Sema: Fix accepts-invalid with throwing function types

We can't unconditionally skip the conformance check if the type contains type
parameters; instead, we only want to skip it in the structural resolution
stage. In interface resolution stage, we proceed by mapping the type into
the generic environment first.

Author: slavapestov

lib/Sema/TypeCheckType.cpp

@@ -4233,12 +4233,15 @@ NeverNullType TypeResolver::resolveASTFunctionType(
     thrownTy = resolveType(thrownTypeRepr, thrownTypeOptions);
     if (thrownTy->hasError()) {
       thrownTy = Type();
-    } else if (!options.contains(TypeResolutionFlags::SilenceErrors) &&
-               !thrownTy->hasTypeParameter() &&
-               !checkConformance(thrownTy, ctx.getErrorDecl())) {
-      diagnoseInvalid(
-          thrownTypeRepr, thrownTypeRepr->getLoc(), diag::thrown_type_not_error,
-          thrownTy);
+    } else if (inStage(TypeResolutionStage::Interface) &&
+               !options.contains(TypeResolutionFlags::SilenceErrors)) {
+      auto thrownTyInContext = GenericEnvironment::mapTypeIntoContext(
+        resolution.getGenericSignature().getGenericEnvironment(), thrownTy);
+      if (!checkConformance(thrownTyInContext, ctx.getErrorDecl())) {
+        diagnoseInvalid(
+            thrownTypeRepr, thrownTypeRepr->getLoc(), diag::thrown_type_not_error,
+            thrownTy);
+      }
     }
   }
 

test/Generics/typed_throws.swift

@@ -14,7 +14,7 @@ func f2<T: P1>(_: T) where () throws(T.A) -> () == () throws -> () {}
 
 protocol P2 {
   associatedtype A where A == () throws(E) -> ()
-  associatedtype E
+  associatedtype E: Error
 }
 
 // CHECK-LABEL: typed_throws.(file).f3@

test/decl/func/typed_throws.swift

@@ -211,3 +211,8 @@ extension Result {
     }
   }
 }
+
+struct NotAnError<T> {}
+
+func badThrowingFunctionType<T>(_: () throws(NotAnError<T>) -> ()) {}
+// expected-error@-1 {{thrown type 'NotAnError<T>' does not conform to the 'Error' protocol}}