Make output -Wunused-parameter-clean, and add real support for _ unnamed parameters, closes #560

Emit `[[maybe_unused]]` on a `that` parameter if the class is empty.
Add -Wunused-parameter to my GCC and Clang regression tests.
Add `_` unnamed parameter support, and use ordinals instead of line/col numbers to keep the generated names stable.

As an example from one of the updated regression tests that had a helper function to just accept (but not use) five generic parameters, that function is now just: `call: (_, _, _, _, _) = { }`

Author: hsutter

regression-tests/mixed-forwarding.cpp2

@@ -8,9 +8,9 @@ struct X {
     X(X &&     that) : i{that.i} { std::cout << "move X " << i << "\n"; }
 };
 
-copy_from: (copy x:_) = { }
+copy_from: (copy _) = { }
 
-use: (x:_) = {}
+use: (_) = {}
 
 // invoking each of these with an rvalue std::pair argument ...
 apply_implicit_forward: (forward t: std::pair<X, X>) = {

regression-tests/mixed-parameter-passing-with-forward.cpp2

@@ -3,12 +3,12 @@
 #include <cstdlib>
 #include <ctime>
 
-copy_from: (copy x:_) = { }
+copy_from: (copy _) = { }
 
 parameter_styles: (
-    in      a: std::string, // "in" is default
+    in      _: std::string, // "in" is default
     copy    b: std::string,
-    inout   c: std::string,
+    inout   _: std::string,
     move    d: std::string,
     forward e: std::string
     )

regression-tests/mixed-parameter-passing.cpp2

@@ -3,12 +3,12 @@
 #include <cstdlib>
 #include <ctime>
 
-copy_from: (copy x:_) = { }
+copy_from: (copy _) = { }
 
 parameter_styles: (
-    in    a: std::string, // "in" is default
+    in    _: std::string, // "in" is default
     copy  b: std::string,
-    inout c: std::string,
+    inout _: std::string,
     move  d: std::string
     )
 = {

regression-tests/mixed-postfix-expression-custom-formatting.cpp2

@@ -1,5 +1,5 @@
 
-call: (v:_, w:_, x:_, y:_, z:_) = { }
+call: (_, _, _, _, _) = { }
 
 test: (a:_) -> std::string = {
     return call( a,

regression-tests/pure2-bugfix-for-discard-precedence.cpp2

@@ -1,8 +1,8 @@
 quantity: type = {
   number: i32;
-  operator=: (out this, _: std::in_place_t, x: i32) = {
+  operator=: (out this, i: std::in_place_t, x: i32) = {
     number = x;
-    _      = _;
+    _      = i;
   }
   operator+=: (inout this, that) -> forward quantity = {
     number += that.number;

regression-tests/pure2-bugfix-for-memberwise-base-assignment.cpp2

@@ -1,7 +1,7 @@
 Base: type = {
   operator=: (out this) = { }
   operator=: (out this, that)       = std::cout << "(out this, that)\n";
-  operator=: (implicit out this, x) = std::cout << "(implicit out this, x)\n";
+  operator=: (implicit out this, _) = std::cout << "(implicit out this, _)\n";
 }
 
 Derived: type = {

regression-tests/pure2-types-basics.cpp2

@@ -19,7 +19,7 @@ myclass : type = {
 
     operator=: (out this, x: int, s: std::string) = {
         this.data = 77;
-        this.more = s + " plugh";
+        this.more = s + std::to_string(x) + " plugh";
         std::cout << "myclass: from int and string\n";
         print();
     }

regression-tests/pure2-types-inheritance.cpp2

@@ -5,7 +5,7 @@ Human: @interface type = {
 
 N: namespace = {
     Machine: @polymorphic_base <I:int> type = {
-        operator=: (out this, id: std::string) = {}
+        operator=: (out this, _: std::string) = {}
         work:      (virtual this);
     }
 }

regression-tests/pure2-ufcs-member-access-and-chaining.cpp2

@@ -21,7 +21,7 @@ main: () -> int = {
     42.no_return();
 }
 
-no_return: (x: int) = { }
+no_return: (_) = { }
 
 ufcs: (i:int) -> int = {
     return i+2;
@@ -37,5 +37,5 @@ get_i: (r:_) -> int = {
 }
 
 //  And a test for non-local UFCS, which shouldn't do a [&] capture
-f: (x)->int = 0;
+f: (_)->int = 0;
 y: int = 0.f();

regression-tests/test-results/clang-12/pure2-types-basics.cpp.execution

@@ -11,7 +11,7 @@ myclass: explicit from string
 myclass: default
     data: 504, more: 3.141590
 myclass: from int and string
-    data: 77, more: hair plugh
+    data: 77, more: hair1 plugh
 x's state before assignments:     data: 1, more: 504
 myclass: implicit from int
     data: 84, more: 504