[clang-tidy] Resolve readability-else-after-return false positive for constexpr if.

Summary:
It fixes the false positive when using constexpr if and where else cannot be removed:

Example:
```
  if constexpr (sizeof(int) > 4)
    // ...
    return /* ... */;
  else // This else cannot be removed.
    // ...
    return /* ... */;
```

Reviewers: alexfh, aaron.ballman

Reviewed By: aaron.ballman

Subscribers: lebedev.ri, xazax.hun, cfe-commits

Differential Revision: https://reviews.llvm.org/D53372

llvm-svn: 344785

1 files changed, 22 insertions, 0 deletions

diff --git a/clang-tools-extra/test/clang-tidy/readability-else-after-return-if-constexpr.cpp b/clang-tools-extra/test/clang-tidy/readability-else-after-return-if-constexpr.cpp
new file mode 100644
index 00000000000..6532940eaf2
--- /dev/null
+++ b/clang-tools-extra/test/clang-tidy/readability-else-after-return-if-constexpr.cpp
@@ -0,0 +1,22 @@
+// RUN: %check_clang_tidy %s readability-else-after-return %t -- -- -std=c++17

+

+// Constexpr if is an exception to the rule, we cannot remove the else.

+void f() {

+  if (sizeof(int) > 4)

+    return;

+  else

+    return;

+  // CHECK-MESSAGES: [[@LINE-2]]:3: warning: do not use 'else' after 'return'

+

+  if constexpr (sizeof(int) > 4)

+    return;

+  else

+    return;

+

+  if constexpr (sizeof(int) > 4)

+    return;

+  else if constexpr (sizeof(long) > 4)

+    return;

+  else

+    return;

+}