Do not insert SPDX identifier if already present in SyntaxTest

test/libsolidity/SyntaxTest.cpp

@@ -51,13 +51,23 @@ TestCase::TestResult SyntaxTest::run(ostream& _stream, string const& _linePrefix
 	return conclude(_stream, _linePrefix, _formatted);
 }
 
+string SyntaxTest::addPreamble(string const& _sourceCode)
+{
+	// Silence compiler version warning
+	string preamble = "pragma solidity >=0.0;\n";
+	// NOTE: this check is intentionally loose to match weird cases.
+	// We can manually adjust a test case where this causes problem.
+	if (_sourceCode.find("SPDX-License-Identifier:") == string::npos)
+		preamble += "// SPDX-License-Identifier: GPL-3.0\n";
+	return preamble + _sourceCode;
+}
+
 void SyntaxTest::setupCompiler()
 {
-	string const preamble = "pragma solidity >=0.0;\n// SPDX-License-Identifier: GPL-3.0\n";
 	compiler().reset();
 	auto sourcesWithPragma = m_sources;
 	for (auto& source: sourcesWithPragma)
-		source.second = preamble + source.second;
+		source.second = addPreamble(source.second);
 	compiler().setSources(sourcesWithPragma);
 	compiler().setEVMVersion(m_evmVersion);
 	compiler().setParserErrorRecovery(m_parserErrorRecovery);

test/libsolidity/SyntaxTest.h

@@ -50,6 +50,9 @@ public:
 	TestResult run(std::ostream& _stream, std::string const& _linePrefix = "", bool _formatted = false) override;
 
 protected:
+	/// Returns @param _sourceCode prefixed with the version pragma and the SPDX license identifier.
+	static std::string addPreamble(std::string const& _sourceCode);
+
 	void setupCompiler();
 	void parseAndAnalyze() override;
 	virtual void filterObtainedErrors();

test/libsolidity/syntaxTests/license/license_missing.sol

@@ -1 +1,2 @@
+// This test is actually useless, as the test suite adds the automatic preamble.
 contract C {}

test/libsolidity/syntaxTests/license/license_unicode.sol

@@ -1,3 +1,4 @@
-// This is just ignored:
 // SPDX-License-Identifier: ⡉⡊⡋⡌⡍⡎⡏⡐⡑⡒
 contract C {}
+// ----
+// Warning 1878: SPDX license identifier not provided in source file. Before publishing, consider adding a comment containing "SPDX-License-Identifier: <SPDX-License>" to each source file. Use "SPDX-License-Identifier: UNLICENSED" for non-open-source code. Please see https://spdx.org for more information.