checker: infer generic type T from matching fn call argument (#6298)

vlib/v/checker/check_types.v

@@ -372,3 +372,20 @@ pub fn (mut c Checker) string_inter_lit(mut node ast.StringInterLiteral) table.T
 pub fn (c &Checker) check_sumtype_compatibility(a, b table.Type) bool {
 	return c.table.sumtype_has_variant(a, b) || c.table.sumtype_has_variant(b, a)
 }
+
+pub fn (mut c Checker) infer_fn_types(f table.Fn, mut call_expr ast.CallExpr) {
+	gt_name := 'T'
+	mut typ := table.void_type
+	for i, arg in f.params {
+		if arg.type_source_name == gt_name {
+			typ = call_expr.args[i].typ
+			break
+		}
+	}
+	if typ == table.void_type {
+		c.error('could not infer generic type `$gt_name` in call to `$f.name`', call_expr.pos)
+	} else {
+		c.table.register_fn_gen_type(f.name, typ)
+		call_expr.generic_type = typ
+	}
+}

vlib/v/checker/checker.v

@@ -56,6 +56,7 @@ mut:
 	inside_sql       bool // to handle sql table fields pseudo variables
 	cur_orm_ts       table.TypeSymbol
 	error_details    []string
+	generic_funcs    []&ast.FnDecl
 }
 
 pub fn new_checker(table &table.Table, pref &pref.Preferences) Checker {
@@ -80,6 +81,7 @@ pub fn (mut c Checker) check(ast_file ast.File) {
 		c.stmt(stmt)
 	}
 	c.check_scope_vars(c.file.scope)
+	c.post_process_generic_fns()
 }
 
 pub fn (mut c Checker) check_scope_vars(sc &ast.Scope) {
@@ -1505,6 +1507,10 @@ pub fn (mut c Checker) call_fn(mut call_expr ast.CallExpr) table.Type {
 			}
 		}
 	}
+	if f.is_generic && call_expr.generic_type == table.void_type {
+		// no type arguments given in call, attempt implicit instantiation
+		c.infer_fn_types(f, mut call_expr)
+	}
 	if call_expr.generic_type != table.void_type && f.return_type != 0 { // table.t_type {
 		// Handle `foo<T>() T` => `foo<int>() int` => return int
 		return_sym := c.table.get_type_symbol(f.return_type)
@@ -3962,10 +3968,16 @@ fn (mut c Checker) fetch_and_verify_orm_fields(info table.Struct, pos token.Posi
 	return fields
 }
 
-fn (mut c Checker) fn_decl(mut node ast.FnDecl) {
-	c.returns = false
-	if node.is_generic && c.cur_generic_type == 0 { // need the cur_generic_type check to avoid inf. recursion
-		// loop thru each generic type and generate a function
+fn (mut c Checker) post_process_generic_fns() {
+	// Loop thru each generic function concrete type.
+	// Check each specific fn instantiation.
+	for i in 0 .. c.generic_funcs.len {
+		mut node := c.generic_funcs[i]
+		if c.table.fn_gen_types.len == 0 {
+			// no concrete types, so just skip:
+			continue
+		}
+		// eprintln('>> post_process_generic_fns $c.file.path | $node.name , c.table.fn_gen_types.len: $c.table.fn_gen_types.len')
 		for gen_type in c.table.fn_gen_types[node.name] {
 			c.cur_generic_type = gen_type
 			// sym:=c.table.get_type_symbol(gen_type)
@@ -3973,6 +3985,24 @@ fn (mut c Checker) fn_decl(mut node ast.FnDecl) {
 			c.fn_decl(mut node)
 		}
 		c.cur_generic_type = 0
+		c.generic_funcs[i] = 0
+	}
+	// The generic funtions for each file/mod should be
+	// postprocessed just once in the checker, while the file/mod
+	// context is still the same.
+	c.generic_funcs = []
+}
+
+fn (mut c Checker) fn_decl(mut node ast.FnDecl) {
+	c.returns = false
+	if node.is_generic && c.cur_generic_type == 0 {
+		// Just remember the generic function for now.
+		// It will be processed later in c.post_process_generic_fns,
+		// after all other normal functions are processed.
+		// This is done so that all generic function calls can
+		// have a chance to populate c.table.fn_gen_types with
+		// the correct concrete types.
+		c.generic_funcs << node
 		return
 	}
 	if node.language == .v && !c.is_builtin_mod {

vlib/v/tests/generic_fn_infer_test.v

@@ -0,0 +1,33 @@
+fn call<T>(v T) {
+}
+
+fn simple<T>(p T) T {
+	return p
+}
+
+fn test_infer() {
+	call(3)
+	i := 4
+	r := simple(i)
+	assert r == 4
+}
+
+fn test_explicit_calls_should_also_work() {
+	call<int>(2)
+	assert true
+	simple<int>(5)
+	assert true
+}
+
+//
+fn choose4<T>(a, b, c, d T) T {
+	// NB: a similar construct is used in prime31's via engine
+	return a
+}
+
+fn test_calling_generic_fn_with_many_params() {
+	x := choose4(1, 2, 3, 4)
+	assert x == 1
+	y := choose4<string>('abc', 'xyz', 'def', 'ghi')
+	assert y == 'abc'
+}