Fix default member initialization in assignment, closes #290

Note this change adds `{` `}` around member assignment statements in assignment operators.

That change now triggers Clang's `-Wbraced-scalar-init` warning, but this warning appears to be spurious. See the open Clang issue here llvm/llvm-project#54702 which mentions the same justification I had in mind (disallow implicit narrowing, that's the #1 reason to recommend using `{` `}` init where possible). Unfortunately that Clang issue still open without comments after a year and so it's hard to know the status.

(BTW, what a coincidence -- I actually hit this on the exact first anniversary of that issue, which also happens to be April 1 but I swear this is not a joke and I think the LLVM issue wasn't either...)

Author: hsutter

regression-tests/test-results/clang-12/pure2-types-smf-and-that-1-provide-everything.cpp.output

@@ -0,0 +1,4 @@
+pure2-types-smf-and-that-1-provide-everything.cpp2:24:16: warning: braces around scalar initializer [-Wbraced-scalar-init]
+        addr = {"123 Ford Dr."};
+               ^~~~~~~~~~~~~~~~
+1 warning generated.

regression-tests/test-results/clang-12/pure2-types-smf-and-that-2-provide-mvconstruct-and-cpassign.cpp.output

@@ -0,0 +1,4 @@
+pure2-types-smf-and-that-2-provide-mvconstruct-and-cpassign.cpp2:24:16: warning: braces around scalar initializer [-Wbraced-scalar-init]
+        addr = {"123 Ford Dr."};
+               ^~~~~~~~~~~~~~~~
+1 warning generated.

regression-tests/test-results/clang-12/pure2-types-smf-and-that-3-provide-mvconstruct-and-mvassign.cpp.output

@@ -0,0 +1,4 @@
+pure2-types-smf-and-that-3-provide-mvconstruct-and-mvassign.cpp2:24:16: warning: braces around scalar initializer [-Wbraced-scalar-init]
+        addr = {"123 Ford Dr."};
+               ^~~~~~~~~~~~~~~~
+1 warning generated.

regression-tests/test-results/clang-12/pure2-types-smf-and-that-4-provide-cpassign-and-mvassign.cpp.output

@@ -0,0 +1,4 @@
+pure2-types-smf-and-that-4-provide-cpassign-and-mvassign.cpp2:24:16: warning: braces around scalar initializer [-Wbraced-scalar-init]
+        addr = {"123 Ford Dr."};
+               ^~~~~~~~~~~~~~~~
+1 warning generated.

regression-tests/test-results/clang-12/pure2-types-smf-and-that-5-provide-nothing-but-general-case.cpp.output

@@ -0,0 +1,4 @@
+pure2-types-smf-and-that-5-provide-nothing-but-general-case.cpp2:24:16: warning: braces around scalar initializer [-Wbraced-scalar-init]
+        addr = {"123 Ford Dr."};
+               ^~~~~~~~~~~~~~~~
+1 warning generated.

regression-tests/test-results/pure2-types-basics.cpp

@@ -84,8 +84,8 @@ namespace N {
     }
 #line 6 "pure2-types-basics.cpp2"
     auto myclass::operator=(cpp2::in<int> x) -> myclass& {
-        data = x;
-        more = std::to_string(42 * 12);
+        data = {x};
+        more = {std::to_string(42 * 12)};
 
 #line 9 "pure2-types-basics.cpp2"
         std::cout << "myclass: implicit from int\n";
@@ -106,8 +106,8 @@ namespace N {
     }
 #line 13 "pure2-types-basics.cpp2"
     auto myclass::operator=(cpp2::in<std::string> s) -> myclass& {
-        data = 99;
-        more = s;
+        data = {99};
+        more = {s};
 
 #line 16 "pure2-types-basics.cpp2"
         std::cout << "myclass: explicit from string\n";

regression-tests/test-results/pure2-types-order-independence-and-nesting.cpp

@@ -116,7 +116,7 @@ namespace N {
 #line 10 "pure2-types-order-independence-and-nesting.cpp2"
     auto X::operator=(cpp2::out<Y> y) -> X& {
         y.construct(&(*this));
-        py = &y.value();
+        py = {&y.value()};
 
 #line 31 "pure2-types-order-independence-and-nesting.cpp2"
         std::cout << "made a safely initialized cycle\n";
@@ -140,7 +140,7 @@ namespace N {
      {  }
 #line 49 "pure2-types-order-independence-and-nesting.cpp2"
     auto Y::operator=(cpp2::in<X*> x) -> Y&  { 
-                                   px = x;
+                                   px = {x};
                                    return *this;
 #line 49 "pure2-types-order-independence-and-nesting.cpp2"
      }

regression-tests/test-results/pure2-types-smf-and-that-1-provide-everything.cpp

@@ -69,8 +69,8 @@ auto main() -> int;
     }
 
     auto myclass::operator=(myclass const& that) -> myclass& {
-        name = that.name;
-        addr = that.addr + "(AC)";
+        name = {that.name};
+        addr = {that.addr + "(AC)"};
 
