[clang] [clang-tools-extra] [clang-tidy][dataflow] Add `bugprone-null-check-after-dereference` check (PR #84166)

[via cfe-commits]

================
@@ -0,0 +1,330 @@
+// RUN: %check_clang_tidy %s bugprone-null-check-after-dereference %t
+
+struct S {
+  int a;
+};
+
+int warning_deref(int *p) {
+  *p = 42;
+
+  if (p) {
+    // CHECK-MESSAGES: :[[@LINE-1]]:7: warning: pointer value is checked even though it cannot be null at this point [bugprone-null-check-after-dereference]
+    // CHECK-MESSAGES: :[[@LINE-4]]:3: note: one of the locations where the pointer's value cannot be null
+  // FIXME: If there's a direct path, make the error message more precise, ie. remove `one of the locations`
+    *p += 20;
+    return *p;
+  } else {
+    return 0;
+  }
+}
+
+int warning_member(S *q) {
+  q->a = 42;
+
+  if (q) {
+    // CHECK-MESSAGES: :[[@LINE-1]]:7: warning: pointer value is checked even though it cannot be null at this point
+    // CHECK-MESSAGES: :[[@LINE-4]]:3: note: one of the locations where the pointer's value cannot be null
+    q->a += 20;
+    return q->a;
+  } else {
+    return 0;
+  }
+}
+
+int negative_warning(int *p) {
+  *p = 42;
+
+  if (!p) {
+    // CHECK-MESSAGES: :[[@LINE-1]]:8: warning: pointer value is checked even though it cannot be null at this point
+    // CHECK-MESSAGES: :[[@LINE-4]]:3: note: one of the locations where the pointer's value cannot be null
+    return 0;
+  } else {
+    *p += 20;
+    return *p;
+  }
+}
+
+int no_warning(int *p, bool b) {
+  if (b) {
+    *p = 42;
+  }
+
+  if (p) {
+    // CHECK-MESSAGES-NOT: :[[@LINE-1]]:7: warning: pointer value is checked even though it cannot be null at this point 
+    *p += 20;
+    return *p;
+  } else {
+    return 0;
+  }
+}
+
+int else_branch_warning(int *p, bool b) {
+  if (b) {
+    *p = 42;
+  } else {
+    *p = 20;
+  }
+
+  if (p) {
+    // CHECK-MESSAGES: :[[@LINE-1]]:7: warning: pointer value is checked even though it cannot be null at this point
+    // CHECK-MESSAGES: :[[@LINE-7]]:5: note: one of the locations where the pointer's value cannot be null
+    return 0;
+  } else {
+    *p += 20;
+    return *p;
+  }
+}
+
+int two_branches_warning(int *p, bool b) {
+  if (b) {
+    *p = 42;
+  }
+  
+  if (!b) {
+    *p = 20;
+  }
+
+  if (p) {
+    // CHECK-MESSAGES: :[[@LINE-1]]:7: warning: pointer value is checked even though it cannot be null at this point
+    // CHECK-MESSAGES: :[[@LINE-9]]:5: note: one of the locations where the pointer's value cannot be null
+    return 0;
+  } else {
+    *p += 20;
+    return *p;
+  }
+}
+
+int two_branches_reversed(int *p, bool b) {
----------------
Discookie wrote:

Indeed - it was testing the check's behavior from back when variables' `Value`s weren't initialized by default.
Merging a variable that only had a `Value` on one of the branches caused weird behavior - same reason the leftover ``initializeFunctionParameters()`` was still in the code.

https://github.com/llvm/llvm-project/pull/84166