[Diagnostics] Don't attempt to synthesize arguments when destructuring a single tuple argument results in an overrun

If arguments are represented by a single tuple it's possible
that the issue is not about missing parameters but instead
about tuple destructuring. Fix `fixMissingArguments` to check for
overruns after destructuring and stop if that produces more
arguments then parameters because such situations are better
diagnosed as a general conversion failure rather than a missing
argument(s) problem.

Resolves: rdar://159408715

Author: xedin

lib/Sema/CSSimplify.cpp

@@ -2747,6 +2747,12 @@ static bool fixMissingArguments(ConstraintSystem &cs, ASTNode anchor,
       args.pop_back();
       for (const auto &elt : tuple->getElements())
         args.emplace_back(elt.getType(), elt.getName());
+
+      // If unpacking a tuple results in more arguments than parameters
+      // it would be diagnosed as a general mismatch because it's unclear
+      // whether it's a problem with missing or extraneous parameters.
+      if (args.size() > params.size())
+        return true;
     } else if (auto *typeVar = argType->getAs<TypeVariableType>()) {
       auto isParam = [](const Expr *expr) {
         if (auto *DRE = dyn_cast<DeclRefExpr>(expr)) {

…rashers/fixMissingArguments-ef29fe.swift …s_fixed/fixMissingArguments-ef29fe.swiftvalidation-test/compiler_crashers/fixMissingArguments-ef29fe.swift renamed to validation-test/compiler_crashers_fixed/fixMissingArguments-ef29fe.swift validation-test/compiler_crashers/fixMissingArguments-ef29fe.swift renamed to validation-test/compiler_crashers_fixed/fixMissingArguments-ef29fe.swift

@@ -1,4 +1,4 @@
 // {"kind":"typecheck","languageMode":6,"signature":"fixMissingArguments(swift::constraints::ConstraintSystem&, swift::ASTNode, llvm::SmallVectorImpl<swift::AnyFunctionType::Param>&, llvm::ArrayRef<swift::AnyFunctionType::Param>, unsigned int, swift::constraints::ConstraintLocatorBuilder)","signatureAssert":"Assertion failed: (Index < Length && \"Invalid index!\"), function operator[]"}
-// RUN: not --crash %target-swift-frontend -typecheck -swift-version 6 %s
+// RUN: not %target-swift-frontend -typecheck -swift-version 6 %s
 func a((Int, Int, Int)) a > {
   b, c in