Fix uninitvar false positive in designed initializers (#5079)

Stop interpreting struct fields in designed initializers as usage of local
variables which can happen if they share the same name.

```
$ cat test.c
struct a { int b; };
int main() {
  char *b;
  extern int foo(struct a *);
  return foo(&(struct a){.b = 0});
}
$ cppcheck --quiet test.c
test.c:5:27: error: Uninitialized variable: b [legacyUninitvar]
  return foo(&(struct a){.b = 0});
```

lib/checkuninitvar.cpp

@@ -1123,6 +1123,10 @@ const Token* CheckUninitVar::isVariableUsage(bool cpp, const Token *vartok, cons
         // (type &)x
         else if (valueExpr->astParent()->isCast() && valueExpr->astParent()->isUnaryOp("(") && Token::simpleMatch(valueExpr->astParent()->link()->previous(), "& )"))
             valueExpr = valueExpr->astParent();
+        // designated initializers: {.x | { ... , .x
+        else if (Token::simpleMatch(valueExpr->astParent(), ".") &&
+                 Token::Match(valueExpr->astParent()->previous(), ",|{"))
+            valueExpr = valueExpr->astParent();
         else
             break;
     }

test/testuninitvar.cpp

@@ -94,6 +94,7 @@ private:
         TEST_CASE(uninitvar_ipa);
         TEST_CASE(uninitvar_memberfunction);
         TEST_CASE(uninitvar_nonmember); // crash in ycmd test
+        TEST_CASE(uninitvarDesignatedInitializers);
 
         TEST_CASE(isVariableUsageDeref); // *p
         TEST_CASE(isVariableUsageDerefValueflow); // *p
@@ -6955,6 +6956,24 @@ private:
         ASSERT_EQUALS("[test.cpp:7]: (error) Uninitialized variable: foo\n", errout.str());
     }
 
+    void uninitvarDesignatedInitializers() {
+        checkUninitVar("struct a { int b; };\n"
+                       "int main() {\n"
+                       "  char *b;\n"
+                       "  extern int f(struct a *);\n"
+                       "  return f(&(struct a){.b = 0});\n"
+                       "}");
+        ASSERT_EQUALS("", errout.str());
+
+        checkUninitVar("struct a { int b, c; };\n"
+                       "int main() {\n"
+                       "  char *c;\n"
+                       "  extern int f(struct a *);\n"
+                       "  return f(&(struct a){.b = 0, .c = 0});\n"
+                       "}");
+        ASSERT_EQUALS("", errout.str());
+    }
+
     void isVariableUsageDeref() {
         // *p
         checkUninitVar("void f() {\n"