Require that floating-point literals write a digit after ., enables UFCS on numeric literals

(1) I find `1.0 and `1.0f` clearer than `1.` and `1.f` anyway, so that's a small plus.
(2) This was the only thing in the way of using UFCS on numeric literals, so that's another maybe-medium plus.

And the latter enables writing unit libraries like `42.gram()`, as proposed in #284's comment thread, which is an interesting use of UFCS on literals. So I'll mark this as closes #284.

Author: hsutter

regression-tests/mixed-float-literals.cpp2

@@ -72,28 +72,20 @@ literals_cpp2: () = {
   123'456LL;
   // float points
   123'456.0f;
-  123'456.f;
   456.0;
-  456.;
   1.0e10;
   1.0e+10;
   1.0e-10;
   1.0e-10f;
-  1.e-10;
-  1.e-10f;
   1e-10;
   1e-10f;
   1e-1'0;
   123'456.0F;
-  123'456.F;
   456.0;
-  456.;
   1.0E10;
   1.0E+10;
   1.0E-10;
   1.0E-10F;
-  1.E-10;
-  1.E-10F;
   1E-10;
   1E-10F;
   1E-1'0;
@@ -108,6 +100,15 @@ literals_cpp2: () = {
   // 1e - 10.0;
   // 1e - 10f;
   // 1e - 10.0f;
+  // 123'456.f;
+  // 123'456.f;
+  // 456.;
+  // 1.e-10;
+  // 1.e-10f;
+  // 123'456.F;
+  // 456.;
+  // 1.E-10;
+  // 1.E-10F;
 }
 
 main: ()->int = { }

regression-tests/test-results/mixed-float-literals.cpp

@@ -61,7 +61,7 @@ void literals_cpp1() {
 
 #line 57 "mixed-float-literals.cpp2"
 auto literals_cpp2() -> void;
-#line 113 "mixed-float-literals.cpp2"
+#line 114 "mixed-float-literals.cpp2"
 [[nodiscard]] auto main() -> int;
 
 //=== Cpp2 definitions ==========================================================
@@ -86,28 +86,20 @@ auto literals_cpp2() -> void{
   123'456LL;
   // float points
   123'456.0f;
-  123'456.f;
   456.0;
-  456.;
   1.0e10;
   1.0e+10;
   1.0e-10;
   1.0e-10f;
-  1.e-10;
-  1.e-10f;
   1e-10;
   1e-10f;
   1e-1'0;
   123'456.0F;
-  123'456.F;
   456.0;
-  456.;
   1.0E10;
   1.0E+10;
   1.0E-10;
   1.0E-10F;
-  1.E-10;
-  1.E-10F;
   1E-10;
   1E-10F;
   1E-1'0;
@@ -122,6 +114,15 @@ auto literals_cpp2() -> void{
   // 1e - 10.0;
   // 1e - 10f;
   // 1e - 10.0f;
+  // 123'456.f;
+  // 123'456.f;
+  // 456.;
+  // 1.e-10;
+  // 1.e-10f;
+  // 123'456.F;
+  // 456.;
+  // 1.E-10;
+  // 1.E-10F;
 }
 
 [[nodiscard]] auto main() -> int{}

source/cppfront.cpp

@@ -2562,7 +2562,7 @@ class cppfront
             }
 
             // Going backwards if we found LeftParen it might be UFCS
-            // expr_list is emited to args variable for future use
+            // expr_list is emitted to 'args' for future use
             if (i->op->type() == lexeme::LeftParen) {
 
                 assert(i->op);

source/lex.h

@@ -1233,7 +1233,7 @@ auto lex_line(
                     auto j = 1;
                     while (is_separator_or(is_digit,peek(j))) { ++j; }
                     if (
-                        peek(j) != '.'
+                        (peek(j) != '.' || !is_digit(peek(j+1)))
                         && peek(j) != 'f'
                         && peek(j) != 'F'
                         && peek(j) != 'e'
@@ -1259,6 +1259,12 @@ auto lex_line(
                         // slurps the digits after '.'
                         if (peek(j) == '.') {
                             ++j;
+                            if (!is_digit(peek(j))) {
+                                errors.emplace_back(
+                                    source_position(lineno, i),
+                                    "a floating point literal must have at least one digit after the decimal point (can be '.0')"
+                                );
+                            }
                             while (is_separator_or(is_digit,peek(j))) {
                                 ++j;
                             }