Don't evaluate reference if it resolves to more than one elements (PY-19856)

python/src/com/jetbrains/python/psi/impl/PyEvaluator.java

@@ -16,6 +16,7 @@
 package com.jetbrains.python.psi.impl;
 
 import com.intellij.psi.PsiElement;
+import com.intellij.psi.ResolveResult;
 import com.intellij.psi.util.PsiTreeUtil;
 import com.intellij.util.containers.ContainerUtil;
 import com.jetbrains.python.PyNames;
@@ -240,7 +241,11 @@ public class PyEvaluator {
       if (myNamespace != null) {
         return myNamespace.get(expression.getReferencedName());
       }
-      PsiElement result = expression.getReference(PyResolveContext.noImplicits()).resolve();
+      final ResolveResult[] results = expression.getReference(PyResolveContext.noImplicits()).multiResolve(false);
+      if (results.length != 1) {
+        return null;
+      }
+      PsiElement result = results[0].getElement();
       if (result instanceof PyTargetExpression) {
         result = ((PyTargetExpression)result).findAssignedValue();
       }

python/testSrc/com/jetbrains/python/PyEvaluatorTest.java

@@ -255,6 +255,15 @@ public class PyEvaluatorTest extends PyTestCase {
     assertTrue(byExpression("not False", Boolean.class));
   }
 
+  public void testMultiResolve() {
+    final PyExpression expression = parseText("if condition:\n" +
+                                              "    a = 1\n" +
+                                              "else:\n" +
+                                              "    a = 3\n" +
+                                              "expr = a < 2");
+    assertNull(new PyEvaluator().evaluate(expression));
+  }
+
   @NotNull
   private <T> T byExpression(@NotNull String expression, @NotNull Class<T> cls) {
     final Object value = new PyEvaluator().evaluate(parseExpression(expression));