Consider mappings return values in control flow analysis.

test/libsolidity/syntaxTests/controlFlow/mappingReturn/named_err.sol

@@ -0,0 +1,5 @@
+contract C {
+    function f() internal pure returns (mapping(uint=>uint) storage r) { }
+}
+// ----
+// TypeError: (53-82): This variable is of storage pointer type and might be returned without assignment and could be used uninitialized. Assign the variable (potentially from itself) to fix this error.

test/libsolidity/syntaxTests/controlFlow/mappingReturn/named_fine.sol

@@ -0,0 +1,5 @@
+contract C {
+    mapping(uint=>uint) m;
+    function f() internal view returns (mapping(uint=>uint) storage r) { r = m; }
+}
+// ----

test/libsolidity/syntaxTests/controlFlow/mappingReturn/unnamed_err.sol

@@ -0,0 +1,5 @@
+contract C {
+    function f() internal pure returns (mapping(uint=>uint) storage) {}
+}
+// ----
+// TypeError: (53-72): This variable is of storage pointer type and might be returned without assignment and could be used uninitialized. Assign the variable (potentially from itself) to fix this error.

test/libsolidity/syntaxTests/controlFlow/mappingReturn/unnamed_fine.sol

@@ -0,0 +1,5 @@
+contract C {
+    mapping(uint=>uint) m;
+    function f() internal view returns (mapping(uint=>uint) storage) { return m; }
+}
+// ----