[clang-format] Fix assertion failure/crash with `AllowShortFunctionsOnASingleLine: Inline/InlineOnly`.

[curdeius]

Summary

Fixes https://github.com/llvm/llvm-project/issues/54147.

When handling AllowShortFunctionsOnASingleLine, we were searching for the last line with a smaller level than the current line. The search was incorrect when the first line had the same level as the current one. This led to an unsatisfied assumption about the existence of a brace (non-comment token).

[owenpan]

clang/lib/Format/UnwrappedLineFormatter.cpp

313

To be safe?

[curdeius]

clang/lib/Format/UnwrappedLineFormatter.cpp

313

Good idea. Will do.
Any idea how to provoke this (if at all possible)?

[owenpan]

clang/lib/Format/UnwrappedLineFormatter.cpp

313

I can't think of a valid C++ example that would trigger this.

[curdeius]

Safer condition.

clang/lib/Format/UnwrappedLineFormatter.cpp

auto ShouldMergeShortFunctions = [this, &I, &NextLine, PreviousLine,
             return false;
 
           // TODO: Use IndentTracker to avoid loop?
           // Find the last line with lower level.
           auto J = I - 1;
           for (; J != AnnotatedLines.begin(); --J)
             if ((*J)->Level < TheLine->Level)
               break;
+          if ((*J)->Level == TheLine->Level)
+            return false;
 
           // Check if the found line starts a record.
           const FormatToken *LastNonComment = (*J)->Last;
           assert(LastNonComment);
           if (LastNonComment->is(tok::comment)) {
             LastNonComment = LastNonComment->getPreviousNonComment();
             // There must be another token (usually `{`), because we chose a
             // line that has a smaller level.

clang/unittests/Format/FormatTest.cpp

verifyFormat("int f() {\n"
                MergeInlineOnly);
 
   // SFS_Inline implies SFS_Empty
   verifyFormat("class C {\n"
                "  int f() {}\n"
                "};",
                MergeInlineOnly);
   verifyFormat("int f() {}", MergeInlineOnly);
+  // https://llvm.org/PR54147
+  verifyFormat("auto lambda = []() {\n"
+               "  // comment\n"
+               "  f();\n"
+               "  g();\n"
+               "};",
+               MergeInlineOnly);
 
   // Also verify behavior when BraceWrapping.AfterFunction = true
   MergeInlineOnly.BreakBeforeBraces = FormatStyle::BS_Custom;
   MergeInlineOnly.BraceWrapping.AfterFunction = true;
   verifyFormat("class C {\n"
                "  int f() { return 42; }\n"
                "};",
                MergeInlineOnly);