[NFC-ish] Do lookahead without llvm::function_ref

Author: beccadax

include/swift/Parse/Parser.h

@@ -487,9 +487,8 @@ class Parser {
   /// \returns the value returned by \c f
   /// \note When calling, you may need to specify the \c Val type
   ///       explicitly as a type parameter.
-  template <typename Val>
-  Val lookahead(unsigned char K,
-                llvm::function_ref<Val(CancellableBacktrackingScope &)> f) {
+  template <typename Fn>
+  decltype(auto) lookahead(unsigned char K, Fn f) {
     CancellableBacktrackingScope backtrackScope(*this);
 
     for (unsigned char i = 0; i < K; ++i)

lib/Parse/ParseDecl.cpp

@@ -3030,7 +3030,7 @@ ParserStatus Parser::parseNewDeclAttribute(DeclAttributes &Attributes,
 
         // Backwards support for @c("stringId"). Remove before enabling in
         // production so we accept only the identifier format.
-        lookahead<bool>(1, [&](CancellableBacktrackingScope &) {
+        lookahead(1, [&](CancellableBacktrackingScope &) {
            return Tok.isNot(tok::string_literal);
         })) {
 

lib/Parse/ParsePattern.cpp

@@ -122,7 +122,7 @@ bool Parser::startsParameterName(bool isClosure) {
     // contextual keywords, so look ahead one more token (two total) and see
     // if we have a ':' that would
     // indicate that this is an argument label.
-    return lookahead<bool>(2, [&](CancellableBacktrackingScope &) {
+    return lookahead(2, [&](CancellableBacktrackingScope &) {
       if (Tok.is(tok::colon))
         return true; // isolated :
 
@@ -235,7 +235,7 @@ Parser::parseParameterClause(SourceLoc &leftParenLoc,
         // is this token the identifier of an argument label? `inout` is a
         // reserved keyword but the other modifiers are not.
         if (!Tok.is(tok::kw_inout)) {
-          bool partOfArgumentLabel = lookahead<bool>(1, [&](CancellableBacktrackingScope &) {
+          bool partOfArgumentLabel = lookahead(1, [&](CancellableBacktrackingScope &) {
             if (Tok.is(tok::colon))
               return true;  // isolated :
 

test/expr/postfix/dot/numeric_literals.swift

@@ -0,0 +1,9 @@
+// RUN: %target-typecheck-verify-swift
+
+func fn() {
+  _ = 1.description
+  _ = 1.5.description
+
+  print(1.description)
+  print(1.5.description)
+}