Replace use of getArchetype() in diagnoseGenericParameterErrors()

This is a quick follow-up to
<#1160>, to replace the becoming
deprecated getArchetype() with doing the same thing via calling through
to ArchetypeBuilder with a declContext.

Uses findGenericSubstitutions() to do so. So this site could take
advantage of destructuring of more complex params containing generics.
Right now, though, that never happens. For complex params the
(unfortunately, worse) diagnosis happens in diagnoseFailureForExpr() on
the argument expression before reaching here. I’d like to improve that
in future work.

Author: gregomni

lib/Sema/CSDiag.cpp

@@ -806,11 +806,13 @@ namespace {
     struct FailedArgumentInfo {
       int argumentNumber = -1;      ///< Arg # at the call site.
       Type parameterType = Type();  ///< Expected type at the decl site.
+      DeclContext *declContext = nullptr; ///< Context at the candidate declaration.
 
       bool isValid() const { return argumentNumber != -1; }
 
       bool operator!=(const FailedArgumentInfo &other) {
         if (argumentNumber != other.argumentNumber) return true;
+        if (declContext != other.declContext) return true;
         // parameterType can be null, and isEqual doesn't handle this.
         if (!parameterType || !other.parameterType)
           return parameterType.getPointer() != other.parameterType.getPointer();
@@ -1186,6 +1188,8 @@ CalleeCandidateInfo::evaluateCloseness(DeclContext *dc, Type candArgListType,
 
       failureInfo.argumentNumber = argNo;
       failureInfo.parameterType = paramType;
+      if (paramType->hasTypeParameter())
+        failureInfo.declContext = dc;
     }
   }
 
@@ -1735,18 +1739,25 @@ bool CalleeCandidateInfo::diagnoseAnyStructuralArgumentError(Expr *fnExpr,
 /// archetype that has argument type errors, diagnose that error and
 /// return true.
 bool CalleeCandidateInfo::diagnoseGenericParameterErrors(Expr *badArgExpr) {
-  bool foundFailure = false;
-  Type paramType = failedArgument.parameterType;
   Type argType = badArgExpr->getType();
-
-  if (auto genericParam = paramType->getAs<GenericTypeParamType>())
-    paramType = genericParam->getDecl()->getArchetype();
 
-  if (paramType->is<ArchetypeType>() && !argType->hasTypeVariable() &&
-      // FIXME: For protocol argument types, could add specific error
-      // similar to could_not_use_member_on_existential.
-      !argType->is<ProtocolType>() && !argType->is<ProtocolCompositionType>()) {
-    auto archetype = paramType->castTo<ArchetypeType>();
+  // FIXME: For protocol argument types, could add specific error
+  // similar to could_not_use_member_on_existential.
+  if (argType->hasTypeVariable() || argType->is<ProtocolType>() ||
+      argType->is<ProtocolCompositionType>())
+    return false;
+  
+  bool foundFailure = false;
+  SmallVector<ArchetypeType *, 4> archetypes;
+  SmallVector<Type, 4> substitutions;
+  
+  if (!findGenericSubstitutions(failedArgument.declContext, failedArgument.parameterType,
+                                argType, archetypes, substitutions))
+    return false;
+
+  for (unsigned i = 0, c = archetypes.size(); i < c; i++) {
+    auto archetype = archetypes[i];
+    auto argType = substitutions[i];
 
     // FIXME: Add specific error for not subclass, if the archetype has a superclass?