[clang-tidy] Fix if-constexpr false-positive in readability-misleading-indentation

When  depend on template parameter,
compiler can use NullStmt instead of CompoundStmt.
This causes issues as we losing information about
end location of that Stmt. To avoid this issue
check now ignores ifStmt with NullStmt on true-branch.

Fixes: https://github.com/llvm/llvm-project/issues/61435

Reviewed By: carlosgalvezp

Differential Revision: https://reviews.llvm.org/D146887

clang-tools-extra/clang-tidy/readability/MisleadingIndentationCheck.cpp

@@ -104,7 +104,8 @@ void MisleadingIndentationCheck::missingBracesCheck(const SourceManager &SM,
 }
 
 void MisleadingIndentationCheck::registerMatchers(MatchFinder *Finder) {
-  Finder->addMatcher(ifStmt(hasElse(stmt())).bind("if"), this);
+  Finder->addMatcher(
+      ifStmt(unless(hasThen(nullStmt())), hasElse(stmt())).bind("if"), this);
   Finder->addMatcher(
       compoundStmt(has(stmt(anyOf(ifStmt(), forStmt(), whileStmt()))))
           .bind("compound"),

clang-tools-extra/docs/ReleaseNotes.rst

@@ -240,6 +240,10 @@ Changes in existing checks
   magic numbers in type aliases such as ``using`` and ``typedef`` declarations if
   the new ``IgnoreTypeAliases`` option is set to true.
 
+- Fixed a false positive in :doc:`readability-misleading-indentation
+  <clang-tidy/checks/readability/misleading-indentation>` check when warning would
+  be unnecessarily emitted for template dependent ``if constexpr``.
+
 - Fixed a false positive in :doc:`cppcoreguidelines-slicing
   <clang-tidy/checks/cppcoreguidelines/slicing>` check when warning would be
   emitted in constructor for virtual base class initialization.

clang-tools-extra/test/clang-tidy/checkers/readability/misleading-indentation-cpp17.cpp

@@ -0,0 +1,29 @@
+// RUN: %check_clang_tidy -std=c++17-or-later %s readability-misleading-indentation %t -- -- -fno-delayed-template-parsing
+
+namespace PR61435 {
+
+template<int N>
+constexpr auto lam_correct = []{
+  if constexpr (N == 1) {
+  } else {
+  }
+};
+
+template<int N>
+constexpr auto lam_incorrect = []{
+  if constexpr (N == 1) {
+  }
+   else {
+  }
+  // CHECK-MESSAGES: :[[@LINE-2]]:4: warning: different indentation for 'if' and corresponding 'else' [readability-misleading-indentation]
+};
+
+void test() {
+  lam_correct<1>();
+  lam_correct<2>();
+
+  lam_incorrect<1>();
+  lam_incorrect<2>();
+}
+
+}