C++: Support flow through LambdaExpression

I've checked with a temporary workaround for the locations problem that
my annotations in the test cpp files are on the correct lines.

cpp/ql/src/semmle/code/cpp/dataflow/internal/DataFlowUtil.qll

@@ -452,6 +452,11 @@ private predicate exprToExprStep_nocfg(Expr fromExpr, Expr toExpr) {
   // initializer `(x)`.
   toExpr.(NewExpr).getInitializer() = fromExpr
   or
+  // A lambda expression (`[captures](params){body}`) is just a thin wrapper
+  // around the desugared closure creation in the form of a
+  // `ClassAggregateLiteral` (`{ capture1, ..., captureN }`).
+  toExpr.(LambdaExpression).getInitializer() = fromExpr
+  or
   toExpr = any(Call call |
       exists(DataFlowFunction f, FunctionInput inModel, FunctionOutput outModel, int iIn |
         call.getTarget() = f and

cpp/ql/test/library-tests/dataflow/dataflow-tests/lambdas.cpp

@@ -11,14 +11,14 @@ void test_lambdas()
 	int w = 0;
 
 	auto a = [t, u]() -> int {
-		sink(t); // flow from source() [NOT DETECTED]
+		sink(t); // flow from source()
 		sink(u);
 		return t;
 	};
-	sink(a()); // flow from source() [NOT DETECTED]
+	sink(a()); // flow from source()
 
 	auto b = [&] {
-		sink(t); // flow from source() [NOT DETECTED]
+		sink(t); // flow from source()
 		sink(u);
 		v = source(); // (v is reference captured)
 	};
@@ -26,7 +26,7 @@ void test_lambdas()
 	sink(v); // flow from source() [NOT DETECTED]
 
 	auto c = [=] {
-		sink(t); // flow from source() [NOT DETECTED]
+		sink(t); // flow from source()
 		sink(u);
 	};
 	c();

cpp/ql/test/library-tests/dataflow/dataflow-tests/test.expected

@@ -1,4 +1,8 @@
 | acrossLinkTargets.cpp:12:8:12:8 | x | acrossLinkTargets.cpp:19:27:19:32 | call to source |
+| file://:0:0:0:0 | t | lambdas.cpp:8:10:8:15 | call to source |
+| file://:0:0:0:0 | t | lambdas.cpp:8:10:8:15 | call to source |
+| file://:0:0:0:0 | t | lambdas.cpp:8:10:8:15 | call to source |
+| lambdas.cpp:18:8:18:8 | call to operator() | lambdas.cpp:8:10:8:15 | call to source |
 | lambdas.cpp:35:8:35:8 | a | lambdas.cpp:8:10:8:15 | call to source |
 | lambdas.cpp:41:8:41:8 | a | lambdas.cpp:8:10:8:15 | call to source |
 | test.cpp:7:8:7:9 | t1 | test.cpp:6:12:6:17 | call to source |

cpp/ql/test/library-tests/dataflow/dataflow-tests/test_diff.expected

@@ -1,3 +1,5 @@
+| lambdas.cpp:8:10:8:15 | file://:0:0:0:0 | AST only |
+| lambdas.cpp:8:10:8:15 | lambdas.cpp:18:8:18:8 | AST only |
 | lambdas.cpp:8:10:8:15 | lambdas.cpp:35:8:35:8 | AST only |
 | lambdas.cpp:8:10:8:15 | lambdas.cpp:41:8:41:8 | AST only |
 | test.cpp:89:28:89:34 | test.cpp:92:8:92:14 | IR only |

