Fix #11389 Do not warn for truncLongCastReturn if operands have known valid int (danmar#8192)

This fix handles the first example in ticket, i.e. when the value is
known to be a valid int:

long f() {
	int n = 1;
	return n << 12;
}

Author: ceJce

lib/checktype.cpp

@@ -383,8 +383,12 @@ void CheckType::checkLongCast()
                     const ValueType *type = tok->astOperand1()->valueType();
                     if (type && checkTypeCombination(*type, *retVt, *mSettings) &&
                         type->pointer == 0U &&
-                        type->originalTypeName.empty())
-                        ret = tok;
+                        type->originalTypeName.empty()) {
+                        if (!tok->astOperand1()->hasKnownIntValue()) {
+                            ret = tok;
+                        } else if (!mSettings->platform.isIntValue(tok->astOperand1()->getKnownIntValue()))
+                            ret = tok;
+                    }
                 }
                 // All return statements must have problem otherwise no warning
                 if (ret != tok) {

test/testtype.cpp

@@ -465,6 +465,26 @@ class TestType : public TestFixture {
         check(code2, dinit(CheckOptions, $.settings = &settingsWin));
         ASSERT_EQUALS("[test.cpp:2:3]: (style) int result is returned as long long value. If the return value is long long to avoid loss of information, then you have loss of information. [truncLongCastReturn]\n", errout_str());
 
+        const char code3[] = "long f() {\n"
+                             "    int n = 1;\n"
+                             "    return n << 12;\n"
+                             "}\n";
+        check(code3, dinit(CheckOptions, $.settings = &settings));
+        ASSERT_EQUALS("", errout_str());
+
+        const char code4[] = "long f(int n) {\n"
+                             "    return n << 12;\n"
+                             "}\n";
+        check(code4, dinit(CheckOptions, $.settings = &settings));
+        ASSERT_EQUALS("[test.cpp:2:5]: (style) int result is returned as long value. If the return value is long to avoid loss of information, then you have loss of information. [truncLongCastReturn]\n", errout_str());
+
+        const char code5[] = "long f() {\n"
+                             "    unsigned int n = 1U << 20;\n"
+                             "    return n << 20;\n"
+                             "}\n";
+        check(code5, dinit(CheckOptions, $.settings = &settings));
+        ASSERT_EQUALS("[test.cpp:3:5]: (style) int result is returned as long value. If the return value is long to avoid loss of information, then you have loss of information. [truncLongCastReturn]\n", errout_str());
+
         // typedef
         check("size_t f(int x, int y) {\n"
               "  return x * y;\n"