C++, parse decltype(<expr>)

Fixes sphinx-doc/sphinx#4094

sphinx/domains/cpp.py

@@ -1931,6 +1931,24 @@ class ASTTrailingTypeSpecDecltypeAuto(ASTBase):
         signode.append(nodes.Text(text_type(self)))
 
 
+class ASTTrailingTypeSpecDecltype(ASTBase):
+    def __init__(self, expr):
+        self.expr = expr
+
+    def __unicode__(self):
+        return u'decltype(' + text_type(self.expr) + ')'
+
+    def get_id(self, version):
+        if version == 1:
+            raise NoOldIdError()
+        return 'DT' + self.expr.get_id(version) + "E"
+
+    def describe_signature(self, signode, mode, env, symbol):
+        signode.append(nodes.Text('decltype('))
+        self.expr.describe_signature(signode, mode, env, symbol)
+        signode.append(nodes.Text(')'))
+
+
 class ASTFunctionParameter(ASTBase):
     def __init__(self, arg, ellipsis=False):
         # type: (Any, bool) -> None
@@ -4413,8 +4431,11 @@ class DefinitionParser(object):
                 if not self.skip_string(')'):
                     self.fail("Expected ')' after 'decltype(auto'.")
                 return ASTTrailingTypeSpecDecltypeAuto()
-            self.fail('"decltype(<expr>)" in trailing_type_spec not implemented')
-            # return ASTTrailingTypeSpecDecltype()
+            expr = self._parse_expression(inTemplate=False)
+            self.skip_ws()
+            if not self.skip_string(')'):
+                self.fail("Expected ')' after 'decltype(<expr>'.")
+            return ASTTrailingTypeSpecDecltype(expr)
 
         # prefixed
         prefix = None

tests/test_domain_cpp.py

@@ -235,6 +235,7 @@ def test_type_definitions():
     check("type", "bool ::B::b", {1:"B::b", 2:"N1B1bE"})
 
     check('type', 'A = B', {2:'1A'})
+    check('type', 'A = decltype(b)', {2:'1A'})
 
     # from breathe#267 (named function parameters for function pointers
     check('type', 'void (*gpio_callback_t)(struct device *port, uint32_t pin)',
@@ -389,6 +390,8 @@ def test_function_definitions():
 
     check('function', 'extern int f()', {1:'f', 2:'1fv'})
 
+    check('function', 'decltype(auto) f()', {1: 'f', 2:"1fv"})
+
     # TODO: make tests for functions in a template, e.g., Test<int&&()>
     # such that the id generation for function type types is correct.
 
@@ -466,6 +469,10 @@ def test_class_definitions():
     check('class', 'A : B, C...', {1:'A', 2:'1A'})
     check('class', 'A : B..., C', {1:'A', 2:'1A'})
 
+    # from #4094
+    check('class', 'template<class, class = std::void_t<>> has_var', {2:'I00E7has_var'})
+    check('class', 'template<class T> has_var<T, std::void_t<decltype(&T::var)>>', {2:'I0E7has_varI1TNSt6void_tIDTadN1T3varEEEEE'})
+
 
 def test_enum_definitions():
     check('enum', 'A', {2:"1A"})