[clang][Diagnostics] Fix diagnostic line numbers

[tbaeder]

Summary

The first line of the code snippet we print is potentially lower than
the caret line, so handle that case.

Fixes https://github.com/llvm/llvm-project/issues/63524

[cjdb]

LGTM. Not sure if the Fixes https://github.com/llvm/llvm-project/issues/63524 line autocloses the issue, but note that Fixes #63524 does.

[tbaeder]

That works as well as far as I know, but I changed it locally to just use the issue ID anyway. Could you set the status to accepted? Thanks.

[tbaeder]

@cor3ntin Thanks. You're very close to getting on my default reviewer list though, so be careful ;)

[cor3ntin]

In D153849#4463001, @tbaeder wrote:

@cor3ntin Thanks. You're very close to getting on my default reviewer list though, so be careful ;)

Thanks for the heads up, I might add you to mine :D

clang/lib/Frontend/TextDiagnostic.cpp

void TextDiagnostic::emitSnippetAndCaret(
   // Arbitrarily stop showing snippets when the line is too long.
   static const size_t MaxLineLengthToPrint = 4096;
   if (CaretColNo > MaxLineLengthToPrint)
     return;
 
   // Find the set of lines to include.
   const unsigned MaxLines = DiagOpts->SnippetLineLimit;
   std::pair<unsigned, unsigned> Lines = {CaretLineNo, CaretLineNo};
+  unsigned DisplayLineNo =
+      Ranges.empty() ? Loc.getPresumedLoc().getLine() : ~0u;
   for (const auto &I : Ranges) {
     if (auto OptionalRange = findLinesForRange(I, FID, SM))
       Lines = maybeAddRange(Lines, *OptionalRange, MaxLines);
+
+    DisplayLineNo =
+        std::min(DisplayLineNo, SM.getPresumedLineNumber(I.getBegin()));
   }
 
   // Our line numbers look like:
   // " [number] | "
   // Where [number] is MaxLineNoDisplayWidth columns
   // and the full thing is therefore MaxLineNoDisplayWidth + 4 columns.
-  unsigned DisplayLineNo = Loc.getPresumedLoc().getLine();
   unsigned MaxLineNoDisplayWidth =
       DiagOpts->ShowLineNumbers
           ? std::max(4u, getNumDisplayWidth(DisplayLineNo + MaxLines))
           : 0;
   auto indentForLineNumbers = [&] {
     if (MaxLineNoDisplayWidth > 0)
       OS.indent(MaxLineNoDisplayWidth + 2) << "| ";
   };

clang/test/Misc/diag-style.cpp

 // RUN: not %clang_cc1 -fsyntax-only %s 2>&1 | FileCheck -strict-whitespace %s
 
 /// empty lines in multi-line diagnostic snippets are preserved.
 static_assert(false &&
 
               true, "");
 // CHECK: static assertion failed
 // CHECK-NEXT: {{^}}    4 | static_assert(false &&{{$}}
 // CHECK-NEXT: {{^}}      |               ^~~~~~~~{{$}}
 // CHECK-NEXT: {{^}}    5 | {{$}}
 // CHECK-NEXT: {{^}}    6 |               true, "");{{$}}
 // CHECK-NEXT: {{^}}      |               ~~~~{{$}}
+
+
+/// #line pragmas are respected
+void printf(const char *a, ...) __attribute__((__format__(__printf__, 1, 2)));
+#line 10
+void f(int x) {
+  printf("%f",
+         x);
+}
+// CHECK: 12:10: warning: format specifies type
+// CHECK-NEXT: {{^}}   11 |
+// CHECK-NEXT: {{^}}      |
+// CHECK-NEXT: {{^}}      |
+// CHECK-NEXT: {{^}}   12 |