Fix #15015 FP knownConditionTrueFalse (bitwise not of unsigned value) (#8831)

Author: chrchr-github

lib/vf_settokenvalue.cpp

@@ -617,7 +617,7 @@ namespace ValueFlow
         // ~
         else if (parent->str() == "~") {
             for (const Value &val : tok->values()) {
-                if (!val.isIntValue())
+                if (!val.isIntValue() || (val.isImpossible() && val.intvalue < 0))
                     continue;
                 Value v(val);
                 v.intvalue = ~v.intvalue;

test/testvalueflow.cpp

@@ -1263,6 +1263,15 @@ class TestValueFlow : public TestFixture {
         ASSERT_EQUALS(1U, values.size());
         ASSERT_EQUALS(~0U, values.back().intvalue);
 
+        // #15015
+        code  = "void f(unsigned u) {\n"
+                "    if (~u) {}\n"
+                "}";
+        values = tokenValues(code, "~");
+        ASSERT_EQUALS(1U, values.size());
+        ASSERT_EQUALS(-1, values.back().intvalue);
+        ASSERT(values.back().isImpossible());
+
         // !
         code  = "void f(int x) {\n"
                 "    a = !x;\n"