cpp/ql/test/library-tests/dataflow/taint-tests/localTaint.expected

@@ -213,7 +213,7 @@
 | taint.cpp:228:11:228:11 | Unknown literal | taint.cpp:228:11:228:11 | constructor init of field t | TAINT |
 | taint.cpp:228:11:228:11 | Unknown literal | taint.cpp:228:11:228:11 | constructor init of field u | TAINT |
 | taint.cpp:228:11:232:2 | [...](...){...} | taint.cpp:233:7:233:7 | a |  |
-| taint.cpp:228:11:232:2 | {...} | taint.cpp:228:11:232:2 | [...](...){...} | TAINT |
+| taint.cpp:228:11:232:2 | {...} | taint.cpp:228:11:232:2 | [...](...){...} |  |
 | taint.cpp:228:12:228:12 | t | taint.cpp:228:11:232:2 | {...} | TAINT |
 | taint.cpp:228:15:228:15 | u | taint.cpp:228:11:232:2 | {...} | TAINT |
 | taint.cpp:228:17:228:17 | `this` parameter in operator() | file://:0:0:0:0 | this |  |
@@ -222,13 +222,13 @@
 | taint.cpp:235:11:235:11 | Unknown literal | taint.cpp:235:11:235:11 | constructor init of field u | TAINT |
 | taint.cpp:235:11:235:11 | Unknown literal | taint.cpp:235:11:235:11 | constructor init of field v | TAINT |
 | taint.cpp:235:11:239:2 | [...](...){...} | taint.cpp:240:2:240:2 | b |  |
-| taint.cpp:235:11:239:2 | {...} | taint.cpp:235:11:239:2 | [...](...){...} | TAINT |
+| taint.cpp:235:11:239:2 | {...} | taint.cpp:235:11:239:2 | [...](...){...} |  |
 | taint.cpp:235:15:235:15 | `this` parameter in operator() | file://:0:0:0:0 | this |  |
 | taint.cpp:238:7:238:12 | call to source | taint.cpp:238:3:238:14 | ... = ... |  |
 | taint.cpp:243:11:243:11 | Unknown literal | taint.cpp:243:11:243:11 | constructor init of field t | TAINT |
 | taint.cpp:243:11:243:11 | Unknown literal | taint.cpp:243:11:243:11 | constructor init of field u | TAINT |
 | taint.cpp:243:11:246:2 | [...](...){...} | taint.cpp:247:2:247:2 | c |  |
-| taint.cpp:243:11:246:2 | {...} | taint.cpp:243:11:246:2 | [...](...){...} | TAINT |
+| taint.cpp:243:11:246:2 | {...} | taint.cpp:243:11:246:2 | [...](...){...} |  |
 | taint.cpp:243:15:243:15 | `this` parameter in operator() | file://:0:0:0:0 | this |  |
 | taint.cpp:243:15:243:15 | `this` parameter in operator() | file://:0:0:0:0 | this |  |
 | taint.cpp:249:11:252:2 | [...](...){...} | taint.cpp:253:2:253:2 | d |  |

cpp/ql/test/library-tests/dataflow/taint-tests/taint.cpp

@@ -208,11 +208,11 @@ void test_swap() {
 	y = 0;
 
 	sink(x); // tainted
-	sink(y);
+	sink(y); // clean
 
 	std::swap(x, y);
 
-	sink(x); // [FALSE POSITIVE]
+	sink(x); // clean [FALSE POSITIVE]
 	sink(y); // tainted
 }
 
@@ -226,35 +226,35 @@ void test_lambdas()
 	int w = 0;
 
 	auto a = [t, u]() -> int {
-		sink(t); // tainted [NOT DETECTED]
-		sink(u);
+		sink(t); // tainted
+		sink(u); // clean [FALSE POSITIVE]
 		return t;
 	};
 	sink(a()); // tainted
 
 	auto b = [&] {
-		sink(t); // tainted [NOT DETECTED]
-		sink(u);
+		sink(t); // tainted
+		sink(u); // clean [FALSE POSITIVE]
 		v = source(); // (v is reference captured)
 	};
 	b();
 	sink(v); // tainted [NOT DETECTED]
 
 	auto c = [=] {
-		sink(t); // tainted [NOT DETECTED]
-		sink(u);
+		sink(t); // tainted
+		sink(u); // clean [FALSE POSITIVE]
 	};
 	c();
 
 	auto d = [](int a, int b) {
 		sink(a); // tainted
-		sink(b);
+		sink(b); // clean
 	};
 	d(t, u);
 
 	auto e = [](int &a, int &b, int &c) {
 		sink(a); // tainted
-		sink(b);
+		sink(b); // clean
 		c = source();
 	};
 	e(t, u, w);