diff --git a/lib/Sema/CSSimplify.cpp b/lib/Sema/CSSimplify.cpp
index 6c9a75fa68c27..3fa6c530a3890 100644
--- a/lib/Sema/CSSimplify.cpp
+++ b/lib/Sema/CSSimplify.cpp
@@ -1349,20 +1349,53 @@ ConstraintSystem::matchTupleTypes(TupleType *tuple1, TupleType *tuple2,
   return getTypeMatchSuccess();
 }
 
+// Determine whether conversion is allowed between two function types
+// based on their representations.
+static bool
+isConversionAllowedBetween(FunctionTypeRepresentation rep1,
+                           FunctionTypeRepresentation rep2) {
+  auto isThin = [](FunctionTypeRepresentation rep) {
+    return rep == FunctionTypeRepresentation::CFunctionPointer ||
+        rep == FunctionTypeRepresentation::Thin;
+  };
+  
+  // Allowing "thin" (c, thin) to "thin" conventions
+  if (isThin(rep1) && isThin(rep2))
+    return true;
+  
+  // Allowing all to "thick" (swift, block) conventions
+  // "thin" (c, thin) to "thick" or "thick" to "thick"
+  if (rep2 == FunctionTypeRepresentation::Swift ||
+      rep2 == FunctionTypeRepresentation::Block)
+    return true;
+  
+  return rep1 == rep2;
+}
+
 // Returns 'false' (i.e. no error) if it is legal to match functions with the
 // corresponding function type representations and the given match kind.
 static bool matchFunctionRepresentations(FunctionTypeRepresentation rep1,
                                          FunctionTypeRepresentation rep2,
-                                         ConstraintKind kind) {
+                                         ConstraintKind kind,
+                                         ConstraintLocatorBuilder locator) {
   switch (kind) {
   case ConstraintKind::Bind:
   case ConstraintKind::BindParam:
   case ConstraintKind::BindToPointerType:
   case ConstraintKind::Equal:
     return rep1 != rep2;
+      
+  case ConstraintKind::Subtype: {
+    auto last = locator.last();
+    if (!(last && last->is<LocatorPathElt::FunctionArgument>()))
+      return false;
+    
+    // Inverting the result because matchFunctionRepresentations
+    // returns false in conversions are allowed.
+    return !isConversionAllowedBetween(rep1, rep2);
+  }
 
   case ConstraintKind::OpaqueUnderlyingType:
-  case ConstraintKind::Subtype:
   case ConstraintKind::Conversion:
   case ConstraintKind::BridgingConversion:
   case ConstraintKind::ArgumentConversion:
@@ -1657,7 +1690,7 @@ ConstraintSystem::matchFunctionTypes(FunctionType *func1, FunctionType *func2,
 
   if (matchFunctionRepresentations(func1->getExtInfo().getRepresentation(),
                                    func2->getExtInfo().getRepresentation(),
-                                   kind)) {
+                                   kind, locator)) {
     return getTypeMatchFailure(locator);
   }
 
diff --git a/validation-test/compiler_crashers_2_fixed/sr12723.swift b/validation-test/compiler_crashers_2_fixed/sr12723.swift
new file mode 100644
index 0000000000000..b04103a9f7a8e
--- /dev/null
+++ b/validation-test/compiler_crashers_2_fixed/sr12723.swift
@@ -0,0 +1,81 @@
+// RUN: %target-swift-emit-silgen %s -verify
+
+func SR12723_thin(_: (@convention(thin) () -> Void) -> Void) {}
+func SR12723_block(_: (@convention(block) () -> Void) -> Void) {}
+func SR12723_c(_: (@convention(c) () -> Void) -> Void) {}
+
+func SR12723_function(_: () -> Void) {}
+
+func context() {
+    SR12723_c(SR12723_function) 
+
+    SR12723_block(SR12723_function) 
+
+    SR12723_thin(SR12723_function) 
+}
+
+struct SR12723_C {
+    let function: (@convention(c) () -> Void) -> Void
+}
+
+struct SR12723_Thin {
+    let function: (@convention(thin) () -> Void) -> Void
+}
+
+struct SR12723_Block {
+    let function: (@convention(block) () -> Void) -> Void
+}
+
+func proxy(_ f: (() -> Void) -> Void) {
+    let a = 1
+    f { print(a) }
+}
+
+func cContext() {
+    let c = SR12723_C { app in app() }
+
+    proxy(c.function)
+    // expected-error@-1 {{cannot convert value of type '(@convention(c) () -> Void) -> Void' to expected argument type '(() -> Void) -> Void'}}
+
+    let _ :  (@convention(block) () -> Void) -> Void = c.function 
+    // expected-error@-1 {{cannot convert value of type '(@convention(c) () -> Void) -> Void' to specified type '(@convention(block) () -> Void) -> Void'}}
+
+    let _ :  (@convention(c) () -> Void) -> Void = c.function // OK
+
+    let _ :  (@convention(thin) () -> Void) -> Void = c.function // OK
+    
+    let _ :  (() -> Void) -> Void = c.function 
+    // expected-error@-1 {{cannot convert value of type '(@convention(c) () -> Void) -> Void' to specified type '(() -> Void) -> Void'}}
+
+}
+
+func thinContext() {
+    let thin = SR12723_Thin { app in app() }
+
+    proxy(thin.function) 
+    // expected-error@-1 {{cannot convert value of type '(@convention(thin) () -> Void) -> Void' to expected argument type '(() -> Void) -> Void'}}
+
+    let _ :  (@convention(block) () -> Void) -> Void = thin.function 
+    // expected-error@-1 {{cannot convert value of type '(@convention(thin) () -> Void) -> Void' to specified type '(@convention(block) () -> Void) -> Void'}}
+
+    let _ :  (@convention(c) () -> Void) -> Void = thin.function // OK 
+
+    let _ :  (@convention(thin) () -> Void) -> Void = thin.function // OK
+
+    let _ :  (() -> Void) -> Void = thin.function 
+    // expected-error@-1 {{cannot convert value of type '(@convention(thin) () -> Void) -> Void' to specified type '(() -> Void) -> Void'}}
+}
+
+func blockContext() {
+    let block = SR12723_Block { app in app() }
+
+    proxy(block.function)
+
+    let _ :  (@convention(block) () -> Void) -> Void = block.function // OK
+
+    let _ :  (@convention(c) () -> Void) -> Void = block.function // OK
+
+    let _ :  (@convention(thin) () -> Void) -> Void = block.function // OK
+
+    let _ :  (() -> Void) -> Void = block.function  // OK
+}