[clangd] Fix a subtle case for GetBeginningOfIdentifier.

[sammccall]

Summary

The following case 1 was incorrectly regarded as case 2:

MACRO(foo->^function())

Also add few more tests for macro.

[sammccall]

(Stealing this to make some revisions after pair-programming)

clangd/ClangdUnit.cpp

SourceLocation clangd::getBeginningOfIdentifier(ParsedAST &Unit,
                                                 const FileID FID) {
   const ASTContext &AST = Unit.getASTContext();
   const SourceManager &SourceMgr = AST.getSourceManager();
   auto Offset = positionToOffset(SourceMgr.getBufferData(FID), Pos);
   if (!Offset) {
     log("getBeginningOfIdentifier: {0}", Offset.takeError());
     return SourceLocation();
   }
-  SourceLocation InputLoc = SourceMgr.getComposedLoc(FID, *Offset);
 
   // GetBeginningOfToken(pos) is almost what we want, but does the wrong thing
   // if the cursor is at the end of the identifier.
   // Instead, we lex at GetBeginningOfToken(pos - 1). The cases are:
   //  1) at the beginning of an identifier, we'll be looking at something
   //  that isn't an identifier.
   //  2) at the middle or end of an identifier, we get the identifier.
   //  3) anywhere outside an identifier, we'll get some non-identifier thing.
   // We can't actually distinguish cases 1 and 3, but returning the original
   // location is correct for both!
+  SourceLocation InputLoc = SourceMgr.getComposedLoc(FID, *Offset);
   if (*Offset == 0) // Case 1 or 3.
     return SourceMgr.getMacroArgExpandedLocation(InputLoc);
-  SourceLocation Before =
-      SourceMgr.getMacroArgExpandedLocation(InputLoc.getLocWithOffset(-1));
+  SourceLocation Before = SourceMgr.getComposedLoc(FID, *Offset - 1);
+
   Before = Lexer::GetBeginningOfToken(Before, SourceMgr, AST.getLangOpts());
   Token Tok;
   if (Before.isValid() &&
       !Lexer::getRawToken(Before, Tok, SourceMgr, AST.getLangOpts(), false) &&
       Tok.is(tok::raw_identifier))
-    return Before;                                        // Case 2.
+    return SourceMgr.getMacroArgExpandedLocation(Before); // Case 2.
   return SourceMgr.getMacroArgExpandedLocation(InputLoc); // Case 1 or 3.
 }

unittests/clangd/ClangdUnitTests.cpp

main.cpp:2:3: error: something terrible happened)");
 
   EXPECT_THAT(
       LSPDiags,
       ElementsAre(Pair(EqualToLSPDiag(MainLSP), ElementsAre(EqualToFix(F))),
                   Pair(EqualToLSPDiag(NoteInMainLSP), IsEmpty())));
 }
 
 TEST(ClangdUnitTest, GetBeginningOfIdentifier) {
+  std::string Preamble = R"cpp(
+struct Bar { int func(); };
+#define MACRO(X) void f() { X; }
+Bar* bar;
+  )cpp";
   // First ^ is the expected beginning, last is the search position.
-  for (const char *Text : {
+  for (std::string Text : std::vector<std::string>{
            "int ^f^oo();", // inside identifier
            "int ^foo();",  // beginning of identifier
            "int ^foo^();", // end of identifier
            "int foo(^);",  // non-identifier
            "^int foo();",  // beginning of file (can't back up)
            "int ^f0^0();", // after a digit (lexing at N-1 is wrong)
            "int ^λλ^λ();", // UTF-8 handled properly when backing up
+
+           // identifier in macro arg
+           "MACRO(bar->^func())",  // beginning of identifier
+           "MACRO(bar->^fun^c())", // inside identifier
+           "MACRO(bar->^func^())", // end of identifier
+           "MACRO(^bar->func())",  // begin identifier
+           "MACRO(^bar^->func())", // end identifier
+           "^MACRO(bar->func())",  // beginning of macro name
+           "^MAC^RO(bar->func())", // inside macro name
+           "^MACRO^(bar->func())", // end of macro name
        }) {
-    Annotations TestCase(Text);
+    std::string WithPreamble = Preamble + Text;
+    Annotations TestCase(WithPreamble);
     auto AST = TestTU::withCode(TestCase.code()).build();
     const auto &SourceMgr = AST.getASTContext().getSourceManager();
     SourceLocation Actual = getBeginningOfIdentifier(
         AST, TestCase.points().back(), SourceMgr.getMainFileID());
-    Position ActualPos =
-        offsetToPosition(TestCase.code(), SourceMgr.getFileOffset(Actual));
+    Position ActualPos = offsetToPosition(
+        TestCase.code(),
+        SourceMgr.getFileOffset(SourceMgr.getSpellingLoc(Actual)));
     EXPECT_EQ(TestCase.points().front(), ActualPos) << Text;
   }
 }
 
 } // namespace
 } // namespace clangd
 } // namespace clang