peek(1), "constraints")) {
+    name = "Lazy Constraints";
   } else if (lower == "general" && name_equals_ci(peek(1), "constraints")) {
     name = "General Constraints";
   }
@@ -609,11 +610,6 @@ typename LpParseEngine<i_t, f_t>::SectionKind LpParseEngine<i_t, f_t>::try_consu
     advance();
     return SectionKind::Constraints;
   }
-  if (lower == "lazy" && name_equals_ci(peek(1), "constraints")) {
-    advance();
-    advance();
-    return SectionKind::LazyConstraints;
-  }
   if (is_bounds_keyword(lower)) {
     advance();
     return SectionKind::Bounds;
@@ -909,8 +905,6 @@ void LpParseEngine<i_t, f_t>::parse_objective_section()
 template <typename i_t, typename f_t>
 void LpParseEngine<i_t, f_t>::parse_constraints_section()
 {
-  // Lazy constraints are treated as regular constraints (matching the MPS
-  // parser's LAZYCONS handling).
   while (!at_section_boundary()) {
     // Optional "name:" label — present iff the first two tokens are Name + ':'.
     std::string row_name;
@@ -1096,8 +1090,7 @@ void LpParseEngine<i_t, f_t>::parse_all()
         parse_objective_section();
         saw_objective = true;
         break;
-      case SectionKind::Constraints:
-      case SectionKind::LazyConstraints: parse_constraints_section(); break;
+      case SectionKind::Constraints: parse_constraints_section(); break;
       case SectionKind::Bounds: parse_bounds_section(); break;
       case SectionKind::Generals: parse_integer_list_section(false); break;
       case SectionKind::Binaries: parse_integer_list_section(true); break;

cpp/libmps_parser/tests/parser_test.cpp

@@ -1748,6 +1748,35 @@ End
                std::logic_error);
 }
 
+TEST(lp_parser, unsupported_lazy_constraints_section_throws)
+{
+  // Lazy constraints and user cuts are scope-limited out: LP/MIP/QP only.
+  EXPECT_THROW(parse_lp_string(R"LP(
+Minimize
+  x
+Subject To
+ c1: x >= 1
+Lazy Constraints
+ lc: x <= 10
+End
+)LP"),
+               std::logic_error);
+}
+
+TEST(lp_parser, unsupported_user_cuts_section_throws)
+{
+  EXPECT_THROW(parse_lp_string(R"LP(
+Minimize
+  x
+Subject To
+ c1: x >= 1
+User Cuts
+ uc: x <= 10
+End
+)LP"),
+               std::logic_error);
+}
+
 TEST(lp_parser, unknown_file_throws)
 {
   auto call = [] { return parse_lp<int, double>("/definitely/does/not/exist.lp"); };