Fix #15080 FP uninitMemberVarNoCtor for const and reference members (#8910)

https://trac.cppcheck.net/ticket/15080

```cpp
struct S {
    const std::array<std::uint8_t, 2> a;
    const std::vector<std::uint8_t>::const_iterator it;
};
S f(const std::vector<std::uint8_t>& v) {
    return S{ {}, v.cbegin() };
}
```
gives `Member variable 'S::it' has no initializer.
[uninitMemberVarNoCtor]`, and `struct S { int a = 0; const int b; int&
c; };` warns for `b` and `c`.

A non-static const or reference member without default member
initializer can't stay uninitialized: for trivial types the implicit
default constructor is deleted, so every instance must initialize it,
and a const class type is default-constructed. These members are now
skipped by the no-constructor check.

Since f90a76f (#15072) this also shows up for class templates whose
member type is a `using` alias.

This PR was created by Claude Code (an AI coding agent) on behalf of
@autoantwort.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

---------

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>
Co-authored-by: claude[bot] <209825114+claude[bot]@users.noreply.github.com>

Author: 3 people

lib/checkclass.cpp

@@ -347,19 +347,25 @@ void CheckClassImpl::constructors()
             // Variables with default initializers
             bool hasAnyDefaultInit = false;
             bool hasAnySelfInit = false;
+            bool isDefaultConstructible = true;
             const bool cpp14OrLater = mSettings.standards.cpp >= Standards::CPP14;
             for (Usage& usage : usageList) {
                 const Variable& var = *usage.var;
+                // isReference() is also true for a function pointer that returns a reference
+                const bool isReferenceMember = var.isReference() && var.nameToken() && Token::Match(var.nameToken()->previous(), "&|&&");
 
                 // check for C++11 initializer
                 if (var.hasDefault()) {
                     usage.init = true;
                     hasAnyDefaultInit = true;
+                } else if (!var.isStatic() && (isReferenceMember || (var.isConst() && !var.isClass()))) {
+                    // the default constructor is deleted, so all members are initialized by every instance
+                    isDefaultConstructible = false;
                 } else if (cpp14OrLater && !hasAnySelfInit && isInitialized(usage, FunctionType::eConstructor)) {
                     hasAnySelfInit = true;
                 }
             }
-            if (!hasAnyDefaultInit && !hasAnySelfInit)
+            if (!isDefaultConstructible || (!hasAnyDefaultInit && !hasAnySelfInit))
                 continue;
 
             handleUnionMembers(usageList);
@@ -373,6 +379,10 @@ void CheckClassImpl::constructors()
                 if (var.typeScope() && var.typeScope()->numConstructors > 0)
                     continue;
 
+                // a const member of class type is default constructed or makes the class not default constructible
+                if (var.isConst())
+                    continue;
+
                 if (diagVars.count(&var) == 0)
                     uninitVarError(var.nameToken(), false, FunctionType::eConstructor, var.scope()->className, var.name(), false, false, true);
             }

test/testconstructors.cpp

@@ -797,6 +797,90 @@ class TestConstructors : public TestFixture {
         ASSERT_EQUALS("[test.cpp:4:9]: (warning) Member variable 'T::j' has no initializer. [uninitMemberVarNoCtor]\n"
                       "[test.cpp:8:9]: (warning) Member variable 'U::k' has no initializer. [uninitMemberVarNoCtor]\n",
                       errout_str());
+
+        check("struct S {\n" // #15080
+              "    int a = 0;\n"
+              "    const int b;\n"
+              "    int k;\n"
+              "};\n");
+        ASSERT_EQUALS("", errout_str());
+
+        check("struct S {\n"
+              "    int a = 0;\n"
+              "    int& c;\n"
+              "    int k;\n"
+              "};\n");
+        ASSERT_EQUALS("", errout_str());
+
+        check("struct S {\n"
+              "    int a = 0;\n"
+              "    const int* d;\n"
+              "};\n");
+        ASSERT_EQUALS("[test.cpp:3:16]: (warning) Member variable 'S::d' has no initializer. [uninitMemberVarNoCtor]\n", errout_str());
+
+        check("struct S {\n"
+              "    int a = 0;\n"
+              "    int* const p;\n"
+              "    int k;\n"
+              "};\n");
+        ASSERT_EQUALS("", errout_str());
+
+        check("struct S {\n" // static members don't affect the default constructor
+              "    int a = 0;\n"
+              "    static const int N;\n"
+              "    static int& r;\n"
+              "    int k;\n"
+              "};\n");
+        ASSERT_EQUALS("[test.cpp:5:9]: (warning) Member variable 'S::k' has no initializer. [uninitMemberVarNoCtor]\n", errout_str());
+
+        check("struct S {\n" // function pointers returning a reference are no references
+              "    int a = 0;\n"
+              "    std::string& (*f)(int, int);\n"
+              "    int& (*g[2])();\n"
+              "    int k;\n"
+              "};\n");
+        ASSERT_EQUALS("[test.cpp:3:20]: (warning) Member variable 'S::f' has no initializer. [uninitMemberVarNoCtor]\n"
+                      "[test.cpp:4:12]: (warning) Member variable 'S::g' has no initializer. [uninitMemberVarNoCtor]\n"
+                      "[test.cpp:5:9]: (warning) Member variable 'S::k' has no initializer. [uninitMemberVarNoCtor]\n",
+                      errout_str());
+
+        check("struct S {\n"
+              "    int a = 0;\n"
+              "    int (*&f)();\n"
+              "    int k;\n"
+              "};\n");
+        ASSERT_EQUALS("", errout_str());
+
+        check("struct S {\n"
+              "    int a = 0;\n"
+              "    int (&arr)[2];\n"
+              "    int k;\n"
+              "};\n");
+        ASSERT_EQUALS("", errout_str());
+
+        check("typedef const unsigned int Mode;\n"
+              "struct S {\n"
+              "    std::vector<int> v;\n"
+              "    Mode m;\n"
+              "    int k;\n"
+              "};\n");
+        ASSERT_EQUALS("", errout_str());
+
+        check("struct S {\n"
+              "    std::string a;\n"
+              "    const std::string s;\n"
+              "    int k;\n"
+              "};\n");
+        ASSERT_EQUALS("[test.cpp:4:9]: (warning) Member variable 'S::k' has no initializer. [uninitMemberVarNoCtor]\n", errout_str());
+
+        check("struct S {\n"
+              "    const std::array<std::uint8_t, 2> a;\n"
+              "    const std::vector<std::uint8_t>::const_iterator it;\n"
+              "};\n"
+              "S f(const std::vector<std::uint8_t>& v) {\n"
+              "    return S{ {}, v.cbegin() };\n"
+              "}\n");
+        ASSERT_EQUALS("", errout_str());
     }
 
     // ticket #4290 "False Positive: style (noConstructor): The class 'foo' does not have a constructor."