Fix RULE-15-1-3 false positives for lambda conversion operators

change_notes/2026-10-08-fix-fp-rule-15-1-3-lambda-conversion.md

@@ -0,0 +1,4 @@
+- `RULE-15-1-3` - `NonExplicitConversionMember.ql`:
+   - Fixed false positives on the compiler-generated conversion operator of a
+     captureless lambda's closure type (conversion to a function pointer). This
+     operator is implicit by definition and cannot be declared `explicit`.

cpp/misra/src/rules/RULE-15-1-3/NonExplicitConversionMember.ql

@@ -42,6 +42,9 @@ where
     exists(ConversionOperator co |
       x = co and
       not co.isExplicit() and
+      // The conversion to a function pointer of a captureless lambda is
+      // compiler-generated and cannot be declared explicit.
+      not co.getDeclaringType() instanceof Closure and
       message = "Conversion operator shall be explicit."
     )
   )

cpp/misra/test/rules/RULE-15-1-3/test.cpp

@@ -44,4 +44,16 @@ class TestClass7 {
   operator double() const;          // NON_COMPLIANT
   operator const char *() const;    // NON_COMPLIANT
   explicit operator void *() const; // COMPLIANT
-};
+};
+
+void test_lambdas() {
+  // The conversion to a function pointer of a captureless lambda's closure
+  // type is compiler-generated and cannot be declared explicit.
+  auto l1 = [](std::int32_t a) { return a; }; // COMPLIANT
+  auto l2 = [](auto a) { return a; };         // COMPLIANT
+  l2(1);
+  std::int32_t captured = 0;
+  auto l3 = [captured](std::int32_t a) { return a + captured; }; // COMPLIANT
+  (void)l1;
+  (void)l3;
+}