Fix #788: Enum value used inside __traits(...) flagged as unused

src/dscanner/analysis/unused_variable.d

@@ -19,6 +19,12 @@ final class UnusedVariableCheck : UnusedIdentifierCheck
 
 	mixin AnalyzerInfo!"unused_variable_check";
 
+	/**
+	Ignore declarations which are allowed to be unused, e.g. inside of a
+	speculative compilation: __traits(compiles, { S s = 0; })
+	**/
+	uint ignoreDeclarations = 0;
+
 	/**
 	 * Params:
 	 *     fileName = the name of the file being analyzed
@@ -42,17 +48,30 @@ final class UnusedVariableCheck : UnusedIdentifierCheck
 		autoDeclaration.accept(this);
 	}
 
+	override void visit(const TraitsExpression traitsExp)
+	{
+		// issue #788: Enum values might be used inside of `__traits` expressions, e.g.:
+		// enum name = "abc";
+		// __traits(hasMember, S, name);
+		ignoreDeclarations++;
+		traitsExp.templateArgumentList.accept(this);
+		ignoreDeclarations--;
+	}
+
 	override protected void popScope()
 	{
-		foreach (uu; tree[$ - 1])
+		if (!ignoreDeclarations)
 		{
-			if (!uu.isRef && tree.length > 1)
+			foreach (uu; tree[$ - 1])
 			{
-				if (uu.uncertain)
+				if (!uu.isRef && tree.length > 1)
-					continue;
+				{
-				immutable string errorMessage = "Variable " ~ uu.name ~ " is never used.";
+					if (uu.uncertain)
-				addErrorMessage(uu.line, uu.column, 
+						continue;
-						"dscanner.suspicious.unused_variable", errorMessage);
+					immutable string errorMessage = "Variable " ~ uu.name ~ " is never used.";
+					addErrorMessage(uu.line, uu.column,
+							"dscanner.suspicious.unused_variable", errorMessage);
+				}
 			}
 		}
 		tree = tree[0 .. $ - 1];
@@ -124,6 +143,15 @@ final class UnusedVariableCheck : UnusedIdentifierCheck
 		testValue.writeln;
 	}
 
+	// Issue 788
+	void traits()
+	{
+		enum fieldName = "abc";
+		__traits(hasMember, S, fieldName);
+
+		__traits(compiles, { int i = 2; });
+	}
+
 	}c, sac);
 	stderr.writeln("Unittest for UnusedVariableCheck passed.");
 }