Disallow the same yul variable occurring multiple times on the LHS of an assignment.

Changelog.md

@@ -21,6 +21,7 @@ Bugfixes:
  * Type Checker: Do not disallow assigning to calldata variables.
  * Wasm backend: Fix code generation for for-loops with pre statements.
  * Yul: Fix source location of variable multi-assignment.
+ * Yul: Disallow the same variable to occur multiple times on the left-hand side of an assignment.
 
 
 ### 0.6.10 (2020-06-11)

docs/yul.rst

@@ -288,6 +288,8 @@ variables at the same time. For this, the number and types of the
 values have to match.
 If you want to assign the values returned from a function that has
 multiple return parameters, you have to provide multiple variables.
+The same variable may not occur multiple times on the left-hand side of
+an assignment, e.g. ``x, x := f()`` is invalid.
 
 .. code-block:: yul
 
@@ -506,6 +508,8 @@ In variable declarations and assignments, the right-hand-side expression
 variables on the left-hand-side.
 This is the only situation where an expression evaluating
 to more than one value is allowed.
+The same variable name cannot occur more than once in the left-hand-side of
+an assignment or variable declaration.
 
 Expressions that are also statements (i.e. at the block level) have to
 evaluate to zero values.

libyul/AsmAnalysis.cpp

@@ -167,6 +167,17 @@ void AsmAnalyzer::operator()(Assignment const& _assignment)
 	size_t const numVariables = _assignment.variableNames.size();
 	yulAssert(numVariables >= 1, "");
 
+	set<YulString> variables;
+	for (auto const& _variableName: _assignment.variableNames)
+		if (!variables.insert(_variableName.name).second)
+			m_errorReporter.declarationError(
+				9005_error,
+				_assignment.location,
+				"Variable " +
+				_variableName.name.str() +
+				" occurs multiple times on the left-hand side of the assignment."
+			);
+
 	vector<YulString> types = std::visit(*this, *_assignment.value);
 
 	if (types.size() != numVariables)

test/libyul/yulSyntaxTests/assignment_duplicate_vars.yul

@@ -0,0 +1,10 @@
+{
+  function f() -> a, b {}
+  function g() -> a, b, c {}
+  let x, y
+  x, x := f()
+  y, x, y := g()
+}
+// ----
+// DeclarationError 9005: (70-81): Variable x occurs multiple times on the left-hand side of the assignment.
+// DeclarationError 9005: (84-98): Variable y occurs multiple times on the left-hand side of the assignment.

test/libyul/yulSyntaxTests/declaration_duplicate_vars.yul

@@ -0,0 +1,6 @@
+{
+  function f() -> a, b {}
+  let x, x := f()
+}
+// ----
+// DeclarationError 1395: (30-45): Variable name x already taken in this scope.