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

[PiotrZSL]

Summary

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

[carlosgalvezp]

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

1 ↗

(On Diff #508346)

We should keep the default value of c++11-or-later, otherwise we are losing coverage on older versions.

199 ↗

(On Diff #508346)

I believe the most common way of solving this is creating another test file where you test the c++17-or-later support. Alternatively you'd need to wrap this code in a #if block, plus create another RUN line with a different check-suffix

[PiotrZSL]

Move tests to new file.

[carlosgalvezp]

Looks great, thanks for fixing!

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

if (SM.getExpansionColumnNumber(InnerLoc) ==
       diag(OuterLoc, "did you mean this line to be inside this '%0'",
            DiagnosticIDs::Note)
           << StmtNames[StmtKind];
     }
   }
 }
 
 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"),
       this);
 }
 
 void MisleadingIndentationCheck::check(const MatchFinder::MatchResult &Result) {
   if (const auto *If = Result.Nodes.getNodeAs<IfStmt>("if"))
     danglingElseCheck(*Result.SourceManager, Result.Context, If);
 
   if (const auto *CStmt = Result.Nodes.getNodeAs<CompoundStmt>("compound"))
     missingBracesCheck(*Result.SourceManager, CStmt);
 }
 
 } // namespace clang::tidy::readability

clang-tools-extra/docs/ReleaseNotes.rst

- Improved :doc:`bugprone-too-small-loop-variable
   <clang-tidy/checks/bugprone/too-small-loop-variable>` check. Basic support
   for bit-field and integer members as a loop variable or upper limit were added.
 
 - Improved :doc:`readability-magic-numbers
   <clang-tidy/checks/readability/magic-numbers>` check, now allows for
   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.
 
 - Improved :doc:`bugprone-use-after-move
   <clang-tidy/checks/bugprone/use-after-move>` to understand that there is a
   sequence point between designated initializers.
 

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

+// 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>();
+}
+
+}