[clang-tidy] Add new hicpp-multiway-paths-covered check for missing branches

Summary:
This check searches for missing `else` branches in `if-else if`-chains and
missing `default` labels in `switch` statements, that use integers as condition.

It is very similar to -Wswitch, but concentrates on integers only, since enums are
already covered.

The option to warn for missing `else` branches is deactivated by default, since it is
very noise on larger code bases.

Running it on LLVM:
{F5354858} for default configuration
{F5354866} just for llvm/lib/Analysis/ScalarEvolution.cpp, the else-path checker is very noisy!

Reviewers: alexfh, aaron.ballman, hokein

Reviewed By: aaron.ballman

Subscribers: lebedev.ri, Eugene.Zelenko, cfe-commits, mgorny, JDevlieghere, xazax.hun

Tags: #clang-tools-extra

Differential Revision: https://reviews.llvm.org/D37808

llvm-svn: 318600

1 files changed, 57 insertions, 0 deletions

diff --git a/clang-tools-extra/test/clang-tidy/hicpp-multiway-paths-covered-else.cpp b/clang-tools-extra/test/clang-tidy/hicpp-multiway-paths-covered-else.cpp
new file mode 100644
index 00000000000..34820b5fcc0
--- /dev/null
+++ b/clang-tools-extra/test/clang-tidy/hicpp-multiway-paths-covered-else.cpp
@@ -0,0 +1,57 @@
+// RUN: %check_clang_tidy %s hicpp-multiway-paths-covered %t \
+// RUN: -config='{CheckOptions: \
+// RUN:  [{key: hicpp-multiway-paths-covered.WarnOnMissingElse, value: 1}]}'\
+// RUN: --
+
+enum OS { Mac,
+          Windows,
+          Linux };
+
+void problematic_if(int i, enum OS os) {
+  if (i > 0) {
+    return;
+  } else if (i < 0) {
+    // CHECK-MESSAGES: [[@LINE-1]]:10: warning: potentially uncovered codepath; add an ending else statement
+    return;
+  }
+
+  // Could be considered as false positive because all paths are covered logically.
+  // I still think this is valid since the possibility of a final 'everything else'
+  // codepath is expected from if-else if.
+  if (i > 0) {
+    return;
+  } else if (i <= 0) {
+    // CHECK-MESSAGES: [[@LINE-1]]:10: warning: potentially uncovered codepath; add an ending else statement
+    return;
+  }
+
+  // Test if nesting of if-else chains does get caught as well.
+  if (os == Mac) {
+    return;
+  } else if (os == Linux) {
+    // These checks are kind of degenerated, but the check will not try to solve
+    // if logically all paths are covered, which is more the area of the static analyzer.
+    if (true) {
+      return;
+    } else if (false) {
+      // CHECK-MESSAGES: [[@LINE-1]]:12: warning: potentially uncovered codepath; add an ending else statement
+      return;
+    }
+    return;
+  } else {
+    /* unreachable */
+    if (true) // check if the parent would match here as well
+      return;
+    // No warning for simple if statements, since it is common to just test one condition
+    // and ignore the opposite.
+  }
+
+  // Ok, because all paths are covered
+  if (i > 0) {
+    return;
+  } else if (i < 0) {
+    return;
+  } else {
+    /* error, maybe precondition failed */
+  }
+}