cql3: introduce similarity functions syntax

The similarity function syntax is:

`similarity_<metric_name>(<vector>, <vector>)`

Where `<metric_name>` is one of `cosine`, `euclidean` and `dot_product`
matching the intended similarity metric to be used within calculations.
Where `<vector>` is either a vector column name or vector literal.

Add `vectorSimilarityArgs` symbol that is an extension of `selectionFunctionArgs`,
but allowing to use the `value` as an argument as well as the `unaliasedSelector`.
This is needed as the similarity function syntax allows both the arguments to be
a vector value, so the grammar needs to recognize the vector literal there as well.

Since we actually support `SELECT`s with constants since this patch,
return true instead of throwing an error while trying to convert the function call
to constant.

cql3/Cql.g

@@ -431,6 +431,7 @@ unaliasedSelector returns [uexpression tmp]
        | K_TTL       '(' c=cident ')'              { tmp = column_mutation_attribute{column_mutation_attribute::attribute_kind::ttl,
                                                                                               unresolved_identifier{std::move(c)}}; }
        | f=functionName args=selectionFunctionArgs { tmp = function_call{std::move(f), std::move(args)}; }
+       | f=similarityFunctionName args=vectorSimilarityArgs            { tmp = function_call{std::move(f), std::move(args)}; }
        | K_CAST      '(' arg=unaliasedSelector K_AS t=native_type ')'  { tmp = cast{.style = cast::cast_style::sql, .arg = std::move(arg), .type = std::move(t)}; }
        )
        ( '.' fi=cident { tmp = field_selection{std::move(tmp), std::move(fi)}; }
@@ -445,6 +446,18 @@ selectionFunctionArgs returns [std::vector<expression> a]
       ')'
     ;
 
+vectorSimilarityArgs returns [std::vector<expression> a]
+    : '(' ')'
+    | '(' v1=vectorSimilarityArg { a.push_back(std::move(v1)); }
+          ( ',' vn=vectorSimilarityArg { a.push_back(std::move(vn)); } )*
+      ')'
+    ;
+
+vectorSimilarityArg returns [uexpression a]
+    : s=unaliasedSelector { a = std::move(s); }
+    | v=value             { a = std::move(v); }
+    ;
+
 countArgument
     : '*'
     | i=INTEGER { if (i->getText() != "1") {
@@ -1683,6 +1696,10 @@ functionName returns [cql3::functions::function_name s]
     : (ks=keyspaceName '.')? f=allowedFunctionName   { $s.keyspace = std::move(ks); $s.name = std::move(f); }
     ;
 
+similarityFunctionName returns [cql3::functions::function_name s]
+    : f=allowedSimilarityFunctionName { $s = cql3::functions::function_name::native_function(std::move(f)); }
+    ;
+
 allowedFunctionName returns [sstring s]
     : f=IDENT                       { $s = $f.text; std::transform(s.begin(), s.end(), s.begin(), ::tolower); }
     | f=QUOTED_NAME                 { $s = $f.text; }
@@ -1691,6 +1708,11 @@ allowedFunctionName returns [sstring s]
     | K_COUNT                       { $s = "count"; }
     ;
 
+allowedSimilarityFunctionName returns [sstring s]
+    : f=(K_SIMILARITY_COSINE | K_SIMILARITY_EUCLIDEAN | K_SIMILARITY_DOT_PRODUCT)
+      { $s = $f.text; std::transform(s.begin(), s.end(), s.begin(), ::tolower); }
+    ;
+
 functionArgs returns [std::vector<expression> a]
     : '(' ')'
     | '(' t1=term { a.push_back(std::move(t1)); }
@@ -2387,6 +2409,10 @@ K_MUTATION_FRAGMENTS:    M U T A T I O N '_' F R A G M E N T S;
 
 K_VECTOR_SEARCH_INDEXING: V E C T O R '_' S E A R C H '_' I N D E X I N G;
 
+K_SIMILARITY_EUCLIDEAN:     S I M I L A R I T Y '_' E U C L I D E A N;
+K_SIMILARITY_COSINE:        S I M I L A R I T Y '_' C O S I N E;
+K_SIMILARITY_DOT_PRODUCT:   S I M I L A R I T Y '_' D O T '_' P R O D U C T;
+
 // Case-insensitive alpha characters
 fragment A: ('a'|'A');
 fragment B: ('b'|'B');

cql3/selection/selectable.cc

@@ -32,7 +32,7 @@ bool
 selectable_processes_selection(const expr::expression& selectable) {
     return expr::visit(overloaded_functor{
         [&] (const expr::constant&) -> bool {
-            on_internal_error(slogger, "no way to express SELECT constant in the grammar yet");
+            return true;
         },
         [&] (const expr::conjunction& conj) -> bool {
             on_internal_error(slogger, "no way to express 'SELECT a AND b' in the grammar yet");