[CS] Avoid emitting duplicate note for generic argument diagnostic

Since we can run CSGen multiple times, we need to guard the emission
of the note on whether the TypeRepr was already marked invalid (i.e
whether we already emitted a diagnostic for it).

Author: hamishknight

lib/Sema/CSGen.cpp

@@ -1957,6 +1957,8 @@ namespace {
     void addSpecializationConstraint(ConstraintLocator *locator, Type boundType,
                                      SourceLoc lAngleLoc,
                                      ArrayRef<TypeRepr *> specializationArgs) {
+      auto &ctx = CS.getASTContext();
+
       // Resolve each type.
       SmallVector<Type, 2> specializationArgTypes;
       auto options =
@@ -1971,6 +1973,7 @@ namespace {
           options |= TypeResolutionFlags::AllowPackReferences;
           elementEnv = OuterExpansions.back();
         }
+        auto alreadyInvalid = specializationArg->isInvalid();
         auto result = TypeResolution::resolveContextualType(
             specializationArg, CurDC, options,
             // Introduce type variables for unbound generics.
@@ -1980,19 +1983,20 @@ namespace {
             OpenGenericTypeRequirements(CS, locator,
                                         /*preparedOverload*/ nullptr));
         if (result->hasError()) {
-          auto &ctxt = CS.getASTContext();
+          if (!alreadyInvalid) {
+            ctx.Diags.diagnose(lAngleLoc,
+                               diag::while_parsing_as_left_angle_bracket);
+          }
           CS.recordFix(IgnoreInvalidASTNode::create(
               CS, CS.getConstraintLocator(argLocator)));
-          result = PlaceholderType::get(ctxt, specializationArg);
-          ctxt.Diags.diagnose(lAngleLoc,
-                              diag::while_parsing_as_left_angle_bracket);
+          result = PlaceholderType::get(ctx, specializationArg);
         }
         specializationArgTypes.push_back(result);
       }
 
       auto constraint = Constraint::create(
           CS, ConstraintKind::ExplicitGenericArguments, boundType,
-          PackType::get(CS.getASTContext(), specializationArgTypes), locator);
+          PackType::get(ctx, specializationArgTypes), locator);
       CS.addUnsolvedConstraint(constraint);
       CS.activateConstraint(constraint);
     }

test/Constraints/generics.swift

@@ -1113,3 +1113,18 @@ func testHolePropagation() {
   _ = { () -> (S<R>, Int) in return (makeT(), 0) } // expected-error {{type 'R' does not conform to protocol 'P'}}
   _ = { () -> (S<R>, Int) in (); return (makeT(), 0) } // expected-error {{type 'R' does not conform to protocol 'P'}}
 }
+
+@freestanding(expression) macro overloadedMacro<T>() -> String = #file
+@freestanding(expression) macro overloadedMacro<T>(_ x: T = 0) -> String = #file
+
+do {
+  func foo(_ fn: () -> Int) {}
+  func foo(_ fn: () -> String) {}
+
+  // Make sure we only emit a single note here.
+  foo {
+    #overloadedMacro < Undefined >
+    // expected-error@-1 {{cannot find type 'Undefined' in scope}}
+    // expected-note@-2 {{while parsing this '<' as a type parameter bracket}}
+  }
+}