Fix issue 9277: FP: Dont warn for knwon conditions in if constexpr (#2085)

lib/checkcondition.cpp

@@ -1345,7 +1345,7 @@ void CheckCondition::alwaysTrueFalse()
                 continue;
 
             const bool constIfWhileExpression =
-                tok->astParent() && Token::Match(tok->astTop()->astOperand1(), "if|while") &&
+                tok->astParent() && Token::Match(tok->astTop()->astOperand1(), "if|while") && !tok->astTop()->astOperand1()->isConstexpr() &&
                 (Token::Match(tok->astParent(), "%oror%|&&") || Token::Match(tok->astParent()->astOperand1(), "if|while"));
             const bool constValExpr = tok->isNumber() && Token::Match(tok->astParent(),"%oror%|&&|?"); // just one number in boolean expression
             const bool compExpr = Token::Match(tok, "%comp%|!"); // a compare expression

lib/token.h

@@ -566,6 +566,13 @@ public:
         setFlag(fIncompleteVar, b);
     }
 
+    bool isConstexpr() const {
+        return getFlag(fConstexpr);
+    }
+    void isConstexpr(bool b) {
+        setFlag(fConstexpr, b);
+    }
+
 
     bool isBitfield() const {
         return mImpl->mBits > 0;
@@ -1098,6 +1105,7 @@ private:
         fIsAttributeNodiscard   = (1 << 23), // __attribute__ ((warn_unused_result)), [[nodiscard]]
         fAtAddress              = (1 << 24), // @ 0x4000
         fIncompleteVar          = (1 << 25),
+        fConstexpr              = (1 << 26),
     };
 
     Token::Type mTokType;

lib/tokenize.cpp

@@ -4233,6 +4233,7 @@ bool Tokenizer::simplifyTokenList1(const char FileName[])
                 tok->deleteNext();
             } else if (tok->strAt(1) == "constexpr") {
                 tok->deleteNext();
+                tok->isConstexpr(true);
             } else {
                 syntaxError(tok);
             }

test/testcondition.cpp

@@ -3206,6 +3206,16 @@ private:
               "    }\n"
               "}\n");
         ASSERT_EQUALS("[test.cpp:3] -> [test.cpp:5]: (style) Condition 'array' is always true\n", errout.str());
+      
+        // #9277
+        check("int f() {\n"
+              "    constexpr bool x = true;\n"
+              "    if constexpr (x)\n"
+              "        return 0;\n"
+              "    else\n"
+              "        return 1;\n"
+              "}\n");
+        ASSERT_EQUALS("", errout.str());
     }
 
     void multiConditionAlwaysTrue() {