Merge branch 'toolchain-overload-diagnostics' of github.com:zygoloid/carbon-lang into toolchain-overload-diagnostics

Author: zygoloid

toolchain/check/context.h

@@ -158,7 +158,7 @@ class Context {
   }
 
   auto cpp_carbon_file_locations()
-      -> llvm::SmallVector<clang::SourceLocation, 0>& {
+      -> llvm::SmallVector<clang::SourceLocation>& {
     return cpp_carbon_file_locations_;
   }
 
@@ -394,7 +394,7 @@ class Context {
 
   // Per-Carbon-file start locations for corresponding Clang source buffers.
   // Owned and managed by code in cpp/location.cpp.
-  llvm::SmallVector<clang::SourceLocation, 0> cpp_carbon_file_locations_;
+  llvm::SmallVector<clang::SourceLocation> cpp_carbon_file_locations_;
 
   // Declaration instructions of entities that should have definitions by the
   // end of the current source file.

toolchain/check/cpp/location.cpp

@@ -20,7 +20,7 @@ struct FileInfo {
 // and Clang's source manager.
 static auto GetFileInfo(Context& context, SemIR::CheckIRId ir_id) -> FileInfo {
   const SemIR::File* sem_ir = &context.sem_ir();
-  unsigned file_index = 0;
+  int file_index = 0;
 
   // If the file is imported, locate it in our imports map.
   if (ir_id != context.sem_ir().check_ir_id()) {
@@ -35,9 +35,9 @@ static auto GetFileInfo(Context& context, SemIR::CheckIRId ir_id) -> FileInfo {
   // If we've seen this file before, reuse the same FileID.
   auto& file_start_locs = context.cpp_carbon_file_locations();
   if (file_start_locs.size() <= file_index) {
+    // Never valid; prepare a slot for the caching below.
     file_start_locs.resize(file_index + 1);
-  }
-  if (file_start_locs[file_index].isValid()) {
+  } else if (file_start_locs[file_index].isValid()) {
     return {.sem_ir = sem_ir, .start_loc = file_start_locs[file_index]};
   }