Fix #11545 FP unreadVariable with std::span (#4792)

Author: chrchr-github

lib/fwdanalysis.cpp

@@ -482,6 +482,8 @@ bool FwdAnalysis::unusedValue(const Token *expr, const Token *startToken, const
         return false;
     if (hasVolatileCastOrVar(expr))
         return false;
+    if (Token::simpleMatch(expr, "[") && astIsContainerView(expr->astOperand1()))
+        return false;
     mWhat = What::UnusedValue;
     Result result = check(expr, startToken, endToken);
     return (result.type == FwdAnalysis::Result::Type::NONE || result.type == FwdAnalysis::Result::Type::RETURN) && !possiblyAliased(expr, startToken);

test/testunusedvar.cpp

@@ -6000,6 +6000,11 @@ class TestUnusedVar : public TestFixture {
                            "[test.cpp:2]: (style) Variable 'p' is assigned a value that is never used.\n"
                            "[test.cpp:3]: (style) Variable 'q' is assigned a value that is never used.\n",
                            errout.str());
+
+        functionVariableUsage("void f(std::span<int> s) {\n" // #11545
+                              "    s[0] = 0;\n"
+                              "}\n");
+        ASSERT_EQUALS("", errout.str());
     }
 
     void localVarClass() {