[CSSimplify] Thin to thin is also allowed

Author: LucianoPAlmeida

lib/Sema/CSSimplify.cpp

@@ -1367,6 +1367,15 @@ static bool matchFunctionRepresentations(FunctionTypeRepresentation rep1,
     if (!(last && last->is<LocatorPathElt::FunctionArgument>()))
       return false;
 
+    auto isThin = [](FunctionTypeRepresentation rep) {
+      return rep == FunctionTypeRepresentation::CFunctionPointer ||
+          rep == FunctionTypeRepresentation::Thin;
+    };
+    
+    // Allowing "thin" (c, thin) to "thin" conventions
+    if (isThin(rep1) && isThin(rep2))
+      return false;
+    
     // Allowing all to "thick" (swift, block) conventions
     // "thin" (c, thin) to "thick" or "thick" to "thick"
     if (rep2 == FunctionTypeRepresentation::Swift ||

validation-test/compiler_crashers_2_fixed/sr12723.swift

@@ -42,9 +42,8 @@ func cContext() {
 
     let _ :  (@convention(c) () -> Void) -> Void = c.function // OK
 
-    let _ :  (@convention(thin) () -> Void) -> Void = c.function 
-    // expected-error@-1 {{cannot convert value of type '(@convention(c) () -> Void) -> Void' to specified type '(@convention(thin) () -> Void) -> Void'}}
-
+    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'}}
 
@@ -59,8 +58,7 @@ func thinContext() {
     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 
-    // expected-error@-1 {{cannot convert value of type '(@convention(thin) () -> Void) -> Void' to specified type '(@convention(c) () -> Void) -> Void'}}
+    let _ :  (@convention(c) () -> Void) -> Void = thin.function // OK 
 
     let _ :  (@convention(thin) () -> Void) -> Void = thin.function // OK