[CodeCompletion] Signature help for braced constructor calls

Implementation is based on the "expected type" as used for
designated-initializers in braced init lists. This means it can deduce the type
in some cases where it's not written:

  void foo(Widget);
  foo({ /*help here*/ });

Only basic constructor calls are in scope of this patch, excluded are:
 - aggregate initialization (no help is offered for aggregates)
 - initializer_list initialization (no help is offered for these constructors)

Fixes https://github.com/clangd/clangd/issues/306

Differential Revision: https://reviews.llvm.org/D116317

1 files changed, 13 insertions, 3 deletions

diff --git a/clang/lib/Parse/ParseInit.cpp b/clang/lib/Parse/ParseInit.cpp
index 9d9c03d..efb162a 100644
--- a/clang/lib/Parse/ParseInit.cpp
+++ b/clang/lib/Parse/ParseInit.cpp
@@ -459,12 +459,22 @@ ExprResult Parser::ParseBraceInitializer() {
       Actions, EnterExpressionEvaluationContext::InitList);
 
   bool InitExprsOk = true;
-  DesignatorCompletionInfo DesignatorCompletion{
-      InitExprs,
-      PreferredType.get(T.getOpenLocation()),
+  QualType LikelyType = PreferredType.get(T.getOpenLocation());
+  DesignatorCompletionInfo DesignatorCompletion{InitExprs, LikelyType};
+  bool CalledSignatureHelp = false;
+  auto RunSignatureHelp = [&] {
+    QualType PreferredType;
+    if (!LikelyType.isNull())
+      PreferredType = Actions.ProduceConstructorSignatureHelp(
+          getCurScope(), LikelyType->getCanonicalTypeInternal(),
+          T.getOpenLocation(), InitExprs, T.getOpenLocation(), /*Braced=*/true);
+    CalledSignatureHelp = true;
+    return PreferredType;
   };
 
   while (1) {
+    PreferredType.enterFunctionArgument(Tok.getLocation(), RunSignatureHelp);
+
     // Handle Microsoft __if_exists/if_not_exists if necessary.
     if (getLangOpts().MicrosoftExt && (Tok.is(tok::kw___if_exists) ||
         Tok.is(tok::kw___if_not_exists))) {