 #line 15 "pure2-types-smf-and-that-1-provide-everything.cpp2"
         std::cout << "assign - copy        ";
@@ -79,8 +79,8 @@ auto main() -> int;
     }
 
     auto myclass::operator=(myclass&& that) -> myclass& {
-        name = std::move(that).name;
-        addr = std::move(that).addr;
+        name = {std::move(that).name};
+        addr = {std::move(that).addr};
 #line 19 "pure2-types-smf-and-that-1-provide-everything.cpp2"
         std::cout << "assign - move        ";
         return *this;
@@ -96,8 +96,8 @@ auto main() -> int;
     }
 #line 22 "pure2-types-smf-and-that-1-provide-everything.cpp2"
     auto myclass::operator=(cpp2::in<std::string> x) -> myclass& {
-        name = x;
-        addr = "123 Ford Dr.";
+        name = {x};
+        addr = {"123 Ford Dr."};
 
 #line 24 "pure2-types-smf-and-that-1-provide-everything.cpp2"
         std::cout << "ctor - from string   ";

regression-tests/test-results/pure2-types-smf-and-that-2-provide-mvconstruct-and-cpassign.cpp

@@ -72,8 +72,8 @@ auto main() -> int;
     }
 
     auto myclass::operator=(myclass const& that) -> myclass& {
-        name = that.name;
-        addr = that.addr + "(AC)";
+        name = {that.name};
+        addr = {that.addr + "(AC)"};
 
 #line 15 "pure2-types-smf-and-that-2-provide-mvconstruct-and-cpassign.cpp2"
         std::cout << "assign - copy        ";
@@ -82,8 +82,8 @@ auto main() -> int;
     }
 #line 13 "pure2-types-smf-and-that-2-provide-mvconstruct-and-cpassign.cpp2"
     auto myclass::operator=(myclass&& that) -> myclass& {
-        name = std::move(that).name;
-        addr = std::move(that).addr + "(AC)";
+        name = {std::move(that).name};
+        addr = {std::move(that).addr + "(AC)"};
 
 #line 15 "pure2-types-smf-and-that-2-provide-mvconstruct-and-cpassign.cpp2"
         std::cout << "assign - copy        ";
@@ -101,8 +101,8 @@ auto main() -> int;
     }
 #line 22 "pure2-types-smf-and-that-2-provide-mvconstruct-and-cpassign.cpp2"
     auto myclass::operator=(cpp2::in<std::string> x) -> myclass& {
-        name = x;
-        addr = "123 Ford Dr.";
+        name = {x};
+        addr = {"123 Ford Dr."};
 
 #line 24 "pure2-types-smf-and-that-2-provide-mvconstruct-and-cpassign.cpp2"
         std::cout << "ctor - from string   ";

regression-tests/test-results/pure2-types-smf-and-that-3-provide-mvconstruct-and-mvassign.cpp

@@ -64,8 +64,8 @@ auto main() -> int;
     }
 #line 4 "pure2-types-smf-and-that-3-provide-mvconstruct-and-mvassign.cpp2"
     auto myclass::operator=(myclass const& that) -> myclass& {
-        name = that.name;
-        addr = that.addr;
+        name = {that.name};
+        addr = {that.addr};
 #line 5 "pure2-types-smf-and-that-3-provide-mvconstruct-and-mvassign.cpp2"
         std::cout << "ctor - copy (GENERAL)";
         return *this;
@@ -83,8 +83,8 @@ auto main() -> int;
 
 #line 18 "pure2-types-smf-and-that-3-provide-mvconstruct-and-mvassign.cpp2"
     auto myclass::operator=(myclass&& that) -> myclass& {
-        name = std::move(that).name;
-        addr = std::move(that).addr;
+        name = {std::move(that).name};
+        addr = {std::move(that).addr};
 #line 19 "pure2-types-smf-and-that-3-provide-mvconstruct-and-mvassign.cpp2"
         std::cout << "assign - move        ";
         return *this;
@@ -100,8 +100,8 @@ auto main() -> int;
     }
 #line 22 "pure2-types-smf-and-that-3-provide-mvconstruct-and-mvassign.cpp2"
     auto myclass::operator=(cpp2::in<std::string> x) -> myclass& {
-        name = x;
-        addr = "123 Ford Dr.";
+        name = {x};
+        addr = {"123 Ford Dr."};
 
 #line 24 "pure2-types-smf-and-that-3-provide-mvconstruct-and-mvassign.cpp2"
         std::cout << "ctor - from string   ";