New tests.

test/libsolidity/semanticTests/arithmetics/checked_called_by_unchecked.sol

@@ -0,0 +1,14 @@
+contract C {
+    function add(uint16 a, uint16 b) public returns (uint16) {
+        return a + b;
+    }
+
+    function f(uint16 a, uint16 b, uint16 c) public returns (uint16) {
+        unchecked { return add(a, b) + c; }
+    }
+}
+// ====
+// compileViaYul: also
+// ----
+// f(uint16,uint16,uint16): 0xe000, 0xe500, 2 -> FAILURE
+// f(uint16,uint16,uint16): 0xe000, 0x1000, 0x1000 -> 0x00

test/libsolidity/semanticTests/arithmetics/checked_modifier_called_by_unchecked.sol

@@ -0,0 +1,15 @@
+contract C {
+    modifier add(uint16 a, uint16 b) {
+        unchecked { a + b; }
+        _;
+    }
+
+    function f(uint16 a, uint16 b, uint16 c) public add(a, b) returns (uint16) {
+        return b + c;
+    }
+}
+// ====
+// compileViaYul: also
+// ----
+// f(uint16,uint16,uint16): 0xe000, 0xe500, 2 -> 58626
+// f(uint16,uint16,uint16): 0x1000, 0xe500, 0xe000 -> FAILURE

test/libsolidity/semanticTests/arithmetics/unchecked_called_by_checked.sol

@@ -0,0 +1,17 @@
+contract C {
+    function add(uint16 a, uint16 b) public returns (uint16) {
+        unchecked {
+            return a + b;
+        }
+    }
+
+    function f(uint16 a) public returns (uint16) {
+        return add(a, 0x100) + 0x100;
+    }
+}
+// ====
+// compileViaYul: also
+// ----
+// f(uint16): 7 -> 0x0207
+// f(uint16): 0xffff -> 511
+// f(uint16): 0xfeff -> FAILURE

test/libsolidity/syntaxTests/unchecked/unchecked_vardecl.sol

@@ -0,0 +1,8 @@
+contract C {
+    uint x = unchecked { f() + 2 }
+    function f() public pure returns (uint) {
+        return 4;
+    }
+}
+// ----
+// ParserError 6933: (26-35): Expected primary expression.