[GSB] Infer requirements from protocol inheritance and where clauses.

Fixes rdar://problem/29231937.

Author: DougGregor

lib/AST/GenericSignatureBuilder.cpp

@@ -4171,7 +4171,7 @@ ConstraintResult GenericSignatureBuilder::expandConformanceRequirement(
 
     auto inheritedReqResult =
       addInheritedRequirements(proto, selfType.getUnresolvedType(), source,
-                               /*inferForModule=*/nullptr);
+                               proto->getModuleContext());
     if (isErrorResult(inheritedReqResult))
       return inheritedReqResult;
   }
@@ -4187,7 +4187,7 @@ ConstraintResult GenericSignatureBuilder::expandConformanceRequirement(
       auto innerSource = FloatingRequirementSource::viaProtocolRequirement(
           source, proto, &req, /*inferred=*/false);
       addRequirement(&req, innerSource, &protocolSubMap,
-                     /*inferForModule=*/nullptr);
+                     proto->getModuleContext());
     }
   }
 

test/Generics/requirement_inference.swift

@@ -1,5 +1,5 @@
-// RUN: %target-typecheck-verify-swift -typecheck %s -verify
-// RUN: %target-typecheck-verify-swift -typecheck -debug-generic-signatures %s > %t.dump 2>&1 
+// RUN: %target-typecheck-verify-swift -typecheck -verify
+// RUN: %target-typecheck-verify-swift -typecheck -debug-generic-signatures > %t.dump 2>&1
 // RUN: %FileCheck %s < %t.dump
 
 protocol P1 { 
@@ -487,3 +487,14 @@ extension X1WithP2 {
     _ = X5<T>() // FIXME: expected-error{{type 'T' does not conform to protocol 'P2'}}
   }
 }
+
+// Inference from protocol inheritance clauses.
+typealias ExistentialP4WithP2Assoc<T: P4> = P4 where T.P4Assoc : P2
+
+protocol P37 : ExistentialP4WithP2Assoc<Self> { }
+
+extension P37 {
+  func f() {
+    _ = X5<P4Assoc>() // requires P2
+  }
+}