fix #13634: Syntax error with decltype in trailing return type (#7301)

Author: ludviggunne

lib/tokenlist.cpp

@@ -1583,10 +1583,15 @@ static Token * createAstAtToken(Token *tok)
     }
     if (Token *const endTok = skipMethodDeclEnding(tok)) {
         if (Token::simpleMatch(endTok, "{")) {
-            const Token *tok2 = tok;
+            Token *tok2 = tok;
             do {
                 tok2 = tok2->next();
                 tok2->setCpp11init(false);
+                if (Token::simpleMatch(tok2, "decltype")) {
+                    AST_state state(cpp);
+                    Token *tok3 = tok2->tokAt(2);
+                    compileExpression(tok3, state);
+                }
             } while (tok2 != endTok);
         }
         return endTok;

test/testtokenize.cpp

@@ -392,6 +392,7 @@ class TestTokenizer : public TestFixture {
         TEST_CASE(astcase);
         TEST_CASE(astrefqualifier);
         TEST_CASE(astthrowdelete);
+        TEST_CASE(asttrailingdecltype);
         TEST_CASE(astvardecl);
         TEST_CASE(astnewscoped);
 
@@ -7020,6 +7021,12 @@ class TestTokenizer : public TestFixture {
         ASSERT_EQUALS("a(", testAst("class a { virtual ~a() throw() = delete; };"));
     }
 
+    void asttrailingdecltype() {
+        ASSERT_EQUALS("Cc& csize.(", testAst("template<class C> constexpr auto s(const C &c) noexcept -> decltype(c.size()) {}"));
+        ASSERT_EQUALS("Cc& MakeSpancdata.(csize.(,(",
+                      testAst("template <typename C> constexpr auto MakeSpan(C &c) -> decltype(MakeSpan(c.data(), c.size())) {}"));
+    }
+
     //Verify that returning a newly constructed object generates the correct AST even when the class name is scoped
     //Addresses https://trac.cppcheck.net/ticket/9700
     void astnewscoped() {