Merge pull request #12150 from slavapestov/csdiag-is-terrible-4.0

Sema: Fix a failure to emit a diagnostic [4.0]

Author: tkremenek

lib/Sema/CSDiag.cpp

@@ -6034,11 +6034,15 @@ bool FailureDiagnosis::diagnoseArgumentGenericRequirements(
 
   auto result = TC.checkGenericArguments(
       dc, callExpr->getLoc(), fnExpr->getLoc(), AFD->getInterfaceType(),
-      env->getGenericSignature(), QueryTypeSubstitutionMap{substitutions},
+      env->getGenericSignature(), substitutionFn,
       LookUpConformanceInModule{dc->getParentModule()}, nullptr,
       ConformanceCheckFlags::SuppressDependencyTracking, &genericReqListener);
 
-  return result != RequirementCheckResult::Success;
+  assert(result != RequirementCheckResult::UnsatisfiedDependency);
+
+  // Note: If result is RequirementCheckResult::SubstitutionFailure, we did
+  // not emit a diagnostic, so we must return false in that case.
+  return result == RequirementCheckResult::Failure;
 }
 
 /// When initializing Unsafe[Mutable]Pointer<T> from Unsafe[Mutable]RawPointer,

test/Constraints/diagnostics.swift

@@ -1080,3 +1080,12 @@ fun_31849281(a: { !$0 }, c: [nil, 42], b: { "\($0)" })
 
 func f_31849281(x: Int, y: Int, z: Int) {}
 f_31849281(42, y: 10, x: 20) // expected-error {{argument 'x' must precede unnamed argument #1}} {{12-12=x: 20, }} {{21-28=}}
+
+// https://bugs.swift.org/browse/SR-5934 - failure to emit diagnostic for bad
+// generic constraints
+func elephant<T, U>(_: T) where T : Collection, T.Element == U, T.Element : Hashable {}
+// expected-note@-1 {{in call to function 'elephant'}}
+
+func platypus<T>(a: [T]) {
+    _ = elephant(a) // expected-error {{generic parameter 'U' could not be inferred}}
+}